Glauconite

Earth-Science Reviews Elsevier Publishing Company, Amsterdam - Printed in The Netherlands

Glauconite S.G. McRae

ABSTRACT

McRae, S.G., 1972. Glauconite. Earth-Sci.Rev., 8:397-440.. The term glauconite has been employed in two senses. It has been used most commonly as a morphological term for sand-sized greenish grains found in sedimentary rocks, but also as a name for a specific mineral species, a hydrated iron-rich micaceous clay mineral. The two uses are not synonymous, since not all morphological glauconite consists exclusively of mineral glauconite, nor is the latter restricted in its occurrence to such pellets. Mineral glauconite in sensu lato is a random interstratification of nonexpanding 10 A layers and expanding montmorillonitic layers. The amount of expandable layers may be over 50 % but it is customary to restrict the name mineral glauconite in sensu stricto to varieties with less than 10 % expandable layers. The variation in amount of expandable layers explains many of the observed variations in the properties of glauconite including chemical composition (especially potassium content), thermal characteristics, cation exchange capacity, colour, refractive index and specific gravity. Mineral glauconite is believed to form by the progressive absorption of potassium and iron by a degraded layer silicate lattice of low lattice charge and elimination of other silicate-lattice types under suitable environmental conditions, of which the most critical seems to be the redox potential. The catalytic activity of marine organisms is no longer thought to be essential, although decaying organic matter and empty foraminiferal tests supply the ideal environment for glauconite genesis. The process of glauconitization is arrested by rapid sedimentation, so that there is a relationship between the variety of mineral glauconite formed and the nature of the host rock. Glauconite is found associated particularly with marine transgressions. Morphological glauconite grains are believed to form as casts, faecal pellets or by aecretionary growth, but may have their morphology modified by subsequent re-working. A number of characteristic internal and external morphologies have been recognised. The wide range of environmental conditions suitable for its formation and its common detrital occurrence debars the use of glauconite in palaeo-environmental studies. Its major use in geology is for the absolute age dating of sedimentary rocks by the K - A r method. Glaueonitic deposits have no present day commercial value, but soils formed on glauconitic parent materials are notable for their fertility. Glauconite weathers by loss of potassium to produce a montmorillonitic or vermiculitic product with the release of, or oxidation of, structural iron, so that the grain has the appearance of having weathered,to limonite.

INTRODUCTION

This review attempts to provide a synthesis of the current views on the nature, m i n e r a l o g y , o c c u r r e n c e a n d f o r m a t i o n o f g l a u c o n i t e . It is n e c e s s a r y first to define t h e use o f t h e t e r m " g l a u c o n i t e " as it h a s a c q u i r e d a dual c o n n o t a t i o n ( B u r s t , 1958a, 1 9 5 8 b ) . It h a s b e e n m o s t c o m m o n l y u s e d b y geologists as a m o r p h o l o g i c a l t e r m for r o u n d e d , sandsized, g r e e n i s h , e a r t h y - l o o k i n g grains f o u n d i n s e d i m e n t a r y rocks. It h a s also b e e n emp l o y e d as a n a m e for a specific m i n e r a l species, a h y d r a t e d i r o n - r i c h m i c a c e o u s clay

398

S.G. McRAE

mineral related to the illites. It is not clear if the first uses of the name as "la glauconite" (Brongniart, 1823) or "glaukonit" (Keferstein, 1828) were meant as morphological or nfineralogical terms, but as Burst (1958b) has noted, the suffixes "-it" or "-ite", by subsequent convention, pressure for a milaeralogical interpretation. Current French nomenclature (Millot, 1964) reserves "glauconite" for the mineralogical interpretation and uses "la glauconie" for the morphological. This distinction is not made in English, and considerable confusion has arisen since the two uses of the word are not synonymous. Not all greenish earthy pellets from sedimentary rocks - morphological glauconite - consist exclusively of mineralogical glauconite, nor is the latter restricted in its occurrence to such pellets. Much of the variability in the reported physical and chemical properties of glauconite is due to the mineralogical variability of the pellets, and the validity of such data is questionable unless mineralogical information is supplied. The term "glauconite" used alone is thus best regarded as a morphological term and will be used as such in this review, while the description "mineralogical glauconite" will be employed, where justified, to distinguish the mineral species. The name glauconite has also been used erroneously for a closely related mineral, celadonite, occurring in altered volcanic rocks. Althotigh structurally very similar to glauconite (Hendricks and Ross, 1941; Pirani, 1963b; Wise and Eugster, 1964) it differs chemically from true mineral glauconite (Wise and Eugster, 1964; Foster, 1969). The term celadonite has been used erroneously for magnesium- (Urban, 1957) and iron(Malkova, 1956) rich varieties of glauconite. Skolite, a name suggested for aluminous varieties of glauconite (Smulikowski, 1936) has not been widely adopted (Lazarenko, 1956). The name glauconite, from the Greek TXavKo¢, bluish or pale green, was first used by Brongniart (1823), although the credit is sometimes given to Keferstein (1828). Previously the material had been called greensand, greenearth, grtinerde, terre verte, etc. Many of the early investigations on glauconite were made on the widespread occurrences in the Cretaceous and Tertiary of the N.E. United States (Bailey, 1856; Cook, 1868; Clark, 1894; Bagg, 1898) but a fresh impetus in the study of glauconite was given by the voyages of oceanographic survey ships (Von Gtimbel, 1886; Murray and Renard, 1891; Collet and Lee, 1906a, 1906b; Murray and Phillippi, 1908 quoted by Hadding, 1932; Correns, 1937) which provided information on glauconite in modern marine sediments.

f

Since graduation in 1965 from the University of Aberdeen S.G. McRae has been lecturer in geology and soil science at Wye College, University of London. His research has been concerned mainly with the relationships between geology and soils in Southeast England, and one aspect of this, the effect of indigenous glauconite on soil fertility, was the subject of a doctoral thesis, presented in 1971.

GLAUCONITE

399

Other notable contributions to the knowledge of glauconite and its occurrence, and often including good historical reviews, have been made by Clarke (1916, 1920, 1924), Mansfield (1919a, 1919b, 1920, 1922), Schneider (1927), Hadding (1932), Gruner (1935), Galliher (1935a, 1935b, 1939), Steenhuis (1937-1939), Hendricks and Ross (1941), Smulikowski (1954), Cloud (1955), Warshaw (1957), Burst (1958a, 1958b), Valeton (1958), Hower and co-workers (Toler.and Hower, 1959; Hower, 1961; Pratt (1961, 1962a, 1962b, 1963a, 1963b), Manghnani and Hower, (1964a, 1964b), Porrenga (1963, 1966, 1967), James (1966), Triplehorn (1966a), and Fairbridge (1967). MORPHOLOGICALGLAUCONITE

External morphology Morphological glauconite occurs mainly in the form of sand-sized grains (Fig. 1) usually in the size range of 100-500 /am (Williams et al., 1954; Teodorovitch, 1961; Pratt, 1963a; Mero, 1965). It may also occur in finely divided form as pigmentary glauconite and as a secondary mineral infilling cracks and hollows or replacing pre-existing minerals. The schemes of Cayeux (1897, 1916, 1932) were the first to attempt to classify glauconite on a morphological basis (see Millot, 1964 for details).

Fig. 1. Typical glauconitepellets from the Chloritic (Glauconitic)Marl, Lower Cretaceous, Folkestone, Kent.

400

S.G. McRAE

A more comprehensive and less genetic classification is that of Triplehorn (1966a): (1) Spheroidal or ovoidal pellets. This variety is very common, and many of the grains comprising the New Jersey greensands are of this type. (2) Tabular or discoidal pellets. A rather rare variety. (3) Mammillated pellets, of irregular shape, consisting of small rounded knobs separated by shallow sutures. Konta (1967) considers these to be a peculiar case of lobate (type 4) pellets, and Triplehorn (1967) has agreed that categories 3 (mammillated) and 4 (lobate) could be combined in a general category of lobate pellets. (4) Lobate pellets, very irregular with deep radial cracks which are triangular in cross section.They are said to resemble pop-corn. Such grains are common. (5) Capsule-shaped pellets (Triplehorn, 1966a, 1967; Konta, 1967). These are commonly thought to have formed from faecal pellets (Takahashi and Yagi, 1929; Takahashi~ 1939) but continued attrition of ovoidal pellets (category 1) could also result in grains with capsule or ellipsoidal shape. (6) Composite pellets. These are normally fairly large, up to 3 - 4 mm in diameter, and consist of smaller grains of glauconite and detrital minerals embedded in a glauconite matrix which is usually paler in colour. These grains are relatively uncommon. (7) Vermicular grains. These have also been called caterpillar, zebra, concertina, accordion or booklet grains, and form by the alteration of a flake of mica, probably biotite (Galliher, 1935a, 1935b, 1939; Carozzi, 1951; Seed, 1965), but possibly muscovite (Ichimura, 1940; Wermund, 1961 ; Hodgson, 1962). (8) Fossil casts and internal moulds, notably of foraminiferal tests, but also of sponge spicules, echinoderm spines and other shell debris. Grains with these morphologies may represent replacement structures or may be the end product of the transformation of detrital material infilling empty tests and shells. (9) Pigmentary glauconite, coating the surfaces and sometimes penetrating cracks and cleavages in other minerals. This may be due to direct replacement by glauconite of other minerals or may be a surface coating of finely divided glauconite derived from precipitation or alteration of finely divided detritus, or from disintegration of larger glauconite grains. Further categories may prove to be necessary to accommodate other distinctive morphologies, for example the "quarter-moon" and "cap" pellets of Wermund (1961). As will be discussed in a later section, the morphology of glauconite may be a valuable guide to its mode of origin. Differences in morphology may also be related to the mineralogical nature of the pellets (Ehlmann et al., 1963), although it is frequently found that pellets of very different morphologies are of similar composition (Barackman, 1964; Tapper and Fanning, 1968).

Internal morphology Associated with the external morphologies of glauconite listed above may be characteristic internal structures. The nomenclature schemes of Carozzi (1958, 1960), Wermund

GLAUCONITE

401

(1961) and Triplehorn (1966a) may be combined to give the following classification of internal structures: (1) Random microcrystalline - homogeneous aggregates of overlapping micaceous flakes without any preferred orientation. This is the commonest internal structure. (2) Oriented microcrystalline or possibly monocrystalline - pellets which tend toward unit extinction in polarized light. They seem (Warshaw, 1957) to be lamellar aggregates with a high degree of parallel orientation. (3) Micaceous or vermicular (Wermund, 1961) structures - similar to (2) but with the presence of incipient micaceous cleavage. All vermicular pellets have such an internal structure but not all pellets with micaceous internal structure are vermicular pellets. (4) Coatings on grains - chiefly on grains of heavy minerals and somewhat resembling oolites (see Carozzi, 1960 p.48). (5) Organic replacement structures - a great variety of fibrous, perforate or lamellar structures, sometimes preserving the internal structure of the organisms (Cayeux, 1897, 1935; Smulikowski, 1954; Triplehorn, 1961 ; Ehlmann et al., 1963), sometimes not. (6) Fibroradiated rims - glauconite grains with rims of different colour and structure compared to the bulk of the grain were reported by several early investigators (Murray and Renard, 1891 ; Cayeux, 1897; Bagg, 1898). These rims, termed the corona by Cayeux (1897) are peripheral zones of very small elongate crystals with radial orientation and apparently subdivided by numerous cleavage planes. In these fibroradiated rims (Triplehorn, 1966a) concentric layering is common and they may be continuous or discontinuous around the periphery, with an abrupt junction to the rest of the pellet. The rims are also more birefringent than the rest of the pellet (e.g., Atanasov, 1962). An extensive study of such rims has been made by Zumpe (1962, 1971) on glauconite from the "Chloritic Marl" of the Isle of Wight, showing the presence of two and sometimes three types of microstructure. Both Zumpe and Triplehorn believe these rims to have formed by accumulation or precipitation on a pre-existing grain rather than by post-depo.~itional alteration of the pellets. Thin sections of glauconlte may also show the presence of inclusions, for example pyrite (Bagg, 1909), calcite (Kharkwal, 1966) or iron oxides (Murray and Mackintosh, 1968). Similar material may be present as coatings to the grains (e.g., Hancock, 1961 and the occurrences of "limonite" weathering rims) or fill cracks, especially in the lobate type of pellet (Adams, 1963; Drobnyk, 1965). These cracks and hollows have been attrib,uted to either e.xpansion of the pellets (Pratt, 1963b; Norris, 1964)or to partial desiccation (Warshaw, 1957; Ehlmann et al., 1963; Bailey and Atherton, 1969). A detailed study of internal cracks in glauconites from Northern Ireland (Bailey and Atherton, 1969) has shown linings of fiorous crystallites of apatite or glauconite enclosing a monocrysta Uine glauconite or haematite filling. Colour and op tical properties

As implied by its name, glauconite is usually green in colour, although the shade may vary from dark green, almost black, to a pale greenish-yellow (e.g., Butt, 1932; Needl 1am,

402

S.G. McRAE

1934; James, 1966). More rarely grains which are red, white or grey have been noted (e.g., Wermund, 1961). The shade of green is related to the relative amounts of iron and aluminium present (Burt, 1932; Dallwitz, 1952; Borchert and Braun, 1963; Caill~re and Lamboy, 1970) and particularly to the ratio of ferrous to ferric iron (Weyl, 1951 ; Keller, 1953, 1958) or the presence of tetrahedral ferrous iron (Hoebeke and Dekeyser, 1955; Dekeyser, 1956). Changes in the amount and form of iron and aluminium accompany the formation of mineral glauconite from a grey or pale-brown precursor (see p.000) and as the mineralogy of the material approaches that of true mineral glauconite so the colour becomes greener (Collet and Lee, 1906a; Alexander, 1934; Galliher, 1935a, 1935b; Glass, 1953; Ehlmann et al., 1963; Porrenga, 1963, 1966, 1967). Mature, well-crystallised mineral glauconite is frequently dark green in colour (e.g., Hoskins, 1895; Pratt, 1963a, 1963b; Murray and Mackintosh, 1968; Caill~re and Lamboy, 1970). Oxidation of glauconite by weathering or by heating in an oxidising atmosphere will change the colour (at least of an outer skin) to a rusty brown. Such grains are frequently reported as limonite. Glauconite in thin section is usually a more vivid green than the grain colour and may be pleochroic (Collet and Lee, 1906a; Ross, 1926; Hendricks and Ross, 1941 ; Hutton and Seelye, 1941; Warn and Sidwell, 1953; Keller, • 958; Atanasov, 1962; Lerbekmo, 1963). The aggregate polarisation shown by most grains renders detailed optical work impossible and reported optical data refer ordy to grains with vermicular or oriented microcrystalline structures. This data includes a maximum birefringence of about 0.020, a biaxizl negative interference figure with 2 V up to about 40 ° and an acute bisectrix nearly perpendicular to (001) (Lacroix, 1893; Ross, 1926; Schneider, 1927; Hadding, 1932; Hutton and Seelye, 1941; Grim et al., 1951; Winchell and Winchell, 1951; Milner, 1962; Bentor and Kastner, 1965). The only optical feature of diagnostic value for glauconite is the index of refraction. This may range from 1.56 (Dallwitz, 1952) to 1.64 (Toler and Hower, 1959) and is related to chemical and mineralogical properties of the grains especially to the percentage of Fe203 (Schneider, 1927; Hendricks and Ross, 1941; Hutton and Seelye, 1941; Toler and Hower, 1959) and to the percentage of expandable layers in the mineral (Sabatier, 1949; Toler and Hower, 1959; Pratt, 1962b) as shown in Fig. 2 and 3.

Other properties The specific gravity of glauconite grains varies trom 2.3 to 2.9 depending on mineralogical composition and degree of drying (Sabatier, 1949; Mursky and Thompson, 1958; Bentor and Kastner, 1965; Murray and Mackintosh, 1968), the presence of impurities and the degree of oxidation of the surface (Pentland, 1931; Gildersleeve, 1932; Needham, 1934; Sidwelt, 1936). Even apparently similar grains may have different specific gravities and it is commonly found during an attempted separation of glauconite grains from other minerals using heavy nquids that some glauconite grains will float while apparently identical grains will sink. In addition, glauconite may interact with some heavy liquids,

GLAUCONITE

403

1"64

1"63 ~ ' ~ +

+ +

+

++ 1"62 nz

++ 1-61

1-60

1 59

1-58

+

5I

10 percent

J 15

expandable

L

20

25I

++ t

30

35

4~0

layers

Fig.2. Relation between index of refraction (nz) and percentage of expandable layers in glauconites (r = - 0.94; from Toler and Hower, 1959).

especially those containing thallium salts, e.g., Clerici solution (Gruner, 1935; Hutton and Seelye, 1941; Hutton, 1950). The magnetic susceptibility of glauconite is relatively high and varies with the content of iron (Sabatier, 1949; Bentor and Kastner, 1965), as shown in Fig.4 below. Glauconite ffains may be effectively separated from other mineral grains using an isodynamic separator (Ehlmann et al., 1963) or even a simple magnetic field (Mansfield, 1920). It is important however to separate every glauconite ~rain from a sample by repeated applications of a magnetic field, i f a bias towards separation of grains high in iron is to be avoided (Sabatier, 1949). Glauconite is reported to have a hardness of 2.0 on Moh's scale (Milner, 1962, p.111), but the hardness is likely to be quite variable (Hutton and Seelye, 1941). MINERAL GLAUCONITE

X-ray diffraction studies of glauconite Early workers, using only the results of total chemical analyses, could conclude little more than that glauconite grains consisted of "hydrous potassium aluminium silicate".

404

S.G. McRAE

1"64

1.63

~

+

+ 1.62 nz

1.60

1"59

1-58

i

i 10

i

J

L

i 15 wt.

percent

i

i 20

i

i

i

*

2;

Fe20 3

Fig. 3. Relation between index of refraction (nz) and wt. percent Fe 203 in glauconites (r = -0.82). The solid lines are estimated curves for change in n Z with change in Fe203 for equal amounts of expandable layers. The number on each line is the amount of expandable layers to which that line refers (from Toler and Hower, 1959). With the advent of X-ray diffraction techniques, however, the similarity of glauconitic material to the micas was established (Ross in Schneider, 1927; Gruner, 1935; Tyler, 1936, Maegdefrau and Hofman, 1937; Galliher, 1939; Mehmel, 1939), but it was generally assumed that glauconite grains were homogeneous. A few early investigators, however, recognised the interstratified nature of mineral glauconite (e.g., Gruner in Galliher, 1939; Nagelschmidt, 1944), but it remained for Warshaw (1957) and Burst (1958a, 1958b) to establish clearly the mineralogical variability of grains classed morphologically as glauconite. These workers found by X-ray diffraction of oriented specimens that many glauconite grains consisted of a random interstratifi'cation of nonexpanding 10 A micaceous layers and expanding layers assumed to be montmorillonitic in nature. This material may be called mineral glauconite in sensu lato, with considerable variation in the amount of interlayering with expandable layers. Only a few samples of morphological glauconite grains were found to consist of material with sufficiently little ( 5 - 1 0 %) expandable layers to be classed as mineral glauconite in sensu stricto. In addition, Burst found some grains which did not consist of mineral glauconite but were instead comprised of heterogeneous mixtures of two or more clay minerals o f unrelated structure. The X-ray classffi-

GLAUCONITE

405

~o weight

/

31

+

Fe203

+

27

+

25

_+_

+

-t-

23

FeO

21 19 17 15 2'1

2~3 2L5 2'7 magnetic

2'9

311 3L3 3'5

susceptibility

3'7

39

C.G.S. × IO - 6

Fig.4. Relationship between magnetic susceptibility and total iron content in glauconite (from Bentor and Kastner, 1965). cation scheme of glauconite pellets proposed by Burst (1958a, 1958b) is shown in Table I together with a similar, but slightly more comprehensive, scheme due to Bentor and Kastner (1965), developed from that of Warshaw (1957). Further studies on the type and extent of interlayering in glauconite were made by Hower (1961) who found that interlayering was "almost exclusively between expandable (montmorillonitic) layers and nonexpandable 10 A layers". That the expandable layers are montmoriUonitic rather than vermicutitic was confirmed by the relationship between the percentage of expandable layers and the percentage of potassium in the samples (Manghnani and Hower, 1964a). Using the cation exchange capacity techniques developed by Alexiades and Jackson (1965) to differentiate between montmoriUonite and vermiculite McRae (1971) also showed the absence of vermiculitic layers in mineral glauconite. Hower (1966) has observed that mineral glauconites are dominantly randomly interstratified whereas illites and illite/montmorillonites which appear to form as a result of the deep burial of argillaceous sediments are dominantly ordered. It is convenient to regard mineral glauconite in sensu lato as a more or less continuous sequence of minerals with increasing amounts of expandable layers, and to restrict the term mineral glauconite in sensu stricto to material in which the amount of expandable layers is less than 10 % (Hower, 1961 ; Bentor and Kastner, 1965), although Burst (1958a, 1958b) used a limit of 5 % expandable layers for this category. The limits of expandable

406

S.G. McRAE

TABLE I X-ray classification of glauconite pellets

Burst (1858a, 1958b)

Hower (1961)

Bentor and Kastner (1965)

Well-ordered - non-swelling high potassium lattice showing sharp symmetrical peaks characteristic ofa micaceous 10 A lattice at 10, 5 and 3.3 A. This constitutes the type mineral glauconite

<~ 10 % expandable layers (montmorillonite)

Class 1: mineral glauconite (a) well-ordered 1 M with symmetric and sharp diffraction at 10.1, 4.53, 3.3A. Reflections (112) and (112-) are always present

Disordered - non-swelling, low potassium lattice, micaceous and monomineralic but with subdued peaks, displaying broad bases and asymmetric sides

10-20 % expandable layers (montmorillonite)

(b) disordered 1 Md with asymmetric basal diffractions broadened at the base. Reflections (112) and (112) are absent

Interlayered clay mineral - extremely disordered, expandable, low potassium montmorillonite type lattice

> 20 % expandable layers (montmorillonite)

Class 2: interlayered glauconite, d (001) > 10.15 A

Mixed mineral ~ mixtures of two or more clay minerals as normal constituents - the most frequent combinations being of illite with montmorillonite, and illite with chlorite

Class 3 : mixed mineral (a) two or more clay minerals (b) mixtures of clay with non clay minerals

There is no separate classification for pellets containing minerals which could be classed as impurities

Class 4 : green pellets containing no glauconite

layers f o u n d by H o w e r (1961) to correspond to the categories o f mineral glauconite recognised by o t h e r workers are also s h o w n in Table I. Fully i n d e x e d X-ray diffraction data o f r a n d o m l y orientated crystals o f mineral glauconite have b e e n given b y Sabatier (1949), G r i m et al. (1951) and Warshaw (1957), the last having been taken as the diffraction pattern for t y p e glauconite by the ASTM p o w d e r index file. R a n d o m l y orientated X-ray diffraction patterns have also been used to determine the p o l y m o r p h s present ( S m i t h and Yoder, 1956), which are invariably IM, an ordered one-layer m o n o c l i n i c p o l y m o r p h , if the mineral glauconite has a low percentage o f e x p a n d a b l e layers, and IMd, a disordered one-layer m o n o c l i n i c p o l y m o r p h , if a higher

GLAUCONITE

407

percentage of expandable layers is present (Warshaw, 1957; Burst, 1958a, 1958b; Hower, 1961; Tyler and Bailey, 1961; Bentor and Kastner, 1965; Velde, 1965; Bailey and Atherton, 1969). These polymorphs are typical of low temperature formation of the lattice (Yoder and Eugster, 1955; Velde, 1965). Values of the (060) line in randomly orientated diffraction patterns suggest a generally dioctahedral nature (Hoebeke and Dekeyser, 1955; Zen, 1959; Tyler and Bailey, 1961 ; Bentor and Kastner, 1965), and may aid in the differentiation of mineral glauconite and illite (Zen, 1959; Bentor and Kastner, 1965). The single crystal X-ray studies of vermicular pellets by Seed (1965) show the more regular internal structure of this type of pellet as compared with other types of pellet. The unit cell parameters of glauconite have been given (Frank-Kamensky, 1960) as: a b c /3

= 5.25 kX = 9.09 kX = 20.07 kX =95 °

Thermal characteristics of mineral glauconite Dehydration curves of mineral glauconite show weight losses between ambient and 150 °C and between 400 ° and 60() °C (Ross, 1926; Takahashi, 1939; Nutting, 1943; Sabatier, 1949; Kampioni-Zakrzewska, 1957). These weight losses show up as endothermic peaks on Differential Thermal Analysis (D.T.A.) curves. The typical shape of the D.T.A. curve was first given by Grim and Rowland (1942) and has been generally confirmed by later workers although an additional exothermic peak at approximately 350 °C has been recognised by Hoebeke and Dekeyser (1954a, 1954b). An idealised D.T.A. curve is shown in Fig.5 together with the actual peak temperatures found by various workers. The reactions giving these peaks have been studied, notably by Hoebeke and Dekeyser (1954a, 1954b): Peak A - due to loss of adsorbed water and not regarded as an important diagnostic peak. McRae and Lambert (1968) have noted that the size of this peak is proportional to the percentage of expandable layers. Sabatier (1949) has shown that if unground glauconite grains are used in D.T.A. a low temperature shoulder appears on this peak representing water sorbed in pores within the grain. Peak B - due to oxidation of structural Fe z+ (Hoebeke and Dekeyser, 1954a, 1954b) associated with a loss of - OH ions. This does not occur in vacuo (Bentor and Kastner, 1965). Peak C - due to dehydroxylation, the peak height and position being affected by the heating rate. McRae and Lambert (1968) have observed that the position of the peak is related to the percentage of expandable layers. Peaks D and E - recrystallisation to spinel minerals (Hoebeke and Dekeyser, 1954a, 1954b; Mikheev and Stulov, 1955).

408

S.C. McRAE

"~-

EXOTHERMIC PEAKS

B

E

AT " ~

_

_

~ ~/ ~

~ \ 5OO~ '

ENDOTHERMICPEAKS

~~

' ~

~

~ Temp°C

c

Fig. 5. G e n e r a l i s e d D . T . A . c u r v e o f g l a u c o n i t e . Investigators

Peak temperatures (°C) reported (A)

(B)

(C)

(D)

(E)

Grim and Rowland

(1942)

110

-

550

-

950

Sabatier

(1949)

190

-

575

-

960

K a u f f m a n a n d Dilling

(1950)

150

-

550

-

-

G r i m e t al.

(1951)

145

340

600

865

915

d e B r u y n a n d v a n d e r Marel

(1954)

100

300

570

900-940

-

Hoebeke and Dekeyser

(1954a)

-

350

570

-

925

Zaprozhtseva

(1954)

150-175

-

560-575

865

-

Honda

(1957)

150

450

510

-

940

C l o o s e t al.

(1961)

170-245

355-390

600-620

-

860-930

Tedrow

(1966)

150

-

560

-

925-975

Drobnyk

(1965)

150

-

560

-

-

Parry and Reeves

(1966)

150

-

650

-

-

GLAUCONITE

409

Infra-red studies of mineral glauconite Few infra-red investigations of mineral glauconite have been made, but the work of Cloos et al. (1960), Owens and Minard (1960), Ehlmann et al. (1963), Manghnani and Hower (1964b) and Young et al. (1968) may be cited. The study of Manghnani and HOwer (1964) is of particular interest since it relates the I.R. spectra to the percentage of expandable layers as determined by X-ray diffraction. Changes brought about in mineral glauconite by, for example, acid attack, heat treatments or reducing conditions have also been studied by I.R. methods (e.g., Touillaux et al., 1960; Cloos et al., 1961; Zabelin, 1962).

Electron microscopy and diffraction studies of mineral glauconite Several electron microscope photographs of mineral glauconite have been published (e.g., Kitazaki, 1951, MiJller and Behne, 1954; Gritzaenko, 1956; Pfefferkon et al., 1956; Burst, 1958b; Aleixandre Ferrandis and Gonzalez Pefia, 1960; Cloos et al., 1961 ; Bentor et al., 1963; Bentor and Kastner, 1965) but, in general, have contributed little to the knowledge of glauconite. Well-ordered mineral glauconites are usually lath-shaped while disordered and mixed mineral pellets tend toward equilateral plates. Electron diffraction studies have been made by Kitazaki (1951).

Chemical investigations of glauconite A very large number of total and partial chemical analyses of glauconite grains have been reported in the literature. Collections of these have been compiled by, amongst others, Clarke ('1915, 1916, 1920, 1924), Ross (1926), Hadding (1932), Hendricks and Ross (1941), Smulikowski (1954), Borchert and Braun (1963) and James (1966). The conversion of these analyses to formQlae has reflected the cUrrent state of knowledge of the structure of glauconite, with early workers (e.g., Ross, 1926; Hallimond, 1928) presenting formulae as mixtures of oxides. Later, chemical analyses were converted to structural formulae similar to those for the micas (e.g., Gruner, ]935; Smulikowski, 1936; Hendricks and Ross, 1941; Hutton and Seelye, 1941). A representative formula for glauconite given by Hendricks and Ross (1941): (K,Cax/2 ,Na)o .s 4 (Alo .47 Fe 0.973 +Fe o. x92 +Mgo.4 o )(8i3.65 Alo. 3 s )O 10 (OH)2 may be compared with that given for illite by Grim et al. (1937): • Ko.s s(All.3sFeo.3v 3+ Feo.o4 2+ Mgo .34)(S13.41Alo .s 9)01 o(OH)2

This shows the general similarity of the two minerals, but with glauconite containing more iron and less aluminium and with greater layer charge requiring more interlayer potassium for charge balance. A serious criticism of most published formulae is that they do not take into account the

410

S.G. McRAE

known interlayering within the structure which may cause large differences in structural formulae, as shown by Hower (1962, quoted by Manghnani and Hower, 1964a) in two representative formulae:

(Alo.2 s Fe i .103 +Ro.6 s 2 ÷XSi3.6 s Alo.3 s )O1 o(OH)2 Xl.o 1+ ( ( 5 % expandable layers containing Ko.9o) (All .30Feo.s o 3 +Ro.2 o2+)(Si3.6 s Alo.3 s )O1 o (OH):

Xo.s s l +

(40 % expandable layers containing Ko :a s) where R 2+ is the sum of magnesium and ferrous iron, and X ~+ is the number of moles of univalent interlayer cation which includes the amount of potassium given for each formula. With a few notable exceptions (Bentor and Kastner, 1965; Murray and Mackintosh, 1968; Bailey and Atherton, 1969) formulae are rarely corrected for the presence of impurities such as non-structural iron oxides (Mehra and Jackson, 1960; Bentor and Kastner, 1965), exchangeable cations (Foster, 1951), inclusions of other minerals, e.g., apatite and calcite, or the presence of non-glauconitic clay minerals (Alexiades and Jackson, 1966). In addition, the trial and error allocation of atoms between tetrahedral and octahedral layers to produce charge balance rarely takes into account the cation exchange capacity which is frequently large, and may also cause misallocation of aluminium (Bentor and Kastner, 1965), ferrous iron (Hoebeke and Dekeyser, 1955; Cloos et al., 1960) and perhaps other atoms. In spite of these objections, however, some marked trends in the chemical composition of glauconites can be observed, in particular the close inverse relationship between the potassium content of mineral glauconite and the percentage of expandable layers present in the lattice (Burst, 1958a, 1958b; Hower, 1961; Manghnani and Hower, 1964a; McRae and Lambert, 1968), as shown in Fig.6. It would appear that all glauconite pellets with > 8% K20 and most with > 7% K20 consist of mineral glauconite sensu stricto, with less than 10% expandable layers. Only half the specimens of Hendricks and Ross (1941) were thus likely to have been genuine mineral glauconite sensu stricto. Their generalised formula, given above, is thus incorrect as a formula for mineral glauconite sensu stricto, for which more representative formulae are:

Ko.79 Cao.o s (Alo.3 s Fe1.063 +Feo.2 s 2 +Mgo.41 )(Si3.61 Alo.a 9)01 o(OH)2 or Ko. 76 (Na,Ca)o. 1a (Alo .4 o Feo .s 73 +Feo .492 +Mgo .4 o)(Sia .42 Alo .s a )01 o (OH)2 i.e., those of the Bonne Terre and Franconia mineral glauconites of Cambrian age, respectively, given by Burst (1958a) as type mineral glauconites. Selective chemical dissolution studies made by Alexiades and Jackson (1966) have shown the Franconia glauconite, which should by defintion contain no chlorite, to be rich in chlorite. There is thus still a need to establish a type mineral glauconite sensu stricto.

GLAUCONITE

5

411

3 samples [-

4-

6

j

....." °."'" ~ " ~ ~ ' M C R a e & Lambert (1968)

-I-y"

~.-'"'" 4-

°"

.....'" \

~

7

§

8 ~-

~ - F . . . ' Y

1

0

10

20

30

40

50

percent expandable layers

Fig.6. The relationship between the percentage of expandable layers determind by X-ray methods and potassium content of glauconites. The dashed line is that given by Manghnani and Hower (1964a) and the solid line refers to the points determined by McRae and Lambert (1968). Mineral glauconites have a rather constant content of about 4 % MgO, i.e., 0.35-0.45 Mg ions per unit half cell (Hendricks and Ross, 1941; Millot, 1964; Bentor and Kastner, 1965). Non-marine occurrences of glauconite (see p.415) are of a variety richer in Mg and poorer in A1 than marine varieties (Tyler and Bailey, 1961). There seems a narrow range for the Fe 3 +/Fe 2 + ratio of 4.1 to 6.2 (Hendricks and Ross, 1941) with non-marine varieties having a rather low ratio (Tyler and Bailey, 1961). Most of this iron is believed to occupy octahedral sites, although evidence, such as the inverse relationship between FeO and A1203 contents (Bentor and Kastner, 1965) suggests that some Fe 2+ or Fe 3+ may occupy tetrahedral sites (Sabatier, 1949; Hoebeke and Dekeyser, 1955; Cloos et al., 1960). The number of octahedral cations frequently exceeds two per unit half cell and it seems likely that mineral glauconite is not a true dioctahedral mineral (Yoder, 1959; Bentor and Kastner, 1965). The amount of aluminium substituting for silicon in tetrahedral sites is normally greater than 0.21 ions per unit half cell. The amount of aluminium in octahedral sites is, however, very variable - from 0.12 to 1.27 ions per unit half cell (Hendricks and Ross, 1941). Traces of other elements are also found in glauconite grains 0-lower, 1961 ; Bentor and Kastner, 1965; Yasyrev, 1966; Balashov and Kazakov, 1968), although they may not be truly structural. The content of strontium, barium, vanadium and calcium seems related

412

S.G. McRAE

to the amount of expandable layers present (Hower, 1961; Bentor and Kastner, 1965) but other elements such as boron, rubidium and chromium do not show this trend. Inclusions of other minerals such as futile (Allen, 1937) and apatite (e.g., Hendricks and Ross, 1941) may account for some or all of the titanium, calcium and phosphate frequently reported in glauconite grains. The chemical relationships between mineral glauconite sensu lato, celadonite and other dioctahedral hydrous 2 : 1 layer silicates may be seen in the triangular diagram given initially by Yoder and Eugster (1955) but modified by Bentor and Kastner (1965) to eliminate from the field of glauconite those species with substantial amounts of expandable layers.

I'O/~$'0

7

@

I "2

~

1.4/~

x~" 0 0

/ / I'6 I I

o 0

_ ~J \-"\

/

~"/"

\

"'"\ \ I .

.......

-

TETRAHEDRAL

¢~ \ O's

o tmor

O.CD

.

o 0~

7"06`

.....9?" \ \"'" I

B

o c9

"j

0 ~

t

% - Ll'Jx

~

o ~

0

R -~3

Fig.7. Plot of tetrahedral R 3+ and octahedral R 3+ in atomic proportions of dioctahedral micas and related minerals. Field A (dotted line) is that originally given for glauconite by Yoder and Eugster (1955); field B (solid line) is the revised field suggested by Bentor and Kastner (1965).

Layer change and cation exchange capacity of mineral glauconite The distribution of ions between octahedral and tetrahedral sites suggests that the two layers contribute more or less equally to the overall charge of the mineral glauconite unit, although the octahedral layer tends to have a slightly higher charge than the tetrahedral layer (Foster, 1956, 1960). The overall negative charge is frequently not balanced by the

GLAUCONITE

413

interlayer potassium, giving rise to a net negative charge (the cation exchange capacity). Several of the mineral glauconites studied by Bentor and Kastner (1965), however, had a higher charge in the tetrahedral layer than in the octahedral, thought to be due to ferrous iron and especially magnesium partly filling vacant spaces within the octahedral layer. An increase in interlayer cation content has been observed (Tyler and Bailey, 1961) to be accompanied by a decrease in octahedral cations, i.e., an increase in net octahedral charge. This is believed to be a real trend whereas the apparently similar trend in montmorillonites has been shown (Foster, 1951) to be due to misallocation of Mg between structural and exchange sites. The cation exchange capacity (c.e.c.) of glauconite varies inversely with the amount of potassium and hence directly with the percentage of expandable layers (Manghnani and Hower, 1964a; McRae and Lambert, 1968). The range of c.e.c.'s found by these investigators is from 5 to 39 milli-equivalents per 100 g, a range easily covering the values earlier given by Grim et al. (1951), Carroll (1959) and Owens and Minard (1960). Manghnani and Hower (1964a) found that the solution of the equation of c.e.c, as a function of % expandable layers gave values of 8.2 and 62 mequiv./100 g for zero and 100 % expansible layers respectively. Exclusion of samples with > 30 % expansible layers which seemed to have anomalously low c.e.c, values gave a 100 % expansible layer c.e.c. value of 90 mequiv./100 g, a result consistent with the expandable layers being montmorillonitic in nature. The low results for the highly expandable glauconites, mostly of geologically recent range, was attributed to blocking of exchange sites by organic material or by the presence of fixed cations in the expandable layers. The selectivity of the exchange sites for monovalent cations has been studied somewhat inconclusively by Sabatier (1949), and for trace element cations by Libor (1962a, 1962b) and Libor and Varga (1963). Studies of glauconite as a "scavenger" for the nuclear waste products caesium and strontium by Schnepfe et al. (1964) have shown that the selective uptake of these from solution can be increased by increasing the pH of the medium. The c.e.c, of glauconite may also contribute to the total c.e.c, of sediments and soils in which it is found, in addition to that from the colloidal fraction. Tedrow (1966), for example, has studied a glauconitic soil whose sand fraction, containing glauconite, had a c.e.c, of 19 to 31 mequiv./100 g. THE OCCURRENCE OF GLAUCONITE Glauconite, both morphological and mineral, is restricted in its occurrence and formation to sedimentary rocks. It is most commonly found in marine sediments, but has also been reported from lacustrine and other alluvial occurrences. The glauconite found in sediments may be authigenic, i.e., may have formed more or less in situ or may be allogenic, i.e., detrital. The distinction between these two origins will usually involve consideration of the petrology of the rock (e.g.,Warn and SidweU, 1953; Wermund, 1961),the morphology of the grains (Light, 1952; Carozzi, 1958, 1960; Owens and

414

S.G. McRAE

Minard, 1960; Wermund, 1961; Adams, 1963; Triplehorn, 1966a; Zumpe, 1971), the mineralogy of the grains and associated sediment (Lebauer, 1964; Bell and Goodell, 1967), or the ratio of ferrous to ferric iron and the amount of interlayer ions (Owens and Minard, 1960). The detrital origin of glauconites can also be shown by K - A r dating (Allen et al., 1964; Bodelle et al., 1969) if the age of the host rock is known. Detrital glauconite may have been derived more or less contemporaneously from a nearby zone in the basin of deposition in which authigenic glauconite is forming especially under alkaline pH conditions. It may alternatively have been derived from submarine outcrops of pre-existing glauconitic rocks (e.g., Pratt, 1962a, 1962b; Bell and Goodell, 1967; Murray and Mackintosh, 1968). Derivation involving fluvio-detrital transport is possible (Allen et al., 1964; Bodelle et al., 1969) but unlikely (Wermund, 1964). The physical and chemical limits of glauconite formation have been discussed by Hadding (1932), Cloud (1955) and Lochman (1957). Their work has formed the basis of the limiting conditions set out in the following sections.

Stratigraphic range Glauconite may be found in sedimentary rocks of all ages from Precambrian to the present day. Cloud (1955) believed that glauconite was not present in Precambrian rocks but Precambrian occurrences have since been noted, e.g., Schaub (1955) and many workers using glauconite for K - A r dating (see below). It is particularly common in sediments of Cretaceous and Tertiary age, and areas of these rocks notably in S.E. England (Hallimond, 1922; Oakley, 1943; McRae and Lambert, 1968), N. France (Cayeux, 1897, 1916, 1935), N.E. United States (Cook, 1868; Mansfield, 1919a, 1919b, 1920, 1922; Zodac, 1945), New Zealand (Hutton and Seelye, 1941), Australia (Feldtmann, 1934; Carroll, 1939, 1941) and Russia (Glinka, 1896) are rich in glanconite. Glauconite is also fairly common in Lower Palaeozoic rocks, but relatively rare in rocks of other geological ages. It has been noted (Conway, 1942; Smulikowski, 1954; Hower, 1961) that glauconites from older rocks have more potassium and are less frequently mineralogically heterogeneous than younger glauconites. Lower Palaeozoic glauconites are thus more likely to consist of mineral glauconite sensu stricto and the two glauconites suggested by Burst (1958a, 1958b) as type examples are both of Cambrian age. This variation in potassium content with age has been attributed to a greater salinity in seas of earlier geological epochs (Tarr, 1920; Conway, 1942), but is probably due to lithological differences in the main types of host rock found in each era (Hower, 1961). There is evidence that glanconite may gain potassium in postdiagenic changes (Hower, 196t). It has also been shown (Smulikowski, 1954) that there is a reduction in the ratio of ferric to ferrous iron with increasing geologic age of glauconites. The use of glanconite in isotopic age dating of rocks will be discussed in a later section.

GLAUCON1TE

415

Present areal distribution Glauconite may be found forming today in all the oceans of the world, the limits of 65 ° S to 80 ° N given by Cloud (1955) delimiting simply the Antarctic continent and Arctic ice. It is found forming mainly on continental shelves away from large streams but does not seem restricted solely to the areas near granite- or biotite-bearing rocks as suggested by Galliher (1935a, 1935b, 1939) and Berthois and Dangeard (1939), although the potassium required for its formation would be plentiful in such areas.

Salinity It was believed for long that glauconite was restricted in its occurrence to a marine environment of normal salinity, although Cloud (1955) did admit of a possible theoretical occurrence in potassium-rich saline lakes. Glauconite from lacustrine and other continental deposits has been described by Jung (1954), Dyadchenko and Khatuntseva (1955, 1956), Keller (1958, 1959), Nicolas (1961), Parry and Reeves (1966) and Porrenga (1968). pH conditions which are slightly alkaline (pH 7 - 8 ) seem to be favourable for glauconite genesis (Dapples, 1967; Fairbridge, 1967).

Oxygenation The formation of glauconite has been reported in environments ranging from moderately anaerobic (Lochman, 1949; Cloud, 1955) to strongly oxidising (Van Andel and Postma, 1954); it is most common around Eh 0, and although some workers (e.g.,Teodorovich, 1946, 1947, 1954 - quoted by Larsen and Chilingar, 1967; Borchert, 1960a, i960b) favour a slightly oxidising environment (Eh +50 to +400 mV) most would follow the view that a slightly reducing environment (Eh 0 to - 2 0 0 mV) is more conducive to glauconitization (Lochman, 1949; Krumbein and Garrels, 1952; Cloud,1955; Burst, 1958a, 1958b; Dapples, 1967; Fairbridge, 1967). Sometimes (Emery, in Cloud, 1955) it can be shown that although the macro-environment may be strongly oxidising, the actual glauconitization occurs in a more reducing micro-environment, such as is accorded by the empty tests of forams. The oxygenation requirements are most commonly met where there is slow bacterial decomposition of organic matter (see next section) although such conditions may be met in other ways, for example by escaping gases from coal (Seed, 1965). More complicated mechanisms have been proposed, including the movement of oxygenated water to reducing areas causing local zones suitable for glauconite formation (Tyler and Bailey, 1961; Leclaire, 1964; Aldinger, 1965). Alternatively, a precursor of glauconite or "pro-glauconite" may itself be moved from an oxidising to a reducing milieu (Millot, 1949) or vice versa (Krotov, 1953, quoted by Chilingar, 1956; Zaporozhtseva, 1954; Adams, 1963).

416

S.G. McRAE

Organic matter of bottom sediments It is generally agreed (Cloud, 1955) that formation of glauconite is facilitated by the presence of decaying organic matter providing the required oxygenation conditions described above. Such an environment is a favourable habitat for sediment-ingesting organisms with low oxygen requirements, but there has been considerable discussion as to whether or not the presence of organisms is essential for glauconite genesis. Many early workers, (e.g., Bailey, 1856; Ehrenberg, 1856; Murray and Renard, 1891) believed that an active, living organic agency was required, whereas others (e.g., Von Giimbel, 1886; Prather, 1905), while admitting the observed fact that glauconite pellets are often of casts of forams or coprolitic pellets, believed that these occurrences were only specific cases in a general mode of origin which did not absolutely require the presence of organisms. The currently accepted view is that the presence of organisms is not essential for the formation of mineral glauconite but that empty tests and shells can form suitable microenvironments for the mineralogical transformations, and the formation of grains of glauconite may in many cases involve organic activity, e.g., faecal pellets.

Temperature The view of Cloud (1955) that glauconite formation is more generally favoured by cool water is incorrect (Fairbrige, 1967) for its most widespread occurrence today is in warm shelf seas. The precise temperature limits of glauconite formation have not been established but a minimum temperature of about 15 °C (Porrenga, 1967) and a maximum of 20 °C (Takahashi and Yagi, 1929) would seem reasonable limits. Water temperature together with oxygenation conditions serve to explain the depths at which glauconite formation is observed (see next section). The relative absence of glauconite in preMesozoic oceans (Conway, 1942) may be due to the prevailing relatively warm conditions favouring the formation of chamosite rather than glauconite (Braun, 1962) or may instead be due to the postulated higher pCO2- (Fairbridge, 1967). Favourable conditions for the formation of glauconite are said to occur where cold and warm currents meet (Murray and Phillippi, 1908 - quoted by Cloud, 1955; Goldman, 1921a, 1921b; Hummel, 1922; Gill, 1927; Takahashi and Yagi, 1929; Borchert, 1960a,1960b) probably because of the high organic productivity resulting from such conditions. In general it seems unwise to regard glauconite as a reliable temperature index (Cloud and Barnes, 1948).

Depth Glauconite is unlikely to form in water shallower than about 15 m because of waveinduced turbulence (Murray and Renard, 1891; Cloud, 1955; Fairbridge, 1967). Below this depth its formation is governed by the temperature and oxygenation conditions (e.g., Borchert, 1960a, 1960b, 1964; Porrenga, 1963, 1966, 1967; Giresse, 1965). In tropical

GLAUCONITE

417

areas, the warm surface water conditions seem to prevent glauconitization until cooler waters at depths of 250 m and deeper are encountered (Porrenga, 1967). In non-tropical areas, glauconitization is commonly found at depths as shallow as 30 m (Cloud, 1955; Porrenga, 1967). Glauconite formation is rarely noted in water deeper than that encountered at the shelf margin (about 500 m - Fairbridge, 1967). Glauconite grains found at depths greater than this (e.g., 4,000 m by Murray and Renard, 1891) are likely to have been transported to these localities and not to represent in situ formation at these depths. It would appear that, unless other conditions such as those of temperature and oxygenation can be established independently, glauconite cannot be used as an indicator of water depth at the time of sediment deposition.

Turbulence Evidence of turbulence is commonly found in the associated sediments (Hadding, t932; Lochman, 1949; Cloud, 1955), although continued marked turbulence would seem incompatible with the requirement for a particular narrow range of oxygenation conditions. It is most probable that where glauconite grains are found in association with agitated sediments they are allogenic (Wermund, 1961), i.e., introduced more or less contemporaneously from other parts of the sea-floor nearby or, less likely, derived from pre-existing rock strata. It is then possible to postulate the absence of turbulence at the actual loci of glauconite formation, or the restriction of glauconite formation to nonturbulent times (Lochman, 1949). Odin (1971) has suggested that the formation of glauconite is favoured by alternating flow of the water which allows formation of the granules and transport of ions and of food for organisms whose catalytic activity is believed essential.

Source materials The starting materials for glauconite formation stated by Cloud (1955) are, in general, micaceous minerals, preferably of a degraded nature, or bottom muds of high iron content. In view of the variety of theories for the formation of glauconite to be reviewed later, it would seem advisable to reserve discussion of the source materials for glauconite formation until then.

Sedimentary influx and nature of associated sediment The formation of glauconite is favoured by a low or even a negative sedimentation rate (Murray and Renard, 1891; Goldman, 1921a, 1922; Hadding, 1932; Cloud, 1955; Miiller, 1967). Such conditions, together with other favourable environmental conditions, may be provided during a marine transgression, so that glauconite is commonly found associated with unconformities and other stratigraphic breaks (Goldman, 1921a, 1921b; ReckFroUo, 1963). In modem shelf areas it is almost invariably found associated with relict deposits dating from the inundation of these shelf areas due to rising sea level following

418

S.G. McRAE

the last glaciation (e.g., Allen, 1965; Clarke, 1969). It has been suggested (Rech-Frollo, 1963) that the large amounts of organic matter from inundated terrestrial soils during a marine transgression may help to provide favourable environmental conditions for glauconite genesis. Glauconite forms most readily at the water/sediment interface and rapid sedimentation would arrest tile glauconitization process by burying the developing glauconite grains. Thus glauconite grains from sediments with high influx of detritus, e.g., argillaceous sandstones and marls frequently contain extraneous clay minerals and the mineral glauconite has a higher percentage of expandable layers (Hower, 1961). By contrast, glauconite grains from clean sandstones, limestones and dolomites with low sedimentation rates are almost exclusively monomineralic, consisting only of mixed-layer illite/ montmorillonite with a low percentage of expandable layers. The glauconite associated with geological unconformities is frequently of a highly ordered form (Burst, 1958b) suggesting slow burial of the glauconite. It would be interesting to establish from modern shelf areas whether the mineralogical nature of the glauconite was related to the rate of sedimentation following the transgression, and if possible to establish the time required for the glauconitization process to reach completion. The lithological rock types in which authigenic glauconite is most common are calcareous detrital sediments such as calcareous sandstones and impure granular limestones, while it is rare in pure clay rocks, pure quartz sandstones or chemically precipitated carbonates. It may occur as detrital glauconite in a wide range of sedimentary rocks, particularly if calcareous, since it seems highly stable in alkaline conditions (Fairbridge, 1967). As a detrital mineral it may become associated with other minerals with which its genesis is incompatible e.g., chamosite (Chilingar, 1956). Glauconite is commonly found associated with phosphatic deposits (e.g., Goldman, 1922; Wetzel, 1937; Kazakov and lsakov, 1940; Carozzi, 1958; Litvinenko, 1965; Rooney and Kerr, 1967; Bailey and Atherton, 1969) and more rarely with other nonclastic deposits such as fluorides (Simpson, 1934) and celestine (SrSO4)(Mogarovsky, 1963a, 1965). It is sometimes found in association with altered volcanic ash (Ross, 1926; Atanasov et al., 1964; Hallam and Sellwood, 1968; Seed, 1968) although such occurrences may really be of celadonite (e.g., Rossetti and Sitzia, 1956; Pirani, 1963a, 1963b; Kubovics, 1964; Peyronel Pagliani and Pagnani, 1965). Green grains, identified as glauconite usually only on morphological evidence, may be found in beds of sedimentary iron ore (e.g., Spurr, 1894; Cayeux, 1906, 1924; Clarke, 1924; Hallimond, 1925; Eckel, 1938; Taylor, 1959; James, 1966; Amstutz and Bubenicek, 1967). Some reported occurrences of glauconite (Spurr, 1894; McCallie, 1908; Galliher, 1935b)have been shown to be of chloritic minerals such as greenalite (Clarke, 1903; Leith, 1903; Jolliffe, 1935) or chamosite (Brown, 1914; Caill~re and Giresse, 1966). Conversely some "chloritic material" in iron ore deposits has proved to be glauconite (Spurr, 1915; Ross, 1916). The name chlorite has also been used erroneously for the well-crystallised mineral glauconite of the Bonneterre Formation in Missouri (Tarr, 1918) and of the "Chioritic Marl" at the base of the Chalk in S.E. England which is in reality a glauconitic marl (Jukes-Browne, 1903, pp.12-14; McRae and Lambert, 1968).

GLAUCON1TE

419

THE FORMATIONOF GLAUCONITE Theories explaining the formation of glauconite have to account for both the mineralogy and the morphology of the material formed. Most theories have concentrated on the mineralogical aspects of glauconite genesis. These will be reviewed first, followed by a review of the mechanisms proposed to account for the morphology.

The formation of mineral gtauconite The early theories of mineral glauconite genesis proposed the co-precipitation of'Mg, Fe, A1 and Si gels to which potassium was attracted (Murray and Renard, 1891 ; Collet, 1908). Equally vague were theories proposing the alteration of mud (e.g., Collet, 1908; Takahashi and Yagi, 1929, Takahashi, 1939). Galliher (1935a, 1935b, 1936, 1939), following earlier observations of Hummel (1923) and Alexander (1934) that there was an association between the presence of biotite and the formation of glauconite, showed that biotite could be converted to glauconite in a marine environment. This theory has received widespread support, and while it is almost certainly correct for booklet or accordion grains of glauconite (e.g., Hough, 1940; BaUance, 1964; Seed, 1965) the view of Galliher that biotite is the universal precursor of glauconite is over-stated, and the theory is best regarded as a rather specific case of the modern "layer lattice theory". According to the "layer lattice theory" (Burst, 1958a, 1958b) the formation of mineral glauconite requires simply: (1) a degraded layer silicate lattice; ( 2 ) a plentiful supply of iron and potassium; and (3) a suitable redox potential. Suitable redox potentials can be produced by decaying organic matter and even in a generally oxidising environment can occur in faecal pellets or within foramiferal tests and other debris, hence the frequent glauconitization of these materials. Other environmental factors, e.g., temperature, rate of sedimentation and parent material may influence the type of mineral glauconite formed. The process of glauconitization consists of absorption of potassium and iron by the degraded lattice leading to a reduction in the amount of expandable layers so that given suitable chemical conditions and sufficient time the material would approach the composition and structure of mineral glauconite sensu stricto. The precise mechanisms involved in the diagenetic ~hange from a degraded layer lattice to mineral glauconite have been further studied by Hower (1961). The starting material for glauconitization should have a low lattice charge since high lattice charge material readily adsorbs potassium from sea water and collapses to a non-expandable "lattice (Weaver, 1958), whereas expandable layers seem to be necessary for the initial adsorption of iron prior to its diffusion into the octahedral layer replacing aluminium ions. The model predicts that the increase in net lattice charge during glauconitization should take place by substitutions in the octahedral layers, and that the corresponding rise in the amount of charge-balancing interlayer cations (chiefly potassium) would be related to the net octahedral charge but be unrelated to the tetrahedral charge. This has been demonstrated by Hower (1961), as has the expected relationship between the contents of

420

i

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iron and potassium in mineral glauconites. This latter relationship has not, however, been confirmed by Bentor and Kastner (1965) or Foster (1969) who consider the incorporation of iron into the structure and the fixation of potassium to be unrelated processes. The model predicts that burial of the mineral glauconite should arrest the glauconitization process to give a low-potassium disordered mineral glauconite, which would explain the higher potassium (Conway, 1942; Smulikowski, 1954; Hower, 1961) and iron contents (Smulikowski, 1954) of glauconites from older geological epochs where the dominant environments were those with slow sedimentation rates. The higher potassium content o f older glauconites may also be due partly to post-diagenetic gains of potassium (Evernden et al., 1961; Hower, 1961; Hurley, 1961; Hower et al., 1963). The process of glauconitization tends to eliminate from the pellets any layer lattices not of the mineral glauconite type, e.g., kaolinite or chlorite, and other extraneous materials, making mature glauconites virtually monomineralic. It is believed (Burst, 1958a; Hower, 1961) that the mineralogical character of the glauconite pellets reflects only in part the source material, for example the tetrahedral charge, but that the evidence for diagenetic changes controlling the resulting mineralogy is overwhelming. During glauconitization the colour of the glauconite becomes darker green and the degree of hydration and refractive index decrease. These and mineralogical changes which confirm the hypotheses of Burst (1958a, 1958b) and Hower (1961) have been observed in modern glauconitic sediments by a number of workers (Glass, 1953; Pratt, 1961, 1962a, 1962b, 1963a, 1963b; Ehlmann et al., 1963; Porrenga, 1966). An alternative theory proposed by Ehlmann et al., (1963) in which weathered mica layers act as templates on which glauconite layers form, seems a specific case of a more general "epigenetic substitution" theory proposed initially to explain the replacement of calcite by mineral glauconite (Cayeux, 1924, 1932, 1935, 1938). Such replacement has been frequently reported (e.g., Dangeard, 1940; Houbolt, 1957; Wermund, 1961; Mogarovsky, 1963b, 1965; Kharkwal,1966; Lamboy, 1968). In situations where both silicate and carbonate materials are present, substitution apparently takes place preferentially in the carbonates (Seed, 1968). In the Qbsence of carbonates, however, mineral glauconite has been observed replacing such diverse non-micaceous silicate lattices as quartz (Hadding, 1932; Ichimura, 1940; Schreyer,1956; Tresise, 1960), orthoclase (Cayeux, 1897; Collet and Lee, 1906a; Murray and Phillippi, 1908; Ichimura, 1940; Dyadchenko and Khatuntseva, 1955, 1956; Gierwielaniec and Turnau-Morawska, 1961 ) plagioclase (Dyadchenko and Ktiatuntseva, 1955, 1956; Tresise, 1960), amphiboles (Glinka, 1896), pyroxenes (Gooch, 1876, quoted by Hadding, 1932; Glinka, 1896; Takahashi, 1939), and olivine (Gooch, 1876 quoted by Hadding, 1932). Fragments of silicate rocks such as andesite (Hendricks and Ross, 1941), phonolite (Ross et al., 1929), rhyolite (Ojakangas and Keller, 1964) and sandstone (Tresise, 1960) may also be replaced by mineral glauconite. It is possible that some of the above observations may not be due to actual replacements but to alteration of detrital clay as coatings of or infillings in cleavages and other cracks. The work of Ojakangas and Keller (1964) and Dapples (1967), however, provides ample evidence of actual replacement. Mineral glauconite may also

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replace phosphatic material (Hadding, 1932; Dyadchenko and Khatuntseva, 1955, 1956; Ojakangas and Keller, 1964) and be itself replaced by carbonate in some instances (e.g., Wermund, 1961; Ojakangas and Keller, 1964). Some examples of secondary glauconite have an unusual mineralogy (Tyler and Bailey, 1961; Wise and Eugster, 1964; Young et al., 1968).

The jbrrnation of morphological glauconite The typical morphology of glauconite may be due to: (1) Alteration of internal fillings to organic debris. (2) Conversion of remains in faecal pellets. (3) Transformation of biotite. (4) Agglomeration of clay-sized material. (5) Precipitation on or alteration of mineral surfaces to give pigmentary glauconite. (6) Replacement processes. These processes are not mutually exclusive. Many workers on glauconite (e.g., Bailey, 1856; Ehrenberg, 1856; Clark, 1894; Bagg, 1898, 1909; Collet, 1908;Mansfield, 1919b; Boswell, 1924; Dryden, 1931; Needham, 1934; Plumley and Graves, 1953; Burst, 1958a; Wermund, 1961; Pratt, 1962b; Ehlmann et al., 1963; Rao, 1964; Drobnyk, 1965; Seed, 1965; Kharkwal, 1966; Bell and Goodell, 1967) have noted that the shapes of many of the glauconite pellets are reminiscent of Foraminifera, and some have postulated that the production of glauconite is by the alteration of infillings in empty foram tests. Other workers (e.g., Von Giimbel, 1886; Prather, 1905; Schneider, 1927; Hadding, 1932; Cayeux, 1938) found that pellets with foraminiferal shapes were rare or absent in the glauconites which they studied, and therefore did not support this hypothesis. As discussed above, empty chambers in Foraminifera and other shell debris can provide microenvironments suitable for the m'ineralogical conversion of detrital clay to glauconite and the resulting material may be expected to reflect this origin in its morphology, at least initially. The relative importance of this mode of origin remains, however, an open question. It may be that many glauconite grains are formed in this way and that the general absence of clearly recognisable casts may be due to subsequent attrition. Conversely, it may be that this mode of origin is in general rather unusual and occurs only in isolated cases. Shapes other than Foraminifera, e.g., Radiolaria (Murray and Mackintosh, 1968), pieces of Echinoderma (Bailey, 1856; Berry, 1940; Wermund, 1961; Pratt, 1962a; Seed, 1965), Mollusca (Bailey, 1856; Mansfield, 1919b; Hadding, 1932; Wermund, 1961 ; Pratt, 1962a), Bryozoa (Bailey, 1856), and Porifera (Hadding, 1932; Pratt, 1962a) have been noted and may be internal casts or may be replacement structures. The origin of morphological glaucolaite as altered faecal pellets, giving ellipsoidal or capsule shaped grains was first suggested by Takahashi and Yagi (1929) and Takahashi (1939). From these and similar observations made on modern occurrences of glauconite (Galliher, 1932; Wetzel, 1937; Moore, 1939; Pratt, 1962a, 1962b; Porrenga, 1963, 1966, 1967; Murray and Mackintosh, 1968) it seems reasonable to ascribe faecal pellet origins

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to capsule-shaped grains from ancient sedimealts (Wermund, 1961; K6ster, 1964; Parry and Reeves, 1966). Passage of biotite flakes through the gut of marine organisms during their conversion to glauconite (Galliher, 1935a, 1935b, 1936, 1939) may produce a bulbous grain of glauconite showing relatively little of its original morphology, although distinctive vermicular grains obviously dependent on the morphology of the parent biotite are often found. It may be expected that alteration of clay galls or shale flakes might also give vermicular grains but it seems more probable that grains with discoidal (Wermund, 1961) or other morphologies (Lebauer, 1964) result. Glauconite grains may be formed by the accretion of material of colloidal dimensions, although workers who have suggested this mode of origin (e.g., Teodorovitch, 1961; Odin, 1971) are rather vague as to how the accretions form, suggesting simply "current action" or similar process. Bailey and Atherton (1969) have provided an attractive hypothesis which also explains the common occurrence of phosphatic material in glauconite pellets. These authors have postulated that there is an initial flocculation of proglauconite (or anteglauconite) and colloidal phosphate as chance encouters in a dispersed suspension and that these primary floccules then serve as centres of accretionary flocculation which sink to the sea floor when of sufficient size (hence the observed minimum size of glauconite grains). On the sea bed, the grains would continue to grow while wave- or current-induced rolling of the accretionary masses during growth would result in a pellet morphology and, by reasons of abrasion, a maxium grain size would be reached. Final maturation of the grain is believed to involve the mineralogical changes as suggested by Burst (1958a, 1958b) and Hower (1961) together with expulsion of water. There is little doubt that the final morphology shown by glauconite grains is dependent not only on their mode of origin but also to the attrition they have undergone. The more delicate vermicular pellets or fossil casts are unlikely to survive much transport and will almost certainly be of local derivation. Spheroidal or ovoidal pellets are frequently of detrital origin. Attrition may also be responsible for pigmentary glauconite (Collet, 1908; Carroll, 1939) although direct precipitation or replacement is more likely (Hadding, 1932). Glauconite morphology may also be modified by compaction of the host sediment (Carozzi, 1960), by accretion to give mammillated or lobate pellets and by inclusion within subsequent composite gtauconite grains.

The experimental synthesis of glauconite No attempts seem to have been made to verify the theories of Burst (1958a, 1958b) and Hower (1961) by experimental synthesis of mineral glauconite. Attempts to verify earlier theories were largely unsuccessful (Calderon et al., 1894; Chaves and del Pulgar, 1896; Collet, 1908; Caspari, 1910; Murray and Hjort, 1912, p.189; Takahashi, 1939) although Birdsall (1951) has claimed to have formed poorly crystalline glauconite by autoclaving co-precipitated gels. No attempts to produce morphological glauconite grains seem to have been made.

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THE VALUE AND USES OF GLAUCONITE It is proposed in this section to discuss the value of glauconite in geological studies, dealing in particular with its common use in the age dating of sedimentary rocks. The commercial uses to which glauconite may be put will also be reviewed although it must be pointed out that glauconite has little commercial value at the present day. Its greatest use has probably been in agriculture and its value in soil fertility will be discussed.

Glauconite in geological studies The value of glauconite as an indicator of particular palaeo-environments has been frequently stressed, although the rather wide range of conditions possible for its genesis (Cloud, 1955) and its common non-authigenic occurrence would seem to make it less useful than commonly supposed. Examples of the possible use of glauconite in palaeoenvironment studies have been given by Lochman (1957), Burst (1958a) and Wermund (1961), including discussions of the disadvantages of glauconite for such studies. Glauconitic sediments may also be used as correlation tools (Burst, 1958a) and even the mere presence or absence of glauconite may be sufficient for correlation (Triplehorn, 1966a). Some workers (Triplehorn, 1961; Wermund, 1961)have been unable, however, to use observed variations in mineralogy or morphology of glauconite to aid correlations. Triplehorn (1966b) has suggested that the common occurrence of concentrations of glauconite at stillstands or marine transgressions may assist in tracing diastems or unconformities. He has also suggested some possible geochemical applications if detailed differences in chemical composition could be related to differences in the chemical and physical conditions prevailing at the time of origin. At present, however, it seems that the view of Wermund (1961) that glauconite "is neither a satisfactory indicator of broad sedimentary environments nor a creditable parameter for stratigraphic correlation" should prevail. In recent years, a great deal of interest in glauconite has centred on its use for K - A r and R b - S r dating, and hence establishing broad stratigraphic correlations on the basis of similar ages. The literature dealing with this aspect of glauconite is voluminous, and no attempt will be made to review it comprehensively here. Good reviews are given by Curtis and Reynolds (1958), Lipson (1958), Kulp (1959), Rubinshtein et al. (1959), Evernden et al. (1960), Hurley et al. (1960), Evernden et al. (1961), Gulbrandsen et al. (1963), Kazakov (1964), Degens (1965), McDougall et al. (1965), Hurley (1966) and Dalrymple and Lamphere (1970) for K - A r dating; and by Cormier (1956), Cormier et al. (1956), Herzog et al., (1958) and Hurley et al. (1960) for R b - S r dating. It is perhaps worth recording the limitations to which the use of glauconite in K - A t dating are subject: (1) The glauconite grains must be syngenetic with the deposit. Glauconite may occur as detrital grains in relatively younger host rocks (e.g., Allen et al., 1964; Bodelle et al., 1969) or as a secondary mineral in relatively older rocks (e.g., Tyler and Bailey, 1961). It

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seems advisable (Wermund, 1961) to precede age determinations of glauconite with a microscopic examination of the rock to establish the relative age relationships of glauconite and host. (2) There should be minimal contamination of the glauconite being used for dating with detrital material (Curtis and Reynolds, 1958; Lipson, 1958; Hurley et al., 1960; Kazakov, 1964; Curtis, 1966). (3) Gains or losses of potassium by the glauconite, other than by conversion to argon, should be negligible or should be predictable. Post-diagenetic gains of potassium have been noted by Evernden et al. (1961), Hower (1961), Hurley (1961) and Hower et al. (1963). Losses of potassium by weathering may also occur (Evernden et al., 1961). (4) Losses of argon, either during the history of the glauconite or during sample pretreatment, should be small or should be predictable. Considerable work has been carried out on the diffusion of argon in glauconite with a view to predicting losses (Amirkhanov et al., 1958; Curtis and Reynolds, 1958; Kazakov and Polevaya, 1958; Hurley et al., 1959, 1960; Evernden et al., 1960, 1961; Murina and Sprintsson, 1961; Hower et al., 1963; Sardarov, 1963; Kazakov, 1964; Fechtig and Kalbitzer, 1966; Kazakov and Teplinsky, 1966; Nikolayeva et al., 1969). From these works it seems that glauconite loses Ar more readily than other micas and that the losses depend on the degree of crystallinity of the glauconite, its water content, and on the temperature at which the glauconite is held. Temperatures as low as 150 °C, if sufficiently prolonged, e.g., by deep burial, may cause substantial losses of argon. (5) Gains of argon may also occur (Curtis and Reynolds, 1958; Kazakov and Teplinsky, 1966) but are thought to be rare. The above factors usually combine so that glauconite gives K - A r ages which are anomalously young (Hurley et al., 1960; Hurley, 1961, 1966; Gulbrandsen et al., 1963; Baadsgaard and Dodson, 1964; Dodson et al., 1964; Kazakov, 1964; Dalrymple and Lamphere,1970). Fortunately the lowering in age is somewhat consistent at about 10-20 % (Hurley et al., 1960). Anomalously high glauconite K - A t ages may also occur (Kazakov, 1964). In spite of these limitations, glauco.nite is the only mineral that can be used routinely to date a sedimentary rock directly (Dalrymple and Lamphere, 1970). It may be usefully employed to date sediments of ages greater than 1" 106 years and may extend well into the Precambrian (Polevaya and Kazakov, 1960; Polevaya et al., 1961; Vinogradov and Tugarinov, 1962; Gulbrandsen et al., 1963; Webb et al., 1963;Kazakov et al., 1965; McDougall et a1.,.1965; Tugarinov et al., 1965; Hurley, 1966). The oldest glauconite so far dated, with an age of 1600" 106 years B.P., is from northern Australia (Webb et a1.,1963; McDougall et al., 1965). Rb-Sr dating of glauconite has received less attention. The ions involved are associated with the expandable layers of glauconite (Hower, 1961 ; Hurley, 1961) and are thus more susceptible to gains and losses than structural potassium and argon. For glauconite it would seem that Rb-Sr dating is inherently less satisfactory than K - A r dating.

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Commercial uses of glauconite

Glauconite has been suggested, usually in times of war (Ashley, 1917; Mansfield, 1920; Anon, 1941 ; Oakley, 1943), as a source of potassium for industrial use and Buck (1918), Charlton ( 1918), Holmes (1919), Shreve ( 1921 ) and Mansfield ( 1922) g'ive details of some extraction processes. Glauconite has also been used as a water-softener (Simpson, 1934; Ingleson and Harrison, 1941; Ingleson and Sullivan, 1941; Rabinovich, 1942; Oakley, 1943; Skow, 1961). Water-softening by glauconite may also occur naturally; waters derived from wells sunk into the Greensand, for example, are much softer than those from wells in the overlying chalk (Milner, 1962, p.589). Recently Schnepfe et al. (1964) have investigated its possible use as a "scavenger" for radioactive isotope wastes. Other suggested uses have been as a colouring agent in glass making (Cook, 1868; Oakley, 1943) and the paint industry (Simpson, 1934; Pocock, 1942; Oakley, 1943), as a sorbent for oils, and to "humanise" or decalcify bovine milk (Simpson, 1934). It may also be employed as a raw material in the manufacture of slag-wool, an insulating material (Carroll-Porcynski, 1958). Glauconite in agriculture

Glauconite or sediments rich in glauconite have been used, or advocated, as fertilisers, particularly in the N.E. United States (Cook, 1868; Bagg, 1898; Skeen, 1925; Skow, 1961) but also in South Carolina (Sloan, 1908), Virginia (Skeen, 1925), Wisconsin (Twenhofel, 1936), Southern England, France and Belgium (Penrose, 1888; Simpson, 1934; Oakley, 1943), Russia (Fedorovsky, 1924), Japan (Swanson, 1949), Australia (Simpson, 1934) and New Zealand (Anonymous, 1941). Glauconite seems to act mainly as a slow-acting source of potassium (Cook, 1868; Lipman and Blair, 1917; True and Geise, 1918; Skeen, 1925; Twenhofel, 1936; Graham and Turley, 1947; McRae, 1971), magnesium (Jurkowska, 1956; McRae, 1971) and occasionally iron (Spencer, 1951). Benefits from additions of greensands or greensand marls may also be due to the presence of adventitious phosphate (Cook, 1868; Topley, 1875; Penrose, 1888; Lipman and Blair, 1917; Joffe, 1923) or calcium carbonate (e.g., McCall and Smith, 1920), or to the improvement of soil texture (Mansfield, 1922). Glauconitic materials are no longer used as fertilisers, but glauconite may contribute to soil fertility where it occurs naturally in the soil. Observations that soils which contain indigenous glauconite are usually very fertile have been made in many parts of the world, e.g., the United States (Morse, 1819 in Cook, 1868; Skeen, 1925; Eckel, 1938), the West Indies (Ahmad et al., 1968), Australia (Simpson, 1934), France (Cayeux, 1905), Belgium (Schreiber, 1907) and England (Topley, 1872, 1875; Oakley, 1943). Although few studies have been made on the actual benefits conferred on soils by indigenous glaueonite, the most likely effect is to improve or modify the potassium status of the soils (Schreiber, 1907; Chaminade, 1936; Joffe and Kolodny, 1937; Levine, 1939; Joffe and Levine, 1947a, 1947b; Levine and Joffe, 1947a, 1947b; Ahmad et al., 1968; McRae,

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1971). Other beneficial effects of the presence of glauconite in soils may be an enhanced cation exchange capacity (Tedrow, 1966), improved water-holding capacity (Tedrow, 1966) and high available phosphate (Ahmad et al., 1968; McRae, 1971), although this last effect may not be due to the glauconite per se. THE ALTERATIONOF GLAUCONITE

Metamorphism Glauconite survives deep burial but is apparently unstable under conditions of folding (Dapples, 1967). Its reaction to more intense metamorphism is unknown.

Natural weathering It seems that, in general, acid weathering of mineral glauconite, especially in soils, involves loss of potassium with the production of expandable layers of either a vermiculitic or montmorillonitic nature (Cole, 1940-1941, 1943; Nagelschmidt, 1944; Wurman, 1960; Hutcheson and Haney, 1963; Ahmad et al., 1968). Further weathering to a kaolinitic mineral may also occur (Hutcheson and Haney, 1963; Carson and Kunze, 1967). During these transformations, structural iron may oxidise to produce a brown colour (De Coninck and Laruelle, 1960, 1964) often present only as a rim to each grain. Some iron may be liberated to accumulate as hydrated iron oxides usually referred to limonite (e.g., Cayeux, 1905; Gildersleeve, 1932; Needham, 1934; Monroe, 1947; Wood, 1956) and also tending to occur as a rim to each grain. Only rarely is there complete alteration to iron oxide (Wolff, 1967). Occasionally other iron-rich weathering products of glauconite have been suggested, e.g., siderite (Jans and Van Calster, 1962), haematite and magnetite (Prather, 1905) and jarosite (Tyler, 1936; Bentor and Kastner, 1965). The rim of oxidised glauconite or iron oxides may serve to protect the grain against further weathering but usually in situ acid weathering of glauconite disrupts the pellet morphology (Wurman, 1960; Cloos et al., 1961; De Coninck and Laurelle, 1960, 1964; Moreau, 1965). Glauconite" is easily destroyed during fluvial transport (Wermund, 1964) but may survive several stages of re-working if the conditions remain alkaline, e.g., in the calcareous Cretaceous sediments of N.E. United States (Fairbridge, 1967). In weathering stability it may be regarded as similar to biotite (Jackson and Sherman, 1953).

Artificial weathering Alteration of glauconite may also be produced, sometimes inadvertently, by a variety of laboratory procedures. Grinding to a fine size causes release of potassium from glauconite (Chaminade and Drouneau, 1936; Rouse and Bertramson, 1949; McRae, 1971) and also oxidation of ferrous iron (Bien and Goldberg, 1956). Shaking in water causes disaggregation of grains (Mansfield, 1920, 1922; Hutton and Seelye, 1941) and ultrasonic

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vibrations in water can cause complete disruption of the grains. The latter treatment is therefore a very useful way to prepare glauconite in a fine grained size for mineralogical examination (McRae and Lambert, 1968). Alternate freezing and thawing releases potassium from glauconite (Graham and Turley, 1947), as does heating to high temperatures (Legg and Axley, 1958). Acids severely attack glauconite, especially if concentrated (Hoskins, 1895; Fraps, 1912; Mansfield, 1922; Leaf, 1958; Stahlberg, 1960; Dell'anna, 1964). It has been noted (Hutton and Seelye, 1941; Cloos et al., 1960, 1961; Gastuche, 1960; Touillaux et al., 1960; Zabelin, 1962) that solution of glauconite by acids selectively removes certain components. This would invalidate any chemical analyses carried out on material pretreated With strong acids (e.g., Schneider, 1927). Acid attack also affects the cation exchange characteristics of glauconite (Schnepfe et al., 1964). Selective removal of potassium from glauconite can be effected by ion-exchange resins, especially if H + saturated (Arnold, 1958) and by the use of various salt solutions (Andr6, 1916; Gruner, 1935; Libor,1962a, 1962b, 1964). In these studies and those involving acids (Leaf, 1958) glauconite behaves similarly to other dioctahedral minerals such as muscovite and illite which release potassium much less readily than trioctahedral minerals such as biotite. REFERENCES Adams, J.K., 1963. Petrology and origin of the lower Tertiary formations of New Jersey. J. Sediment. PetroL, 33:587-603. Ahmad, N., Jones, R.L. and Beavers, A.H., 1968. Genesis, mineralogy, and related properties of West Indian soils. I. Montserrat Series derived from glauconitic sandstone, Central Trinidad. J. Soil Sci., 19:1-8. Aldinger, H., 1965. Uber den Einfluss yon Meeresspiegelschwankungen auf Flachwassersedimente in Schw~ibischen Jura. Tschermaks Mineral. Petrogr. Mit., 10:61-68. Aleixandre Ferrandis, V. and Gonzalez Pefia, J.M., 1960. Contribuci6n al estudio de la glauconite. An. Edafol. AgrobioL, 19:615-634. Alexander, A.E., 1934. A petrographic and petrologic study of some continental shelf sediments. J. Sediment. PetroL, 4: 12-22. Alexiades, C.A. and Jackson, M.L., 1965. Quantitative determination of vermiculite in soils. Proc. Soil Sci Soe. Am., 29:522-527. Alexiades, C.A. and Jackson, M.L., 1966. Quantitave clay mineralogical analysis of soils and sediments. Clays Clay Miner., 14:35-52. Allen, J.R.L., 1965. Late Quaternary Niger Delta, and adjacent areas: sedimentary environments and lithofaeies. Bull. Am. Assoc. Pet. Geologists, 49:547-600. Allen, P., Dodson, M.H. and Rex, D.C., 1964. Potassium-argon dates and the origin of Wealden giauconites. Nature, 202:585-586. Allen, V.T., 1937. A study of Missouri glauconite. Am. Mineralogist, 22:1180-1183. Amirkhanov, K.I., Brandt, S.B., Bartnitzky, E.N., Gurvich, V.S. and Gasanov, S.A., 1958. On the preservation of radiogenic argon in glauconites. DokL Akad. Nauk. S.S.S.R., 118:328-330 (in Russian). Amstutz, G.C. and Bubenicek, L., 1967. Diagenesis in sedimentary mineral deposits. In: G. Larsen and G.V. Chilingar (Editors), Diagenesis in Sediments. Elsevier, Amsterdam, pp. 417-475. Andr6, G., 1916. D~placement de la potasse et de l'acide phosphorique contenus dans certaines roches par quelques substances employ6es comme engxais. C.R- Acad. ScL, 162:133-136.

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Anonymous, 1941. Potash supplies in wartime. N.Z.J. Agric., 62:171 - 172. Arnold, P.W., 1958. Potassium uptake by cation-exchange resins from soils and minerals. Nature, 182:1594-1595. Ashley, G.H., 1917. Notes on the greensand deposits of the eastern United States. Bull. U.S. Geol. Surv., 660: 27-49. Atanasov, G., 1962. Glauconites from the Jurassic in Bulgaria. God. Sofii. Univ., Biol.-Geol. Geogr. Fak., 55:142 157 (in Bulgarian, with English abstract). Atanasov, G., Stefanov, D., Trashliev, S. and Goranov, A., 1964. Bentonitic clays from near Dimitrovgrad. God. Sofii. Univ., Geol. Geogr. Fak., 5 7 : 7 5 - 9 1 (in Bulgarian, with English abstract). Baadsgaard, H. and Dodson, M.H., 1964. Potassium-argon ages of sedimentary and pyroclastic rocks. Q.J. Geol. Soc. Lond., 120S:119 127. Bagg, R.M., 1898. The Cretaceous Foraminifera of New Jersey. Bull, U.S. Geol. Surv., 88:11-89. Bagg, R.M., 1909. Casts of Foraminifera in the Carboniferous of Illinois. Bull. Ill. State Geol. Surv., 14:263-271. Bailey, J.W., 1856. On the origin of greensand and its formation in the oceans of the present epoch. Proc. Boston Soc. Nat. Hist.,.5:364-368. Bailey, R.J. and Atherton, M.P., 1969. The petrology of a glauconite sandy chalk. J. Sediment. Petrol.; 39: 1420-1431. Balashov, Yu.A. and Kazakov, G.A., 1968. Source of the rare earths in Pacific Ocean glauconite. Dokl. Akad. Nauk. S.S.S.R., 179: 181-183. Ballance, P.F., 1964. Streaked-out mud ripples below Miocene turbidites, Puriri formation, New Zealand. J. Sediment. Petrol., 34:91-101. Barackman, M.A., 1964. A Study of the Mineral Glauconite in Apalachicola Bay, Florida. Thesis, Florida State Univ., Tallahassee, Fla., 44 pp. Bell, D.L. and Goodell, H.G., 1967. A comparative study of glauconite and the associated clay fraction in modern marine sediments. Sedimentology, 9: 169-202. Bentor, Y.K. and Kastner, M., 1965. Notes on the mineralogy and origin of glauconite. J. Sediment. Petrol., 35:155 166. Bentor, Y.K., Bodenheimer, W. and Heller, L., 1963. A reconnaissance survey of the relationship between clay mineralogy and geological environment in the Negev (Southern Israel). J. Sediment. Petrol., 33: 8 7 4 - 903. Berry, C.T., 1940. Glauconite pseudomorphs after Ophiuran plates. Science, 91:449. Berthois, L. and Dangeard, L., 1939. La glauconie dans les s~diments n~ritiques de la M6diterran~e. Geol. Meere Binnengewiiss. , 3: 5 3 2 - 541. Bien, G.S. and Goldberg, E.D., 1956. Polarographic determination of ferrous and ferric iron in refractory minerals. Anal. Chem., 28:97-98. Birdsall, P., 1951. Recherches sur les conditions de formation des aluminio-silicates ferreux d'origine secondaire. C.R. Acad. Sci., 233:1371-1372. Bodelle, J., Lay, C. and Parfenoff,. A., 1969. Age des glauconies cr~tac~es du sud-est de la France (2: ValiSe de l'Est~ron Alpes-Maritimes). R6sultats pr~liminaires de la m~thode potassium-argon. C.R. Acad. ScL, 268D:1576-1579. Borchert, H., 1960a. Genesis of marine sedimentary iron ores. Trans. Inst. Min. Metall., 6 9 : 2 6 1 - 279. Borchert, H., 1960b. Geosynklinale Lagerstfitten, was dazu gehiSrt und nicht dazu gehiSrt, sowie deren Beziehungen zu Geotektonik und, Magmatismus. Freiberg. Forsehungsh., C79:8-61. Borchert, H., 1964. Uber Faziestypen yon marinen Eisenerzlagerst~itten. Ber. Geol. Ges. D.D.R., 9:163-193. Borchert, H. and Braun, H., 1963. Zum Chemismus yon drei Glaukonittypen. Chem. Erde, 23: 82-90. Boswell, P.G.H., 1924. The petrography of the sands of the Upper Lias and Lower Inferior Oolite in the West of England. Geol. Mag., 61 : 2 4 6 - 264. Braun, H., 1962. Zur Entstehung der marin-sedimentaeren Eisenerze. Z. Erzbergb. Metalhiitten wes., 15:613-623. Brongniart, A., 1823 M~moire sur les Terrains de S~diments Sup~rieurs Calear~o-Trapp~ens du Vicentin. Levrault, Paris, 85 pp. Brown, T.C., 1914. Origin of oolites and the oolitic texture in rocks. Geol. Soc. Am. Bull., 25:745-780.

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