Harkerite from the Alban Hills, Italy

Harkedte from the Alban Hills, Italy M. BARBIERI, D. COZZUPOLI, M. FEDERICO, M. FORNASERI, 8. MERLINO, P. ORLANDI & L. TOLOMEO

LITHOS

Barbieri, M., Cozzupoli, D., Federico, M., Fornaseri, M., Merlino, S., Orlandi, P. & Tolomeo, L. 1977: Harkerit¢ from the Alban Hills, Italy. Lithos 10, 133-141. Oslo. ISSN 0024-4937. Harkerite, found in metamorphic ejecta of the Alban Hills associated with cuspidine, grossular, phlogopite, vesuvian, biotite, and minor amounts of diopside, aegirinaugite, leucite, magnetite and calcite, shows cubic Laue symmetry m3m, possible space groups Fm3m, F432, F~3m, ao= 14.82 A. On the basis of isomorphous replacements suggested by the crystal structure analysis, chemical data may be represented by the formula: Ca4s Mgl~(AISi(O16)4(BO3h2 (CO3)20 • 4H20. Refractive index riD= 1.6490. The relations between harkerite from Albano, harkerite from Skye and other known harkerites

and sakhaites are discussed. M. Barbieri, M. Fornaseri, L. Tolomeo, Institute of Geochemistry, University of Rome. D. Cozzupoli, M. Federico, Institute of Mineralogy and Petrography, University of Rome. S. Merlino, P. Orlandi, Institute of Mineralogy and Petrography, University of Pisa.

A preliminary report on the presence of harkerite in the ejectites imbedded in a grey tuff of the Alban Hills area, locally referred to as 'peperino', has been given earlier (Barbieri et al. 1973). The sample was collected and submitted by Mr. L. Liotti from Rome, who called our attention to the presence in it of small, brilliant and colourless octahedral crystals of unusual appearance. The first summary identification was made on the basis of a powder photograph and on the value of the refractive index. Further investigation showed that the mineral from the Alban Hills, particularly in its chemical composition, is doubtless related to harkerite, but from a structural point of view can also be connected with sakhaite, from which it can be derived through a mechanism of substitution which will be discussed later. On the other hand, the mineral shows some difl'erences from the original harkerite from Skye (Tillley 1948, 1951) and for this reason des¢,rves special consideration. The harkerite bearing rock of the Alban ]:/ills is composed of cuspidine, which appears to be the most abundant mineral, grossular, phlogopite, vesuvian, biotite and in minor amounts diopside, aegirinaugite, leucite, magnetite and calcite. The harkerite, in small octahedral crystals with

maximum diameter of 2-3 ram, is clearly recognizable (Fig. 1) in the vugs of the rock, associated with large crystals of cuspidine, sometimes also with phlogopite. It is also present in the groundmass.

The crystals often appeal corroded and coated with a film of a substance light brown in colour. Sometimes the corrosion process penetrates deeply into the crystals, leaving inside the film of the above mentioned product an octahedral cavity still containing some lamellar relicts of harkerite (Figs. 2, 3).

Optical properties The harkerite from the Alban Hills is isotropic in thin section and in grains but sometimes optical anomalies are present. Crystals in this case appear divided into defined doubly refracting areas in which undulating extinction is frequently visible. The refractive index, measured with the temperature-variation immersion method, is riD= 1.6490. In Table 1 tne values of the refractive indexes of the harkerite from Albano are shown, together with those of harkerite and sakhaitcs from different l~,~.alities. The refractive index of Alban harkerite i: lower than that of the harkerite

134 M. Barbieri et al.

Fig. 1. Octahedrai harkerite crystals (h) accompanied by cuspidine (c) and phlogopite (phi). Length of bar 1 m m .

LITHOS 10 (.i977)

Fig. 3. Fragment of film of the alteration product coating the harkerite crystals. Length of bar 0.10 m m .

compositions are rather different as can be seen from the chemical ,analyses reported in Table 5.

Chemical analysis

Fig. 2. Corroded harkerite crystals. Length of bar 0.25 mm.

from Skye and more similar to the value reported by Pertsev (197i) for the harkerite from Lou-Lou creek in the central part of the Chersky ridgc, Tas-haiaiah, NE USSR, although the chemical

Owing to the extremely small amount of available material a selection of methods of microchemical e nalysis have been used, coupled with electron ~aicroprobe and spark source-mass spectrometer analyses. Silicon dioxide was determined spectrophometrically on a 7 mg sample by measuring the absorbance at 650 nm of the silico-molybdate complex reduced to molybdenum blue (Bunting 1944; Shapiro & Brannock 1962). Boron was also determined spectrophotometrically in the same 7 mg sample by measuring the absorbance at 660 nm of the coloured compound formed by methylene blue and fluoboric acid (Pasztor et al. 1960; Barbieri & Penta 1968). Aluminium, irou, manganese, magnesium, calcium and chlorine

Table 1. Refractive indexes of harkerite and sakhaites.

Harkerite

Sakhait,:

Albano Broadford, Skye, Scotland Broad ford, Skye. Scotland Broa~ford, Skye, Scotland Moral'n'j creek, NE USSR kou-Lou creek, NE USSR Moral'n'j creek kou-kou creek Efka~an creek (V-0791) Doku6an river (V-01330) Solongo dep., Transbaikal, USSR

1,6490 1,653 1,653 1,653 i,652 1,650 1,6412--I,646 1,6386 1,642 1,644 1,648

This paper Tiiley, 1951 Ostrovskaya et al. 1966 This paper Pertsev, 1971 Pertsev, 1971 Pertsev et al. 1968, 1971 Pertsev el al. 1968 Alexandrov & Malinko 1975 Aiexandrov & Malinko 1975 Alexandrov & Malinko 1975

LITHOS 10 (1977)

Harkerite

Table 2. Chemical analysis of harkerite from Albano.

Weight percent

SiO2 A!203 Fe203 MgO CaO MnO SrO Ce203(°) B203 CO2 H20

The major element composition of the harkerite from the Alban Hills will be discussed later. In Fig. 4 the ratio of the abundances of the REE in the harkerite to the average chondrites is shown. F r o m this figure a considerable bulk enrichment and a particular enrichment of the light REE can be noticed, together with a slight negative anomaly for europium and a significant positive anomaly for gadolinium. This unexpected anomaly is hard to explain and needs perhaps further investigation in the sakhaite-harkerite minerals; such anomalies however are not unusual in selective minerals as v~own by Semenov et al. 1958.

Contents of the unit cell a

b

16.82 3.13

Si AI

16.18 3.5.5

16.46 3.61

1.78 10.01 44.13 0.13 0.2 0.8 7.0 15.07 1.25

Fe Mg Ca Mn Sr Ce B C H20

1.29 14.35 45.49 0.11 0.13 0.66 11.62 19.79 4.01

1.31 14.60J 15"91 46.27 j 0 1!~47.18 0113 / 0.67J 11.82 20.13 4.08

C! F

0.06 0.15 100.53 - 0 = F, C! 0.07 100.46

CI F

0.06 0.46

135

0.06 0.47

X-ray crystallography Crystals of harkerite from Albano were studied by rotation, Weissenberg and precession pllotographs. The Laue symmetry (m3ra) and systematic absences were consistent w,th th~ space groups: Fm3m, F432 and F43m. The rotation and precession photographs gave a = 1~,.82 ,~ (e.s.d 0.02/~,).

(a) From the analytical data. (b) Recalculated on the basis of the crystal chemical considerations discussed in the text. (°) Total REE oxides expressed as Ce203.

have been determined with the Jeol ISM 50 A electron microprobe analyser. Water and carbon dioxide have been obtained i~y using the 'elemental analyser' rood. 1104 of the firm Carlo Erba (Milan). Fluorine and other minor and trace elements including the P, EE, have been determined with the spark source mass spectrometer MS 7 at the Dept. of Geology of the University of Manchester; precision of this method is considered to be within :k 5 % and accuracy within the limits of the precision (Nicholls et al. 1967). The analytical data concerning the major elements are given in Table 2. In this table the btLik value of the percentage of the R E E oxides is also reported, expressed as Ce2Oa, the cerium being the most abundant RE element. In Table 3 individual values of the REE and of the other trace elements are shown.

Crystal-chemical considerations Table 2 gives the chemical conposition of harkerite from Albano together with the unit cell contents (column a) calculated on the basis of the unit cell volume V = 3254.0 A a and the observed density d = 2 . 9 5 g cm -a. As was previously mentioned, harkerite from AIbano presents a close crystal-chemical relationship not only with harkerite from Skye but also with sakhaite; this relationship is also reflected in the fimilarity of the powder spectra (Table 4). The :hemical data for sakhaite and harkerite from different sources are reported in Table 5: account should be taken of this table in discussing the relations among the various minerals.

Table 3. Rare earth elements and trace elements content of harkerite from Albano.

RRE ppm

La Ce Pr 2639 3547 52

Nd Sta Eu Gd Dy 202 23 0.86 354 3

Ho 2

Yb 2

Trace elements

Sc

V

Cr

Co

Ni

Cu

Zn

Ga

Ge

As

9pm

27

11

15

208

11

6

39

15

10

12

Br 1

Rb Y 2 6

Zr 5

Nb Sb 1 0.1

Cs

Ba

0.5 76

136

LITHOS 10 (1977)

M. Barbieri et al.

Table 4. Interplanar spacings for harkcrites and sakhaite (copper radiation). 1

2

d

I

5.22 4.45 4.17 3.84 3.69 3.61 3.58 3.39 3.30 3.24 3.15 3.01 2.97 2.95 2.84 2.81 2.66 2.61 2.53 2.50 2.46 2.39 2.37 2.33 2.25 2.22 216 2.13 2.07 2 03 2 O0 I 97 1 92 1 84

s w vw vw vw vw vw m

1.82 1.80

"w

d

3 I/Io

d

8.50 5.21 4.45

8.3 37.2 4.3

11

3.39 3.33

18.6 3.1

2.98

3

3.02

13.5

7

2.81

13

2.85

10

2.66 2.61

2 ![00

2.58

100

2.62

100

2.49 2.46 2.38

1 4 2

2.47 2.44

0.5 3

2.33 2.25 2.22

6 l 2

2.31 2.23 2.20

5 5

2.124 2.065

22 4

2.108 2.04~

1.990 !.968 1.924 1.843

1 4 2 5

1.951

1.902 1.827

1.737 1.704

5 4

8.5 5.22 4.45 4.24 3.82 3.67

20 2 1 1 1

8.40 5.16

9 22

4.21

5

3.65

3

3.39

7

3.35

3.22

2

3.00 2.96

8 2

2.84

5

v~v

w vw m vw vw m vw w vs vw vw w vw vw w vw vw vw s m vw vw m w s

1.74 1.70

vw m m

1.65

m

11Io

I/Io

1.722 1.686 1.676

2.47

4.7

2.34 2.26

7.1 2.6

56 10

2.14 2.07

38.6 9.7

14 5 21

2.00 1.93 1.85

10.2 4.2 3O

1.75 1.71

4.3 4.4.

1.66

5.3

1

1.633 1. 2. 3. 4.

Harkerite, Broadford, Skye, Scotland (Tilley 1951). Harkerite, Broadford, Skye, Scotland (Ostrovskaya et al. 1966). Sakhaite, Moral'n~j river, Tas-ha[atah, USSR (Ostrovskaya et al. 1966). Harkerite, Albano, Aiban Hills, Italy (this p~tper).

Relations between sakhaite and harkerite from Albano Sakhait, ~, is magnesium the f o r m u l a 4H20 was determined

a carbc+nate-borate o f c a l c i u m a n d ( O s t r o v : : k a y a et al. 1966) for w h i c h Ca4s Mgls(BOa)~s (COa)le ( O t t ) 8 C!4" p r o p o s e d . C h i c h a g o v et al. (1974) the crystal s t r u c t u r e o f the cor-

r e s p o n d i n g synthetic c o m p o u n d Ca4s Mg16(BOs)a~ (COa)16 • 2 H z O , s p a c e g r o u p IF 4132, a = 14.690,~: in this s t r u c t u r e t h e ( B O a ) - " a n i o n s w e r e f o u n d in g r o u p s o f f o u r (BOa) -a ions, w i t h t h e b o r o n a t o m s t e t r a h e d r a l l y l o c a t e d a r o u n d t h e 23 (T) site. I n cubic h a r k e r i t e , w h o s e s t r u c t u r e , a t present, is being refined~ o n e h a l f o f these g r o u p s a r e s u b s t i t u t e d by t h e p o l y a n i o n (AISi,O~e) -~3

LITHOS 10 (1977)

Harkerite

137

D

&

,:

3

l

' >:

--:0o I I I

I 57 L,a

I S8Ce

I 59 Pr

I 60 Nd

I 62 s m

I 63 Eu

I B4 Gd

\

i

/ I

a_

Fig. 4.

/ /

'1/

l..

I

65 T b

6¢DY

39 Y

I

67 Ho

I

I

68 Er

69 Tm

I , 70Yb

Ratio o f abundances of R E E in harkerite from Albano vs. chondrkes.

with structure similar to that of polyanion (SisO.) -1~ in zunyite (Pauling 1933; Kamb 1960; Lou~snathan & Gibbs 1972). The substitution of sixteen (BOs) -s anions for four (AlSi40~s)-'3 anions gives an excess of four negative charges which can be balanced by the further substitution of four (BOs) -8 anions with four (COs) -~ anions. These substitutions reflect in the data given in column b of Table 2: the cell contents were recalculated on the basis of the previously discussed structural relations, assuming nn + nc + 4/5(ns, + hA0 = 48, where n,, indicates the number of atoms of the element x in the unit cell. The resulting crystal-chemical formula for harkerite from A1bano is: Ca~sMg.6~dSi4Ols)4 (BO3). (COBbÜ" 4H.O

Relations with harkerite from Skye Harkerite was first described by Tilley (1951) as a cubic calcium and magnesium borosilicatecarbonate with chloride ions, Laue symmetry m3m, cell parameter a = 29.53 A and successively studied by Ostrov~kaya (1969) and Davies & Machin (1970). On the basis of the recent chemical analysis given by C)strovskaya et al. (1966), the formula: Ca, aMg~6AIs(BOs)~ (SiO,)~ (CO3)1s (OH)6C12" 3H20 was proposed, which, taking account of the preceding crystal chemical considerations, can be written" 10 ~ Lithos 2/77

C-.a4sMgls(AISi~O.)a (BOs)15 (COshs (OH)sCIs • 3HsO This formula clearly shows the similarities as well as the differenc~ between the harkerites from the two sources.

Alteration processes The harkerite from Albano behaves as a rather unstable mineral, as witnessed by the intense corrosion and alteration processes whose products are found in the form of the previously mentioned thin films. Information on the nature of these products is very scarce. The X-ray pattern does not show appreciable diffraction effects except for two broad and diffu~ reflections with an intensity maximum at 11.05 and 3.56 A. Electron microprobe analysis has revealed as major elements Si, Fe, AI, Mg, Ca in the atomic ratios of 4.9: 2.2: 2.0: 5.5: 0.56, respectively, leading to a composition which can be represe-',ed by: Ca0.3z(Mg3aaFel.aT) (ALL, Siz.,) Ol0(OH)4.~s something intermediate between a saponite and a chlorite member. The very weak reflections observed may suggest the presence of both products. The possibility of transformation of b.~rkerite into chlorite under endogenous conditions has been demonstrated by Alexandrov & Malinko (1975) in their paper in which the geochemical peculiarities of the endogenic and hypergenic alteration of carbona-

138

M. Barbieri et 02.

LITHOS 10 0 9 7 7 )

Table 5. Chemical analyses of sakhaites and harkerites. Sakhaites

1 SiO2 TiO2 CO2 B203 A1203 Ce203 Fe203 FeO MnO MgO CaO SrO Na20 K20 H20H2 O+ C!

2 0.25 . 14.33 17.65 0.24

0.33 0.05 12.35 49.55

I. 10 0.37 0.10 11.43 49.30

0.58 0.22 12.60 49.06

0.47 0.72 0.13 12.50 47.40

1.06 0.51 0.08 12.40 48.51

10.84 50.88

u -~ 2.75 2.63

0.04 0.03 ~ 2.52 2.73

m -0.20 1.38 2.91

--~ 1.26 3.22

0.22 0.04 0.20 1.40 3.09

-~ 0.10 3.78 --

3.04 2.03

Si Ti C B AI Ce Fe3 • Fe z+ Mn Mg Ca Sr Na K H C! F

.

.

.

.

.

.

.

.

.

.

1.54 . 13.84 18.00 0.70

7

0.31 -12.54 50.18

- .

Total

6

14.82 17.73 0.75

.

0.50

5

14.87 18.07 0.25

Ins. res.

0.25

4 0.98 . 13.61 18.88 0.33

F

Less O = F + C!

3

.

1.65

2.03

14.87 17.45 0.28

14.20 15.80 1.42

.

.

0.76

~

,

100.88

101.04

101.09

100.74

101.11

100.75

100.24

0.58

0.62

0.66

0.74

0.70

--

0.44

100.30

100.42

100.43

100.00

100.41

100.75

99.80

*

*

atoms per cell 0.22

*

0.22

0.45

0.87

1.37

1.47

1.86

17.28 26.91 0.25

17.93 27.55 0.26

18.08 27.34 0.78

16.53 28.98 0.34

16.87 27.74 0.74

18.11 26.87 0.29

17.81 25.05 1.54

0.21

0.22

0.39

16.52 47.50

0.04 16.27 46.90

0.74 0.28 0.08 15.23 47.68

0.17 16.70 46.74

0.32 0.54 0.10 16.64 45.34

0.71 0.38 0.06 16.49 46.36

I i.85 51).08

16.20 3.94

0.07 0.03 14.85 4.09

7.47 4.85

0.38 0.05 8.34 4.68

14.64 2.81 5307.9

14.64 2.81 5307.9

8.22 4.41

22.49

18.62 3.16 la

ao D M 1. 2. 3. 4. 5. 6. 7. 8.

Sakhaite, Mora~'n'~ river, Tas-haiatah, N E LISSR (Ostrovskaya et al. 1966). Sakhaite, Morai'n'j river, Tas-haiatah (Ostrovskaya et al. 1966). Sakhaite, Efka~an river, Selenniahskij ridge, NE USSR (V 0791), (Pertsev 1971). Sakhaite, Lou-Lou creek, Tas-haiatah (Perts¢.w et al. 1968). Sakhaite, Moral'n'j creek, Tas-haiatah (Pertsev et al. 1968). Sakhaite, Doku6.an river, Tas-haiatah (V 01330), (Pertsev et al. 1968). Sakhaite, Solongo deposit, Transbaikal, (I 2~/3) (Alexandrov & Malinko 1975). Harkerite, l.ou-Lou creek, Tas-Haiatah (Pertsev et al. 1968).

14.64 2.84 5364.6

14 66 2.91 3519.4

Harkerite

L I T H O S 10 (1977)

139

Herkerites

8

9

10

11

12

13

8.50 0.11 15.71 1.50 3.20

9.35 -14.65 11.17 2.59

12.74 ~ 13.90 9.20 2.51

14.17 ~ 14.94 7.77 2.84

16.54 0.02 13.38 11.73 2.81

1.00 0.84 0.12 9.90 45.10

0.13 0.77 0.08 11.05 46.92

0.87 0.02 11.32 46.31

0.85 0.46 ~ 11.15 46.23

0.03 0.71 -11.00 41.58

0.20 0.06 0.45 2.14 2.12 .

0.17

.

0.07

.

0.15 1.01 2.12 0.72

~ !.92 1.26 . .

100.95

100.95

100.13

100,71

0.48

0.48

0.27

0.31

100.47

100.47

99.86

100.40

8.77

13.51

.

.

.

.

.

16, 82 -15.07 7.0 3.13 0.8 1.78 0,13 11.01 44.13 0.20 .

.

.

.

0.11 0.81 1.36

.

.

1.42 2.16 0,32 9.5C 43.84

1.04 2.41 0.23 8.39 43.33

0,17 0.41 ~ ~

0.30 0,84

100.34

100.44

0.07

99.80

~

100.46

ID

8.35 . 21.09

19.46 0.21 12.93 8.80 2.46

.

100.53

~

,

17.55 0.22 12.36 9.53 2.86

.

~ 1.25 0.06 0.15 .

. 99.80

15

.

0.70 1.30 ~

. .

14

100.34

100.44

*

*

18.76

12.06 . 17.97

19.45

17.19 0.02 18.98

19.52

18.09

15.04

12.79

21.04

11.62

16.34

15.66

3.71

2.~7

2.80

3.19

3 44

3.55

3.34

3.00

0.66

m

.

.

0.74 0.69 O. 10 14,51 47..~0 . 0.38

.

.

.

0.09 0.60 0.06 15.46 47.17 .

. .

0.08

.

. 14.73 2.93 5637.3

.

0.61 0.37 -15.85 47.23 .

.

0.31 6.32 3.37

17.42 0.17 i 6.75

20.06 0.16 18.20

.

--0.02 15.99 47.0 .

0.08

14.03 3.53 .

.

16.18 N 19.79

12.13 2.02 . 14.76 2.94 5691.2

0.02 0.64 -17.67 48.0

.

.

.

.

5.15 2.20

. .

1.29 -O.I 1 14.36 45.49 0.13 . .

9,01 ---

14.76 2.96 5729.9

14.82 2.95 5780.5

9. Harkerite, Moral'n'j creek, Tas-haiatah (Pertsev et al. g968). 10. Harkerite, Broadford, Skye, Scotland (O;trovskaya et ~d. 1966). 11. Harkerite, Broadford, Skye, Scotland (Tilley 1951). 12. Harkerite, Doku~an river, Tas-haiatah ('7' 01171) (Alexandrov & Malinko 1975). 13. Harkerite, Albano, Alban Hills, Italy (Tais pape0. 14. Harkerite, Doku6an river, Tas-haiatah, (V 01334) (Alexandrov & Malinko 1975). 15. Harkerite, Doku~an river (V 01332) (Alexandrov & Malinko 1975). * On the basis of Mg + Fe + Ti + ... + Ca = 64.

10" ~ Lithos 2/77

0.81 2.o8 0.20 12.89 47.87

2.71 ---

5.77

. .

8.02 0.10 0.46

.

1.06 1.80 0.27 14.06 46.64 --

140

M. Barbieri et al.

toborates have been discussed. In any case the alteration product as compared with the original harkerite does show an enrichment in silica, alumina, iron and magnesium oxides, a depletion of calcium and a total removal of boron and of carbonates.

Discussion and conclusions The presence of a harkerite member in the metamorphic ejected masses in the Alban Hills area fits well into the observed mineralogical associations. These associations ~Lre characteristic of a high temperature, low l':.ressure contact metamorphism and metasorfmtism. This occurred during the interaction of the parent magma with the country rocks, chiefly represented by limestones and dolostones, silicon boron and fluorine were added in the presence of an aqueous fluid. Typical indicators of this kind of l:,rocess are the well known minerals of the eject~L from the Alban Hills such as humite (de' Medici Spada 1845) vesuvian, diopside (Strtiver 1877), cuspidine (Stella Starrabba 1913), forsterite (Fornaseri 1951), phlogopite (Federico & Fornaseri 1952) and the monticellite recently described by Orlandi (1975). Within this framework one could say that the presence of harkerite was to be expected. In this respect the paragenetic situafio~ in the ejecta of the Alban Hills is remarkably similar to that observed by Tilley (1951) in the boronfluorine contact skarns of the Broad['ord area, Skye, in which harkerlte is found associated with magnetite, forsterite, diopside, montice! lite, chondrodite and clinohumite. Similar associations (harkerite-vesuvian-cuspidine; harkerite-phlogopite-diopside, with calcite or monticellite) have been described by Pertsev (1971), Alexandrov & Malioko (1975)in many boron-bearing skarns of the Chersky ridge in NE USSR and, with reference to sakhaite, by Maiinko (~974) in Transbaikai. The crystal-chemical data discussed show the connections between the harkerite frora Albano, the sakhaite and the known harkerites. The idealized fiJrmula Ca4sMgt6(BO3).,~ (CO3)1e " 2H20 has been attributed (Chichagov et al. 1974) to sakhaite. As a matter of fact chemica.! analyses (Ostrovskaya et ai. 1966; Pertsev et al. 1968; Alexandrov & Mal:inko 1975) show that sakhaite also contains a signilicant amount of chlorine

LITHOS 10 (1977)

and of OH groups. Therefore the actual composition of sakhaite corresponds more closely to the formula Ca4aMg16(BO3)2s (COa)le (OH)sCI4 • 4H~O originally reported by Ostrovskaya et al. (1966). This means that in the sakhaite the presence of chlorine is not negligible, whatever its position in the structure may be. This is also the case for chlorine and fluorine in the harkerite from Albano and indicates that the presence or absence of chlorine is not to be considered as an absolutely distinctive feature between the harkerite of Skye type and the harkerite of Albano type, as could be inferred by the idealized formulas previously reported. Considering the analytical data reported in Table 5 it is easy to recognize that in the harkerites the silicon content varies from a minimum of 8.44 to a maximum of 20.12 atoms per cell, while the aluminium content is less variable, from a minimum of 2.80 to a maximum of 3.57 atoms per cell. This implies a variability of the number of the substituted (AI, Si)50le groups and of the Si: AI ratio also. In the harkerite from Albano there are four (AI, Si)5016 groups and this number is also found in samples ;'-01171 and V-01334 of the Doku~an river. In the harkerite from Broadford (Skye) there are approximately three (AI, Si)50~6 groups (from the average of the analyses of Tilley & Ostrovskaya), in the harkerites from Lou-Lou creek and Moral'n'j creek we have slightly more than two groups, whereas the maximum substitution of five groups occurs in sample V-01332 of the Doku~an river. Chemical analyses also show that with increasing number of S i t A l atoms the chlorine content decreases systematically. The comparative uniformity of the aluminium content implies also that the Si: AI ratio increases systematically with the increasing Si content. For the above reasons the minerals of the sakhaite-harkerite group can well be expressed by the general formula: Ca48Mgle(AlpSi5.pOle)xm - (B O 3)y3 - (COa)z~(OH, Cl)w" nH20 with m = 1 2 + p and m x + 3 y + 2 z + w = 128 with x variable between 0 and 5 and Si: AI variable between 2.2 and 6.7 ca. Since this work was completed we have been made aware of two new .~tructural studies on harkerites of different sources, one by Giuseppetti et al. (1976) on harkerite frc rn Skye, t[:e other by

LITHOS 10 (1977)

Machin & Miehe (1976) on a ]harkerite. sample from Siberia donated by Pertsev. The results obtained by these: authors can be summarized by comparing the iidealized crystalchemical formulas they propos,-, with that previously given for harkerite from Albano: Harkerite from Siberia (Machin & Miehe) Ca4s Mg~s(AISi4Ox,)~ (BOa)~v (COa)~9 • 4H~O Harkerite from Skye (Giuseppetti et al.) Ca4s Mg~e(AISi40~e)4 (BOa)~6 (CO3)~e (H20, OH, CI)~

Harkerite from Albano (present paper) Ca4sMg~s(AISi,O~s)4 (BO3)~z (CO3)20 • 4 H 2 0 The comparison clearly indicates the different degree of substitution between AlSi4Oxe-t3 and 4BO~-8 groups as well as the different amount of substitution between BOa -3 and COa -3 groups. There is also strong suggestian that different structural types may correspond to different chemistry. As a matter of fact Giuseppetti et al. (1976) found that harkerite from Skye, previously assumed to be cubic, is actually rhombohedral with a cubic pseudocell with a period of 14.76 A, whereas Machin & Miehe (1976) assumed a cubic cell with a = 29.46 A, with the same pseudocell edge a ' = a/2 = 14.73 A. Acknowledgement. - This work was supported by Consiglio Nazionale deile Ricerche, Rome.

References Alexandrov, S. M. & Malil~ko, S. V. 1975: Geochemic,,! peculiarities of endogenic and hypergenic alteration of carbonatoborates. Geokhimiya i, 3-16 (Russian). Barbieri, M. & Penta, A. 1968: Osservazioni geochimiche sulla serie evaporitica miocenica di San Cataldo (Caltanissetta). Per. Min. 37, "!77-807. Barbieri, M., Cozzupoli, D., Federico, M., Fornaseri, M. & Tolomeo, L. 1973: Harkerite negli inclusi del peperino dei Coili Albani. Per. Min. 42, 687-688. Bunting, W. E 1944: The determination of soluble silica in very low concentration Indus. and Eng. Chem. Anal. lz~l. 16, 612-615. Chichagov, A. V., Simonov, M, A. & Belov, N. V. 1974: The crystal structure of sakhaite. Ca3Mg(BO3)2" CO3xH20. Dokt A.N. S S S R 218, 576-579 (Russian). Davies, W. O. & Machin, M, P. 1970: Isomorphous replacements in harkerite and the relation of sakhaite to harkerite. Ca:~. Mineral. 10, 689-695. De Medici Spada, L. 1845: Sopra alcune specie minerali non in prima osservate hello Stato Pontificio. Raccolta Scientifica, 1-7. Federico, M. & Fornaseri, M. 1932: Le miche dei proietti del Vulcano Laziale. Per. Min. 21, 209-227. Fornaseri, M. 1951: Ricerche petrografiche sul Vulcano Laziale. ~ proiet~i inclusi nei tuff Per. Min. 20, 21 !-235.

Harkerite

141

Giuseppetti, G., Mazzi, F. & Ta~dini, C. 1976: La struttufa cristallina della harkerite di Skye (Scozia). Soc. It. Min. PetroL, Perugia meeting, abstr. 22-23. Kamb, W. B. 1960: The cr'.:stal structure of zunyite. Acta Cryst. 13, 15-24. Louisnathan, S. J. & Gibbs, G. V. 1972: Aluminiumsilicon distribution in zu:qyite. Am Mineral. 57, 1089-1108. Machin, M. P. & Miehe, G. !976:Al(SiO4)413- tetrahedral pentamers in harkerite. Neues Jahrb.f~r Mineral. Monatsh. 5, 223-232. Malinko, C. V. 1974: Hydrothermal alteration of kurchatovite-sakhaite minerals of Solongo deposit. Minerals and minerals paragenesis in hydrothermal deposits. A.N. SSSR, Nauka, Leningrad (Russian). Merlino, S. 1977: The crystal structure of harkerite from Albano (Italy). in prep. Nicholls, G. D., Graham, A. L., Williams, E. & Wood, M. 1967: Precision and accuracy in trace element analysis of geological materials using solid source spark mass spectrography. Anal Chem. 39, 584-590. Orlandi, P. 1975: La monticeilite di Alhano. Per. Min. 44, 80-82. Ostrovskaya, I. V. 1969: Crystal structure of sakhaite and harkerite. New data on minerals of SSSR. Trans. Mineral. Museum A.N. S S S R 19, 197-201 (Russian). Ostrovskaya, I. V., Pertsev, N. N. & Nikitina, I. B. 1966: Sakhaite, a new carbonate-borate of calcium and magnesium and its correlation with harkerite. Zapiski Vseross. Mineral. Obshch. 93, 193-202 (Russian). Pasztor, L., Bode, D. J. & Fernando, A. 1960: Determination of microquantities of boron in steel by solvent extraction method. Anal. Chem. 32, 277-281. Pauling, L. 1933: The crystal structure of zunyite. AII3SisO20(OH, FhsCI. Z. Krist. 84, 442-452. Pertsev, N. N. 1961: Harkerite and kotoite in skarns of Polar Yakutia. Geol. Geophys. A.N. SSSR, Sibirsl, Otd. 7, 102-105. Pertsev, N. N. 1971: Faragenesis of boron minerals in magnesian skarns. Nauka, Mescow (Russian). Pertsev, N. N., Bogomolov, M. A. & Nikitina, !. B. 1968: New finds of harkerite and sakhaite, their paragenesis and alterations. Trans. Mineral. Museum A.N. S S S R 18, 105-122. Semenov, E. !. & Barinskii, R. L. 1958: The composition characteristics of the rare earths in minerals. Geochemistry, 398-410. Shapiro, L. & BrannocL W. W. 1962: Rapid analysis of silicate, carbonate and phosphate rocks. Geol. Surv. Bull. 1144-A. S~ella Starrabba, F. 1913: Sulla cuspidina degli inclusi nel peperino dei Monti Aibani. Atti R. Ace. Lintel Rend, CI. Sc. Fis. Mat. Nat., Ser. 5, 22, 871-875. Striiver, G. 1876: Studi sui minerali del Lazio. Parte I. Atti R. Acc. Lincei Mem. Ci. Sc. Fis. Mat. Nat., Set. 2, 3, 205-224. Striiver, G. 1877: Studi sui minerali del Lazio. Parte II. Atti R. Acc. Lincei Mere. Ci. Sc. Fis. Mat. Nat., Scr. 3, l, 93-I12. T,Iley, C. E. 1948: Dolomite contact skarns of the Broadford Area, Skye, a preliminary note. Geol. Mag. 85, 215. Tilley, C. E. 1951: The zoned contact-skarns of the Broadford Area, Skye: a study of boron-fluorine metasomatism in dolon:~:.es. Mineral. Mag. 29, 621-666. Accepted for publication October 1976 Printed April 1977