GANGUE MINERALS AND NON-OPAQUE

GANGUE MINERALS AND NON-OPAQUE OXIDE ORE MINERALS The positive identification of non-opaque minerals in reflected light is very difficult if not impossible, especially for the great number of minerals of relatively low refractive indices. The methods which are developed in this chapter give little promise of truly practical results. Effort in this direction is also unnecessary, because a thin section or even a tiny fleck removed with a needle and investigated in index oils will permit identification. I n oxidized ores, the small residual particles of original sulphide minerals often provide an important clue. In spite of this, one will naturally wish to t r y to identify the non-opaque minerals in a polished section, at least approximately, using all available criteria: hardness and cleavage, color of the internal reflection (e.g., in copper ores), the very high birefringence of the carbonates, idiomorphic form (e.g., quartz), the layered structure of the micas, incipient decomposition (e.g., olivine by serpentine), exsolved matter (in diallage and hypersthene), variations in reflectivity or its disappearance on immersion in oil. Etch reactions, which we have seen can easily cause misidentifications of ore minerals, are generally worthless in distinguishing non-opaque minerals in polished section. However, some carbonates can be distinguished very well by this technique. Immersion in oil decreases the reflectivity strongly or causes it to disappear altogether. Where gangue minerals are not very clouded, one can often see deep beneath the polished face and recognize the crystal form of the enclosed ore minerals, especially the more characteristic features such as the striation of sulphosalts. One can, of course, also observe the color, form, and cleavages for the gangue minerals themselves. The reflectivity of non-opaque crystals in reflected light depends entirely on the refractive index of the wave in the reflecting plane. With a little practice one can make a rough estimate of the relative reflectivity, and therefore of the refractive indices, especially if one observes the section first in air, then in oil. A refractive index of n corresponds to a reflectivity in air and oil of approximately the following percentages : 1075

1076

T H E ORE MINERALS n 1-5 1-625 1-75 1-875 200 2125 2-25 2-5

air

oil

0% 4% 6% %% 9%% iV«% 2% n% 13% 3% 4% ~15% 18% βν·%

On very careful observation, one can clearly recognize, as a light veil, a reflectivity as low as 1 / 4 %, the equivalent in oil, e.g., of the n 0 of calcite; values of 1/2% are already more noticeable. This property is especially important in differentiating the carbonates of low refractive index, calcite and dolomite, from siderite, rhodochrosite, and smithsonite. I t is also important t h a t the difference between R 0 and R E in the carbonates can become very striking. The behaviour of silicate and carbonate gangue minerals during the polishing process is, strictly speaking, quite similar and surprisingly good, even with very easily cleavable or very hard minerals. This is due primarily to the relative uniformity of the gangue minerals, in t h a t they offer considerably less variation t h a n the ore minerals, and also due to the elimination of some gangue minerals such as kaolin and talc during preparation of the specimens, whenever possible. Only the most important of the gangue minerals and oxide ores will be discussed. QUARTZ General properties are well known. The rarely recognized cleavage of quartz is never seen in polished section. The polish is much better and easier achieved t h a n t h a t of the very few ore minerals of similar hardness (pyrite, cassiterite). Reflectivity is very low, and in oil the surface of the quartz seems to disappear completely. SCHNEIDERHΦHN and his students standardized their photometer ocular on the basis of the calculated reflectivity of quartz in air: 4-61% green, 4-58% orange, and 4-54% red. Fabric. Very much pιtrographie work has been done on the structure and texture of quartz. I n hydrothermal ore deposits of all kinds it is one of the principal gangue minerals ; in general it is older t h a n the sulphides with the exception of some early pyrite and arsenopyrite ; however, it is also relatively common for it to occur in several generations, some of them contemporaneous with various sulphides. Because it usually forms early it often presents good idiomorphic forms (fig. 629). I n many moderate-temperature ore deposits, especially in those formed near the surface, it is often seen to be very nicely

1077

CALCITE

zoned with liquid inclusions. With increasing alkalinity of the hydrothermal solutions, quartz is sometimes typically replaced by ore minerals or calcite (fig. 629). Inclusions of the tiniest rutile needles, which cause the "blue quartz phenomenon" in hand specimen, are very easily observed in polished sections of granite, especially in oil. I n some instances it has been observed t h a t these rutile needles clearly display subsequent migration (e.g., occasionally in the Witwatersrand reefs).

F I G . 629

70 x

Mina San Fernando, Zacatecas, Mexico

RAMDOHR

Quartz (dark grey) well developed crystals of different orientation, partly younger than sphalerite (grey) but always older than galena (white). The irregular inclusions of sphalerite in quartz are not relics of replacement

A very large number of sections investigated had quartz as the principal gangue mineral. Identification: Hardness, clarity, lack of cleavage, and the often idiomorphic form make it relatively easy to recognize. CALCITE General data. CaC0 3 with rather extensive isomorphous miscibility with FeC0 3 , MnC0 3 , b u t only limited miscibility with MgC0 3 . Properties well known. The refractive indices of the purest material are n 0 = 1-658 and n E = 1·486, b u t are somewhat higher in common calcite due to substitution for Ca. Carbonates as hydrothermal gangue minerals are mostly clouded, from milky to porcelaineous, partly from very fine liquid inclusions, partly as a result of pervasive pressure twinning.—Calcite takes a good polish, better

1078

T H E ORE MINERALS

t h a n the other carbonates which are all considerably harder. However, in coarsely crystalline calcite masses, small cleavage fragments break out from the intersections of twinning planes. The polishing hardness is the same as t h a t of chalcopyrite. Reflection behavior. R is very low, but its variation depending on crystal orientation is striking even without the ni col. The color is dull grey. I n oil the extraordinary ray disappears completely, but the ordinary ray is still visible as a fine veil. The bireflection in air is an especially useful distinguishing characteristic, and permits grain boundaries, twinning lamellae, crystals under torsion, etc., to be recognized easily. The high anisotropy under + nie. is observable through internal reflections. Etching of calcite is characterized by rapid attack by all acids, even very dilute acetic acid ; this distinguishes calcite from other carbonates, especially from dolomite. Accord, to CISSARZ, etching with concentrated hydrofluoric acid for one or two minutes produces a thick coating of CaF 2 , varying with the section cut, which makes possible not only its differentiation from dolomite b u t permits its recognition in mixed aggregates. Other specific etching media are mentioned in numerous publications; they appear to be unnecessary.

F I G . 630

170

X

Niηois about 10% turned from crossed position Yxsjφberg, Sweden

RAMDOHR

Calcite with its structure technically deformed. Two systems of twinning lamellae are developed which partly w a r p each other, and are partly superimposed

DOLOMITE

1079

The fabric is quite variable. Usually it can be determined better in polished section than in thin section, where the very strong double refraction obscures many details. For example, the phenomenon of the controversial ''hollow canals" is often observed in polished section. The canals are readily demonstrated to occur at the intersection of two lamellae at (0112). According to calculations of L. W E B E R and P . M. HOLMQTJIST, which are surely accurate, canals ought not to occur; instead the phenomenon should represent zones with CaC0 3 m a t t e r under especially high compaction. Since hollow canals do in fact occur, this shows t h a t they evidently were formed secondarily by the preferential solution of previously existing regions which were under stress and were perhaps in a more or less isotropic condition. Quite often the space in question is closed, since the lamellae penetrate it, wedging out against each other. Frequently in such cases, a very fine recrystallization product has formed. The ''stressed" condition has never been observed with certainty; it may not be stable. Calcite texture is easily modified by pressure, developing twinning lamellae (fig. 630) and recrystallizing very easily. Both can occur in close proximity and in widely varying intensity. Replacement by sulphides is very common, often with very attractive structures. The sulphides follow cleavage planes and twinning lamellae. The reverse relationship is also not rare. With respect to other carbonates, calcite generally occurs as the replacing mineral (fig. 254), b u t understandably it is likewise often replaced by them (dolomitization, sideritization, etc.). Recognition in polished section is comparatively easy; finely laminated twinning structures in a carbonate primarily point to calcite, since these structures are much more rare in other carbonates. Other carbonates are also considerably more resistant to acetic acid. DOLOMITE General data. CaMgC 2 0 6 , often with a considerable portion of the Mg, sometimes nearly all, replaced isomorphously by iron ("ankerite"). Ca is not replaced. The properties are well known. Hardness and density are considerably higher than in calcite; the refractive indices, especially in the iron-rich isomorph, are essentially higher than in calcite: n 0 > 1-681, n E > 1-500. Color water-clear to a transparent brownish yellow, often weathered brown. Pressure twinning, (0221) in contrast to calcite, is rare. Polishes very well, with a polishing hardness greater than calcite and chalcopyrite. Reflection behavior is only a little higher than calcite. Pleochroism and anisotropic effects are similar to calcite. The etching properties characteristically differentiate dolomite from calcite. Dolomite (and ankerite) in corresponding grain size are much more slowly attacked by all acids than is calcite. Etching with H F is particularly distinctive. As with calcite, the fabric is quite variable. The tendency to develop idiomorphic forms is much stronger than for calcite. The uncommon twinning lamellae parallel to (0221), through their trace parallel to R E , are a principal means of distinguishing dolomite from calcite, which has its (0112) parallel to R 0 Confusion with other rhombohedral carbonates is possible, but the reflectivities of smithsonite, rhodochrosite, and siderite are considerably higher ; these minerals remain visible in oil even for their R E . Ankerite of high iron content, however, approaches these minerals in its refractive indices.

1080

T H E OBE MINERALS

S I D E R I T E (occas. chalybite) [Eisenspat] General data. FeC0 3 , often with considerable MnC0 3 , also MgC0 3 and CaC0 3 . Properties well known. The density of 3-9 puts it undesirably close to sphalerite and chalcopyrite, creating a problem in milling by gravity separation. The refractive indices n 0 = 1-875, n E = 1-633, vary somewhat with the chemical composition. Light yellow, rarely colorless, generally somewhat brown as a result of some weathering. Transparent. Pressure twinning on (0112) as in calcite is occasionally distinct. Polishes very well, with a polishing hardness very similar to sphalerite. Cleavage parallel to (10Ο1) is often visible. Beflection behavior distinctly higher t h a n calcite, dolomite, and magnetite, but very similar to sphalerite and rhodochrosite, which can be distinguished from siderite in polished section only by further testing. On the other hand, cerussite, anglesite, and pyromorphite are somewhat brighter, although often very little. The difference between R 0 and R E is very high. R E remains recognizable in oil. The very high bireflection can be very much concealed by internal reflections, as can the anisotropic effect under crossed niηois. Interior reflections are very common; they appear to be doubled through the double refraction. Etching (accord, to SCHNEIDERHΦHN). Etching with strong acids during the usual etching time is astonishingly small ; hot acids act much faster. At first the acid appears to be almost ineffective, b u t suddenly the etch reaction begins vigorously, perhaps depending upon the rupture of the polishing film. Fabric. Twinning lamellae can be very distinct locally, e.g., in some occurrences in the Siegerland, but they are much less common t h a n in calcite. Zoning is common, although difficult to recognize in polished section; in thin section it is striking. Some "red spar" from the Siegerland shows strong zonal variations arising from dust-like inclusions of hematite. Deformation by tectonic action leads to twinning lamellae, cataclastic texture, and recrystallization quite similar to t h a t of calcite, b u t siderite is more brittle. Texture. The grains are very often idiomorphic, more rarely also idioblastic. The grain size ranges widely; in the veins of the Siegerland it reaches 10, even 20 cm, b u t in many clay-iron rocks, oolitic siderites, or carbonaceous iron rock (black band), it remains small, sometimes under 0-1 mm. Since this latter type of occurrence is derived for the most part from gels, it often exhibits the corresponding textures. Replacement is common. Siderite often replaces other carbonates or is itself replaced by them. During weathering to brown iron ore, expecially beautiful replacement structures are often formed. Goethite (Nadeleisenerz) is always formed in these circumstances, sometimes also pyrolusite or psilomelane. More rarely, siderite may be replaced by spιculante (Bou Doucka Mine, Algeria).

SMITHSONITE

1081

The recognition of siderite as a carbonate is easy through its anisotropy. Smithsonite and rhodochrosite can resemble fresh siderite very closely but the weathering products can facilitate recognition. Calcite, dolomite, and magnesite have weaker reflectivity. Any confusion with cerussite would usually be eliminated through the paragenesis. Paragenetic position. Siderite is ubiquitous. It occurs in cryolite pegmatite and as a very early mineral in many tin ores, but also in the lowest temperature spring deposits. The principal occurrences of siderite, however, are in the middle and low-temperature deposits. It is also very widespread in sedimentary sequences, precipitating under conditions of air deficiency. In the oxidation zone it occurs rarely, but sometimes forms in considerable amounts through the reaction of FeS0 4 on CaC03, therefore as a replacement of calcite. The occurrences are not detailed here. The literature often mentions siderite; SCHNEIDERHΦHN has made it the subject of systematic ore-microscopy (1922 a, 1923). RHODOCHROSITE General data. MnC0 3 , but with very wide miscibility with FeC0 3 , MgC0 3 and CaC0 3 , so that all properties range within wide limits. The nearly pure end member has D = 3-70, n 0 = 1-817, n E = 1·597; generally these values are considerably lower, but in iron-rich varieties they are somewhat higher. The color varies, even for pure MnC0 3 , from almost colorless to rose to brownish red, and to black in weathered specimens. I t takes a good polish, is harder than siderite, softer than smithsonite. Reflection behavior is about the same as siderite or smithsonite (see there). The frequent occurrence of black weathering products helps differentiate rhodochrosite from them. Only occasionally does the internal reflection permit recognition through the rose color. Fabric is quite similar to those of siderite, with no further noteworthy distinctive characteristics. Replacement by pyrolusite, psilomelane, and other manganese oxides is striking and widespread (fig. 618). Paragenetic position. Several quite different parageneses: 1. especially characteristic in near-surface vein deposits with gold, silver, alabandite, fine-grained manganese silicates, also tellurides ; 2. metasomatic replacement deposits of a mesothermal character; 3. sedimentary in siliceous beds (Kieselschiefern), associated with fine-grained rhodonite —after oxidation these deposits can be economically important; 4. redeposition caused by the local reduction of oxidized manganese ores, here more often brownish red ("raspberry spar") than in the first-mentioned occurrences. Literature on ore-microscopic properties is very limited.

SMITHSONITE [Zinkspat] General data. ZnC0 3 , often with Fe, Mn and Cd. Properties are known. Most unusual for a carbonate is the hardness of 5; also the density at 4-4 is especially high. The refractive indices lie a little under those of siderite, at n 0 = 1-818, n E = 1-618. Transparent white, blue, green, especially brownish and cloudy. Takes a good polish, with the polishing hardness notably high; because of the usual fine grain, fragmentation along cleavages is rare.

1082

T H E ORE MINERALS

Reflection behavior is similar to siderite (see there). The pleochroism can be recognized even in very fine-grained aggregates and is important to notice in distinguishing smithsonite from calamine. However, it must also be noted that even extremely fine-grained hydrozincite is also strongly anisotropic. The anisotropic effect under + nie. is always clouded by the numerous white, light green, or yellow internal reflections. Etching is not accurately known. In strong or hot HC1, smithsonite is quickly attacked, but in acetic acid and cold HC1 it is not affected. Calamine is attacked at least as rapidly by hot HC1, but instead of effervescing it coats itself with a film of gelatinous silica. Fabric has yet been very little investigated; rhythmic structures as in wurtzite appear to be common. Twinning lamellae have not yet been observed. Recognition of smithsonite as a rhombohedral carbonate is usually easy. Confusion with calcite and dolomite is not possible with careful work, since both have considerably weaker reflectivity. Siderite and rhodochrosite are in general similar in almost all their properties, but they occur as primary minerals in non-oxidized occurrences, while smithsonite is apparently limited exclusively to the zone of oxidation. Cerussite has similar high pleochroism, but its lowest refractive index is about the same as the highest index for smithsonite ; this is particularly apparent in oils.

CERUSSITE General data. PbC0 3 , mostly quite pure. Cryst. D2h> pseudohexagonal. No striking cleavage. H = 3, Ο) = 6-5. Opt. transparent and colorless, or clouded by inclusions. na = 1-804, n^ = 2-076, ny = 2-078. High, greasy, glassy lustre. Takes an excellent polish, with no fragmentation on cleavage. Hardness equal to or greater than galena, less than calcite. Reflection behavior is low but always higher than the true gangue minerals. The color appears to be pure grey. In oil the surface remains easily visible in all positions, but the reflectivity is strongly diminished. TheOncEL photocell value for blue is 14, or 11-5%, in air; calculated to yellow it is Ra = 8-2, Rj = 12-3, and Ry = 12-3%. Reflection pleochroism in air is strong, and in oil very strong since the Ra is naturally relatively strongly lowered. The strong anisotropic effects under + nie. in air are almost completely obscured in oil by internal reflections. As a result of the high dispersion, the internal reflections often have wide, colored borders. Etching (accord, to SCHNEIDERHΦHN). Easily attacked by even very dilute acids. H 2 S0 4 quickly forms a protective coating of PbS0 4 , limiting further attack. Cold HC1 forms a similar coating of PbCl2 which often shows large crystals. Fabric. Cerussite forms most often as a weathering product of galena or many other sulphides containing lead, of which relics are often preserved. The cleavage of galena is often preserved as a relic texture (fig. 228). The grain size ranges within wide limits. Characteristically the copper from fahlore inclusions in galena appears as fine, disseminated dustlike covellite, more rarely as chalcocite. Contained silver may often be recognized as fine argentite dust, more rarely as a dust of native silver. Cerussite is often enrich-

B A M T E , BARYTES

1083

ed in the vicinity of the boundaries of primary ore, in t h e hand specimen as well as in the ore deposit. Occasionally cerussite reverts to galena in magnificent rhythmic structures (fig. 125). Identification as a carbonate is very easy. All non-carbonate oxide ore minerals of lead are much less pleochroic, with t h e exception of t h e very rare, highly reflective mineral crocoite (na = 2-3, n y = 2-66), which is distinguished by its red internal reflections. Among t h e carbonates, confusion is always possible. I n some cases t h e paragenesis is decisive, and in others t h e lack of cleavage. Since R^ and R y are considerably higher t h a n the R 0 in siderite, rhodochrosite, and smithsonite, in which on the other hand t h e R E is much smaller t h a n t h e R a in cerussite, the distinction should be possible with careful work. Furthermore, in contrast to m a n y other lead ore minerals, t h e ready susceptibility to etching in acids is very characteristic. MALACHITE

AND

[Malachit and

AZURITE

Kupferlasur]

General data. Malachite Cu 2 [(OH) 2 /C0 3 ], azurite Cu 3 [(OH)/(C0 3 )] 2 . Gryst. both monoclinic, malachite mostly in small fibers, azurite in larger, more isometric crystals. Cleavage in malachite is perfect // (001), lacking in azurite. H = S1^-^ D = 3-8-4-0. Opt. in thin layers quite transparent, malachite green, azurite blue. n a = 1-655, n« = 1-875, n y = 1-909 in malachite; n a = 1-730, n^ = 1-758, n y = 1-838 in azurite. Both minerals polish easily and well. The polishing hardness lies between t h a t of calcite and dolomite. The reflection behavior is almost the same for the two minerals and is low, about the same as siderite. Characteristic and common are the internal reflections in air and especially in oil, but they are also common in many copper minerals (e.g., green: pseudomalachite, atacamite, brochantite; blue: linarite, boleite, etc.). Malachite, however, is so strongly pleochroic t h a t misidentification is scarcely possible. The blue minerals, except water soluble compounds, mostly have considerably higher refractive indices and correspondingly higher reflectivity. I n malachite the angle of extinction is often recognizable in the birefringence (bireflection). Anisotropie effects under -f- n * c · are almost always completely masked by internal reflections. In addition to the foregoing criteria concerning minerals with blue and green inner reflections, the ready etchability in dilute acids with liberation of C0 2 is important in identification. The paragenetic position is known. Malachite is by far the most frequent of the oxide ore minerals of copper and occurs in outcrops (Ausbissen) of all copper bearing ore deposits as soon as carbonates form in the primary ores or in the country rock. Malachite has enormous capacity to color, and often simulates a copper ore deposit when in fact only traces of copper are present. Azurite occurs similarly, but it is considerably rare and forms almost exclusively from enargite, famatinite or fahlore —a phenomenon which still needs to be explained, but fits to experimental results of STSHERBINA.

BARITE,

BARYTES

[Heavy spar] General data. BaS0 4 . Gryst. D 2 h , cleavage // (001) perfect, // (110) distinct. Opt. : n a = 1-636, n^ = 1-637, n y = 1-648. Colorless or cloudy white. Takes a good polish without difficulty. Cleavage cracks after (001) are often noted. Polishing hardness is > calcite, < siderite and almost all silicates.

1084

T H E ORE MINERALS

Reflection behavior is very low, with no pleochroism and no anisotropic effect. Inner reflections are colorless. Not etchable. Identification in polished section is difficult. The lack of anisotropic effects (bireflection) excludes carbonate; the low hardness excludes most silicates ; the layered silicates of similar or lesser hardness have considerably better cleavage and are often bent. Celestite, apatite, and anhydrite appear in polished section to be quite similar ; generally they must be distinguished in thin section. Paragenetically barite ranges from early to late in deposition in ore deposits of all kinds and among the gangue minerals. I n magmatic rocks it is indeed rare. In uraninite bearing veins it gets often a deep brown color by the radioactivity.

ANGLESITE General data. PbS0 4 . Cryst. D 2h , cleavage // (001) and (110), but not very distinct. D = 6*3. Opt. n a = 1-877, n^ = 1-882, n y = 1-894. Greasy, vitreous lustre, transparent or clouded. Takes a very good polish. Cleavage is scarcely visible in polished section. The polishing hardness is low, almost the same as that of cerussite. Reflection behavior is low, and is strongly lowered in oil. Cerussite is very similar, but is strongly anisotropic, especially in oil, and is somewhat brighter. No pleochroism or anisotropic effect. Not etchable. Structures and textures like cerussite ; especially common are fine grained, rhythmic encrustations on galena, often with interlayered secondary galena. Identification of anglesite from the numerous other oxide lead ore minerals, is not thoroughly investigated. In comparison to cerussite, anglesite lacks pleochroism and is chemically resistant to etching media. Compared with pyromorphite, which has no = 2-050, n E = 2-042, and a somewhat higher reflectivity, an etch test is generally necessary.

FLUORITE [Fluorspar] General data. CaF 2 , sometimes with YF 3 and rare earths. Properties well known — D = 3-18, n = 1-434, which is very low. Colorless, transparent. Polishes very well; if coarse grained, it easily yields characteristic cleavage fragments. Polishing hardness ~ chalcopyrite. The reflection behavior is lower than in all other common gangue minerals. Fluorite is especially distinguished by being isotropic. Although resistant to ordinary etching media, it is etched by concentrated H 2 S0 4 with the release of H F vapor. Identification — in many instances fluorite can be confused with certain zeolites and with gypsum.

CERARGYRITE [Chlorargyrite] General data. AgCl, completely miscible with AgBr; small amounts of Hg and Na can be present. Properties well known. Lattice: halite t y p e ; a 0 = 5-540 ΐ. Opt.—when completely fresh it has an adamantine lustre and is transparent, with n ^ a = 2-061 ; however, in light it quickly turns dull grey to brown. "Embolite", Ag(Br, Cl), has a slight "pharmacy odor". Cerargyrite is polished with great difficulty, since it is always deeply scratched unless given the lightest possible polish. Good polished surfaces have been produced by MATTHES with microtome blades. Polishing hardness ~ argentite. The reflection behavior is only moderate, and can be further reduced through poor polishing. The calculated reflectivity is 11-3 percent. Isotropic, with internal reflections common.

JAROSITE

1085

Etching (accord, to DAVY & FARNHAM). Positive : KCN quickly tarnishes to reddish brown, washing to a dark pitted surface; FeCl 3 quickly tarnishes dark brown,which remains when rubbed; K O H tarnishes, and a pale tarnish remains when rubbed. Negative : H N 0 3 , HCl, HgCl 2 . Not noticeably etched by light in polished section except after a very long time. Fabric. Structure and texture. Cerargyrite forms shells, crusts, and fracture fillings, also beautiful individual crystals, mostly (100) with (111): it also occurs like satinspar with the fibers perpendicular to the fracture surface. I t also occurs as the matrix in carbonate and sandy material. Often it replaces native silver. Because of the lack of a suitable etching medium, little has been written on the detailed properties of the individual crystals or the microtextures of the aggregates. Diagnostic features. The very low hardness and the difficulties in achieving a polish, as well as the typical occurrence, more or less exclude any misidentification. The lead and zinc gossan minerals which can be somewhat similar are largely notably anisotropic and are much more easily polished. Paragenetic] position. Cerargyrite is the commonest of the compounds of silver with halogens and was formerly of great economic significance. In ore deposits it is generally inconspicuous and easily overlooked. Cerargyrite is typically present in the gossan of silver-rich ore deposits, but is only present if chlorine is available for the precipitation of silver from its solutions or for action upon native silver resulting from an intermediate step in the replacement process. The origin of the chlorine is often difficult to explain, is disputed, and may not always be the same. K E Y E S especially has assembled and described the different possibilities. He thinks t h a t the chlorine is derived from deposits in desert basins and is transported by the wind or other means. In humid areas, the chlorine may come from the weathering of country rock, from wind-borne sea spray, from volcanic exhalations, etc. The association with unusual amounts of silver iodide supports the concept of a wind-borne marine origin for the chlorine. The marked predominance of Cl over Br or I often causes the precipitation of AgCl before AgBr and Agi (e.g., at Tonopah, Nevada), although the normal solubility is in the reverse order. Associated minerals are especially those of the silver gossan and cementation ores, including silver, bromyrite, iodyrite, argentite, and the others, as well as the gossan minerals of lead, zinc and copper such as cerussite, anglesite, smithsonite, malachite, and covellite. Investigated occurrences. Huantajaya near Iquique, Chile; the Bocona and Manto de Peralta mines, and the Mina de la Florido, all near Chanarcillo, Chile.

JAROSITE m

General data. Chem. X F e 3 [(OH) 6 (S0 4 ) 2 ], where X = K (in simple jarosite), Na 9 NH 4 , Ag or 1/2 P b . Cryst. Tiny tabular rhombohedrons, or nearly cubical grains in aggregates. Opt. Properties depend considerably on the chemical composition, but no is > 1*8, An ~ 0-1 ; yellow to dark brown in hand specimen, light yellow powder. Jarosites are extremely widespread in the gossan of pyrite ore deposits and are beside limonite microscopically easily overlooked. On careful observation, one can recognize t h a t the internal reflections are nearly colorless: also the pleochroism, as observed in oils, is relatively higher than t h a t of goethite. The reflectivity is naturally considerably lower, but the apparent reflectivity of goethite can be strongly lowered by bad polishing or by occlusions of water. In many instances jarosite accumulates in the rhythmically banded limonitic crusts of pyrite and chalcopyrite. Under certain, apparently common, climatic conditions, jarosite breaks down into hematite and goethite. Since "argentojarosite" and "plumbojarosite" can carry considerable amounts of valuable metals, recognition and subsequent analytical determination is of importance. Powder diagram (G. FRIEDRICH). (8) 3-10, (10) 3-06, (5) 1-97, (5) 1-82 Β.

1086

T H E ORE MINERALS

OLIVINE General data. Mixed crystals of (Mg, Fe, Mn) 2 Si0 4 , mostly with Mg predominant. Pure Mg 2 Si0 4 is called forsterite, Fe 2 Si0 4 fayalite, Mn 2 Si0 4 tephroite. Among the many named intermediate members is hortonolite, with Fe ~ Mg, typical of a particular type of platinum ore deposits. Fayalite accompanies some contact-met amorphic iron ore deposits, and tephroite many manganese ore occurrences. They are therefore important in ore-microscopy. Cryst. D 2 h . # (010) distinct, (100) less distinct, both especially in coarse-grained aggregates. H = 6-7, D = 3-2-4-3. The refractive indices rise from ß = 1-651 in forsterite to 1-869 in fayalite. Colorless to dark bottle-glass green. Polishes well despite the hardness little greater than magnetite, distinctly less than quartz. Beflection behavior is strongly dependent upon the composition, but is always low. Fayalite is somewhat comparable with anglesite or cerussite, forsterite with barite. The color is always dull grey, without recognizable color tint. Pleochroism and anisotropic effects are not recognizable. Internal reflections are always visible. Fabric. I n peridotite the grains are polygonal, mostly without a suggestion of idiomorphism, and are relatively large (up to 10 cm for fayalite!). The most important characteristic is the serpentinization suffered by all members. Since the various serpentine minerals are considerably softer than olivine, the latter always stands out in relief. Serpentine also has distinctly weaker reflectivity than the corresponding olivine. Sometimes fine and widely distributed magnetite forms during serpentinization, which is in general a hypogene process. In some instances, well formed skeletal crystals of magnetite form in olivine t h a t appear to be oriented. Since olivine is pseudohexagonal (a0 pseudohexagonal c-axis), this is understandable. The lattice dimension c0 in olivine (corresponding to the hexagonal oxygen packing) at 6-0 Β is ~ 1 / 2 the octahedral edge in magnetite, 11-8 Β, and a 0 in olivine at 4-8 Β is = 1/3 of the cube diagonal of magnetite at 14-4 Β. I n fayalite and hortonolite, similar magnetite stars are also observed in fresh material. Ilmenite also occurs occasionally in olivine exsolved from solid solution, in a form quite similar to its occurrence in diallage or hypersthene. This intergrowth is also structurally understandable, since c0 = 27-4 and a0 = 4-98 Β in ilmenite, compared with 6 a0 = 28-7 and °° · /ΊΓ = 5 1 A in olivine.

MICA AND RELATED MINERALS In ore-microscopy the number of minerals described simply as mica is very large. Their properties in reflected light vary so little t h a t they cannot be differentiated in polished section. In addition to biotite, phlogopite, muscovite, and sericite, other minerals such as the brittle micas belong in this class. A few are common among the gangue minerals, especially sericite which is one of the commonest alteration products of country rock by hydrothermal processes. Polishing. Micas are not quite as easily polished as many other gangue minerals, notably less so than the harder gangue minerals such as quartz, hornblende, etc. The polishing hardness is about that of galena or chalcopyrite, varying with the chemical composition, the grain size, and also the cutting angle. Reflection behavior and color. Reflectivity is very low, lower than one might wish; apparently the extremely fine, partly submicroscopic cleavages are to blame. In any case, most quartz and certainly augite and hornblende are definitely brighter, on careful observation. The color is a very dull grey. In oil the surface completely disappears, but internally one may often recognize a pearly iridescence and characteristic internal reflections caused by the cleavage.

1087

MICA AND R E L A T E D MINERALS

F I G . 631

150

X

RAMDOHB

Langenberg near Seeheim, Odenwald, Germany

Corundum (grey) in clean, good crystals, next to corundum which contains an approximately homogenous amount of about 20% hematite-ilmenite (a part of this corundum is decomposed to hydryrgillite or margarite, the latter somewhat darker). The white is magnetite with an ilmenite exsolution intergrowth

F I G . 631a

70 x

Senze de Itombe, Angola

RAMDOHR

A "titanomagnetite ore" contains crystals of corundum, in that case a twin, showing in both halfs fine glide-lamellation // (1011) giving way for alteration products. — Magnetite, light grey is heavily martisized 70

1088

T H E ORE MINERALS

Beflection pleochroism and anisotropic effects under + nie. are actually less striking than one might expect in view of the layered lattice structure of the mica group minerals. The different micas, chlorite (except some chlorites t h a t are iron rich and reflect more strongly), kaolin, etc., are indistinguishable in reflectivity and color. Etching. Not yet systematically investigated; certain of the iron-rich members appear to be readily attacked. Fabric: Thick and thin tabular, generally with well developed base but ragged prism zone faces. Very often uniformly oriented over wide distances. Sometimes occurs as a synantectic reaction product, e.g., with sulphides in gabbros, etc. Diagnostic features. The exceptional cleavage, the tabular form, and the low hardness are also striking in polished section ; likewise the low reflectivity. Distinction between individual members of the groups must be undertaken in thin section or in refractive oils.

CORUNDUM General data. Chem. A1203, other constituents insignificant. Cryst. D 3 d Parting and twinning plane (1011). Translation (0001). H = 9. D = 4-0. Essentially transparent, n 0 = 1-769, An = 0-008. Takes an extraordinarily poor polish; without the greatest care, not only does the corundum remain almost unpolished, but a few grains can prevent the proper polishing of all accompanying minerals. Polishing is only possible with the expenditure of much time. Compact aggregates take a very good polish with diamond powder. The reflection behavior is low, corresponding to the refractive index ; in oil it is a little over V 2 %. Anisotropic effects are not recognizable. Fabric. Corundum is inclined to be idiomorphic. With magnetite it can occur in an almost myrmekitic intergrowth. In ilmenite, magnetite, and apparently also in chromite, it occurs as exsolution particles. Near Seeheim (fig. 631) clear corundum crystals occur side by side with what appears to be a decomposition product, a fine intergrowth of corundum with about 20-30% specular hematite. Twinning is often very striking, partly by growth, partly by pressure (Fig. 631a). Diagnostic features. The enormous hardness makes recognition easy. Paragenetic position. Since corundum commonly occurs in many ores derived from ultrabasic rocks, also in latιrites which have been contact metamorphosed, ("emery rocks"), its discussion is necessary. The associated minerals are ordinarily magnetite, ilmenite, and especially ilmenite-hematite. Investigated occurrences: Seeheim, Odenwald; Naxos; Smyrna; Chester, Mass.; Cortland, N.Y. ; Kussa, Ural ; Senze de Itombe, Angola.

ZIRCON General data. Chem. ZrSi0 4 , with some Hf, rare earths, U, or Th substituting for Zr, P for Si. Cryst. tetragonal D 4 h . Almost always idiomorphic, generally very small crystals. Zircon is found in all stages of a process of "isotropization" by radioactivity, and is then lighter and softer. H = 7Va (-6), D = 4-7 (-3-8), n 0 = 1-960, n E = 2-01 (maximum). Transparent in all colors, often practically colorless. Polishes very well. Zonal structure is often recognizable through polishing. The reflection behavior is somewhat variable, corresponding to the refractive index; in air it is about 10%, in oil 1V2%» therefore very similar to cassiterite, titanite, and scheelite. Compared to the first two, however, the reflection pleochroism is considerably lower. The reflectivity of monazite and xenotime in oil is markedly lower. Fabric. In almost all cases, in contrast with other placer or black-sand minerals of the same size that have undergone transportation for great distances, zircon remains essentially idiomorphic. The habit of the crystals varies considerably, depending upon

ZIRCON

1089

the origin ; it can often be very characteristic for certain igneous rocks, e.g., two different granites in the upper part of the river. I n rare cases in plutonic rocks it is not idiomorphic, and may be sieve-textured. This occurs, e.g., in the Rayfield-Gona alkali granite in Nigeria.

F I G . 632 a

250

X, imm.

RAMDOHR

Crown Deep, Central Witwatersrand Zircon, strongly zoned

F I G . 632b

600 x , imm.

Bodenmais, Bavaria

RAMDOHR

Zircon, a crystal from a "gneiss", strongly zoned. It shows radioactive blasting cracks radiating from the isotropized ("metamict") zone. — Many internal reflections

Zircon often shows zoning. I n younger rocks this zoning can be seemingly missing; the older the rock, the more pronounced it is, for it is caused to a large extent on variable contents of U and Th causing varying radioactive effects. Since central parts are often 70*

1090

T H E ORE MINERALS

particularly rich in U and Th, which causes a complete "isotropization", the core of old zircons often "bursts" the rim (fig. 632a). Oriented intergrowth with xenotime is very widespread, but definitely rarer than the opposite case. Occurrences. Zircon is a principal component of all heavy mineral sands and their fossil, cemented equivalents. I t is therefore often observed in polished sections of concentrates of gold-magnetite-ilmenite-platinum-monazite-uraninite. The study of its special properties can lead to most valuable indications of the origin of these sands.

TITANITE (Sphene) General data. Chem. CaTi(0/Si0 4 ) with partial substitution of Ca by Y, rare earths, and Th, and of Ti by Fe 1 1 1 . Cryst. Monoclinic, idiomorphic often as acute rhombs, or with rounded cross sections, or xenomorphic. H = or > 5 , D = 3-5. Light transparent yellow to dark brown. n y = 1-907, An = 0*13. Polishes very well. Reflection behavior is moderate, but much higher than all "gangue minerals", similar to cassiterite, zircon, scheelite, but much lower than rutile or anatase. The reflection pleochroism, at least in oil, is usually easy to recognize. Fabric. Idiomorphic sphene is present in many rocks, especially in the immediate wall rock of sulphide ore deposits. However, most sphene forms from ilmenite or the ilmenite network of titanomagnetite. Superb pseudomorphs occur. Sometimes these are extremely fine grained and correspond then in part to "leucoxene"; it should, however, be noted t h a t much more "leucoxene" has formed from other titanium minerals. Sphene in the vicinity of ore deposits is often replaced by anatase, especially along the irregular glide-twinning surfaces (fig. 632 c).

F I G . 632 c

250

X, imm.

Besshi mine, J a p a n

RAMDOHB

Sphene crystal being converted to anatase (lighter) especially along the pressure twinning lamellae

Occurrences. Sphene is widely distributed. As a replacement of ilmenite it indicates an introduction, or at least a transfer, of Si0 2 and CaO. I n ilmenite ores its presence is undesirable, since it withdraws a certain amount of Ti from the normal chemical ore beneficiation process. In such a case, its recognition by ore-microscopy is naturally of importance.

P Y R O X E N E AND AMPHIBOLE

1091

PYROXENE AND AMPHIBOLE augite and hornblende

General data. Chain and band structure silicates of the formula n(Si0 3 ) ~2 or n(Si 4 O n ) ~6 with Ca, Mg, Fe, Mn, also Na and Al, as cations in quite variable proportions. Gryst. Monoclinic and rhombic. Characteristic are the cleavage angles of 87° for augite, 124° for hornblendes. The cleavage of augite is much less distinct. The refractive indices vary markedly with the chemical composition, ranging in general between 1*60 and 1-70. Augite and hornblende polish rather well, although some thin bladed amphiboles shred and splinter easily. These must be fixed by impregnation during the preparation of the section. The polishing hardness is very high, although generally < pyrite. In polished section the cleavages of hornblende are always very distinct, especially in cross section; the cleavages of augite are quite variable in quality. Reflection behavior and color. In keeping with the low refractive indices, the reflectivity is very low. The differences in index due to variations in chemical composition cannot be determined without measurements. The reflection color is dull grey. In oil the surface almost completely disappears except in some of the very strongly absorbing varieties. Pleochroism and anisotropic effects under + nie. are not recognizable. Etching. Augite and hornblende are not attacked by any etching medium within the usual etching time duration. Fabric. Much augite and hornblende, which occurs widely as gangue type minerals in magmatic ores, in pneumatolytic replacements, and in metamorphosed ore deposits, and also, e.g., as components of the country rock in the different kinds of important hydrothermal ore deposits, show replacement related in general to the ore formation. Augite becomes hornblende or is serpentinized ; olivine receives reaction rims of augite and hornblende (the "synantectic" coronas are surprisingly noticeable in polished section, e.g., fig. 3) ; or chlorite and epidote masses develop from them. The alteration often results in the development of excellent replacement patterns. Hornblende, by the way, is in general more resistant than augite. Zoning is very widespread (fig. 633).

F I G . 633

35 x

Pitkδranta, Karelia

R A M D O H R - EHRENBERG

Zoned augite in magnetite (white). The zonal structure reveals a gradual habit change by the presence of decreasing or increasing faces

1092

T H E ORE MINERALS

Recognition. In polished section augite and hornblende are quite hard. Hornblende especially may generally be recognized by its extraordinarily noticeable cleavages in its basal section, and by its bladed to needleform development. Augites give much more difficulty ; for some, the distinguishing characteristic is the polished basal section with its peculiar net-like cleavage pattern ; for others, especially hypersthene, bronzite, and some diallage, it is the characteristic ilmenite exsolution intergrowth, partly as discs, partly as needles.

FELDSPARS The very important rock-building group, the feldspars, occurs only sparingly as gangue minerals in ores, especially in sulphide ores. Only the magmatic ores, the pegmatites, and some gold quartz veins carry large amounts of plagioclase and orthoclase; a few hydrothermal veins carry some albite; and finally there is some "valencianite", an adularia-like mineral, at the boundary between extrusive and intrusive hydrothermal deposits. On the other hand, it may frequently be seen t h a t feldspar is widely decomposed by the ore forming solutions. Information on the chemical, crystallographic, and optical properties is given in detail in textbooks on mineralogy. From the viewpoint of oremicroscopy, the significant features are the hardness (— 6), the transparency, the low refractive indices, and the low double refraction. Polishing properties. Feldspars take a very good polish, although those t h a t are sericitized, kaolinized, epidotized, etc., polish imperfectly. The polishing hardness is about the same as for augite and hornblende. The cleavage in polished section is often visible, as in hornblende, b u t is only quite insignificant. Reflection behavior and color. All members have similar, low reflectivity and dark grey color. Some characteristic details are not visible. I n oil the surface disappears entirely. Reflection pleochroism and anisotropic effects cannot be seen under + nie. The typical twinning lamellae of plagioclase seen in thin section are not noticeable in polished section. Etching. Little investigated. Orthoclase and the soda plagioclases are chemically very resistant; calcic plagioclase is rather rapidly attacked by H F and HCL Fabric. The feldspars present in ore deposits, as described especially by LINDGREN (1934), are almost always widely altered to topaz, albite, sericite, epidote, kaolin, alunite, carbonates, etc., each according to the material being removed and the kind and temperature of the ore bearing solutions. I n order to determine the decomposition product, thin sections are necessary. However, the pattern or structure of the typically very fine-grained replacement minerals, and their relationship to the original feldspar, is often much better seen in polished t h a n in thin section. Recognition. I n polished section, recognition of feldspar is difficult and cannot be made with certainty. Often sericitic and other replacement can give a clue.

COALS

1093

COALS The constitution of the most important of mineral raw materials, coal, lies outside the scope of this book. This knowledge of coal and its complicated interrelationships forms a specialized branch of science known as coal petrography. However, we must go briefly into the subject since coal is widely distributed in sedimentary ore deposits and its alteration products also occur in metamorphic deposits. I n addition there are hydrocarbons similar to coal b u t of different origin which recently have aroused special interest. On superficial observation, or in working with poorly prepared material, it is possible to confuse coaly substances with many ore minerals —one more reason to give at least a little attention to coal in the present context. I n many cases coal becomes anisotropic through the action of radioactivity. THE

INDIVIDUAL

COALS

On the basis of microscopy a large number of coal constituents are now differentiated, as discussed, e.g., in " t h e Atlas" 1951 and in literature cited there. For our purpose here, we may only sketch the three principal components of coal, namely, vitrite, durite and fusite. 1. V I T R A I N ( V I T R I T E ) [Glanzkohle] The brilliantly lustrous black megascopic element of coal, exhibiting occasionally light fringes of almost reddish coloration in coal approaching anthracite grade. I n polished surface the reflectivity is relatively high, increasing with the rank of the coal. The brittleness and conchoidal fracture show t h a t at least the principal component has passed through a colloidal condition. Vitrite easily takes a very fine polish. The polishing hardness is less t h a n t h a t of durite, and may be about the same as t h a t of galena. Reflectivity, as measured in air with the photometer ocular, in green 7·5, orange 6-5, and red 7%. 2. D U R A I N

(DURITE)

[Mattkohle] Less uniform in composition t h a n vitrain. The reflectivity in polished section and on fractures is low, although highly variable; it is always much less t h a n t h a t of vitrite. Polishing hardness is considerably higher t h a n t h a t of the latter; one can easily differentiate vitrain from durain by relief polishing. A more detailed analysis of the very low reflectivity would serve no purpose. With increasing rank, the properties of durite change more rapidly t h a n for vitrite. The reflectivity increases so sharply t h a t at the rank of semi-anthracite [Magerkohle], e.g., durain becomes almost as lustrous macroscopically as vitrite.

1094

T H E ORE MINERALS

3. F U S I N I T E ( F U S I T E ) [Faserkohle] Fusinite is similar to charcoal—black, strongly finger coloring, and very brittle. I t takes a good polish and is harder t h a n glancecoal and mat coal. The distinction between soft and hard fusite made with the hand lens is of little microscopic significance. Fusite almost always shows woody cell structure in polished surface section without need for special care in preparation. The measured reflectivity is 5 % . I n coking (natural coking as at Coly Hill Dyke, Newcastle, as well as artificially) fusite remains intact for the most part ; however, a reorientation, such as described on p . 390, is occasionally recognizable. "CARBONACEOUS

MATTER"

Volatile or liquid carbohydrates can be polymerized and converted into materials similar to coal through various known and unknown natural processes. They are given various names according to their properties, their chemical composition, or their geological environment, without complete logic or consistency. E.g., a material t h a t resembles anthracite both megascopically and microscopically, occurring in fractures and congested tectonic zones in the Devonian diabase iron ore of Central Germany, may still be very rich in volatile constituents so t h a t the name is completely misleading. I t appears to be best to use the general term i£ coaly substance" or carbonaceous matter.

F I G . 634

250 X , imm., one nicol

Baldwyn Township, Blind River, Ontario

RAMDOHR

Uraninite is enveloped and partly penetrated by "coaly substances" which are notably pleochroic and strongly anisotropic in reflected light as a result of radioactive r a y s

1095

COALS

F I G . 635a

250 X, imm.

Kimberly Reef, Far East Rand

RAMDOHR

"Coaly substance", stripe-like, with streaks of relict uraninite. Tension cracks

FIG. 635b

Same as 635a, but with crossed niηois

The pronounced texture of pregraphitization is very clearly discernible

1096

T H E ORE MINERALS

F I G . 636

350

x , imm.

Geduld mine, Far East Rand

RAMDOHB

"Coaly substance", in which former uraninite is completely digested, but which shows the general occurrence of uranium by very uniform dispersal of radiogenic galena. The uraninite must have been dispersed a very long time ago. The coaly substance is strongly pregraphitic

I n many cases the conversion of carbohydrates to carbonaceous matter is a normal process of various stages of metamorphism. I n others the process is chemically activated through some catalytic effect. Of great importance, however, is the conversion by means of radioactivity. I n all cases the first effect is compaction to a splinty, conchoidally breaking mass, with a gradual increase in reflectivity. At a certain stage the material begins to assume a graphitic pre-orientation which can lead to a distinct anisotropy almost like t h a t of graphite. I n extreme cases, (fig. 637), coaly substance may be difficult to distinguish from graphite—perhaps there actually is a transition. Carbohydrates and uranium have great mutual affinity ; uraninite may be enveloped by migrating carbohydrates (fig. 634), while uranium solutions may be adsorbed by coal. I n the first instance, uraninite would be always somewhat dispersed (as is Ce0 2 and Th0 2 ). This forms the noteworthy "mineral", thucholite*, which contains up to 60% ash consisting predominantly of U 0 2 . Uranium-bearing coals, or polymerized bitumen, have a very much lower uranium content and low reflectivity; sometimes, however, it too can be rich enough to be considered a source of uranium. "Thucholite" * "Thucholite" is a synthetic word formed from Th, U, C, H, and O, designating the principal components. I t is very inaccurate, in that Th is often absent ; also the uraninite is often not dispersed at all, so that "thucholite" is mineralogically a mixture.

1097

COALS

F I G . 637

250 X , imm.

Basal Reef, Orange Free State

RAMDOHR

"Coaly substance", somewhat pregraphitized and generally somewhat anisotropic

has especially high reflectivity (up to about 15%), very strong anisotropy, and great hardness, very often containing uraninite fragments in varying number and sizes (figs. 635, 636). I t would be rather useless to discuss properties in detail, since there is an extraordinarily wide range of variation in hardness, chemical composition, reflectivity, and anisotropy. 4 'Coaly substance" as defined above is surprisingly widespread, occurring even in granite, pegmatite and hydrothermal veins where one would truly not expect it. The great mobility of carbohydrates a t higher temperatures causes extraordinary migration. The writer and many others have investigated minutely the problem of ''carbonaceous m a t t e r " . I t is discussed in detail in RAMDOHR (1954b; 1957 a; 1958 a, b) and LIEBENBERG (1956).