Contr. Mineral. and Petrol. 15, 67--92 (1967) Optical Properties and Cell Parameters in the Glaucophane-Riebeckite Series* I. Y. Bo~ Lawrence Radiation Laboratory, University of California, Livermore, California Received February 2, 1967 Abstract. A complete set of new optical and x-ray data is given for eleven analyzed alkali amphiboles [Na2(Mg, Fe")3(Al, Fe'")~SisO22(OH)2]. Nine new wet chemical analyses are report- ed. Using additional selected data from the literature, variation in refractive indices, extinc- tion angles (y--s), optic angles, density, lattice constants and cell volume are expressed graphically as a function of composition in the glaucophane-riebeckite and magnesiorie- beckite-ferroglaucophane series. Four orientations (G, C, O, and R) of the optical indicatrix within the structure are described and shown to be characteristic of the chemical species glaucophane (G), crossite (C), magnesioriebeckite (0), riebeckite (0), and riebeckite-arfved- smfite (R and 0). Optical properties of the pure end members by extrapolation are: fl y (r--~) c A n e Glaueophane 1.594 1.612 1.618 0.025 b = fl c A y = 6 ~ 3.03 Riebeckite 1.702 1.712 1.719 0.015 b=~ cA~= 6 ~ 3.40 Magnesioriebeekite 1.655 1.671 1.67~ 0.02 b = y c A ~ = 32 ~ 3.15 X-ray parameters of the end members referred to the C 2/m space group are: a0(A ) b0(A) c0(A) fl(o) v(A3) Glaueophane 9.50 17.67 5.29 103.7 864 Riebeckite 9.78 18.08 5.34 103.5 918 Magnesioriebeckite 9.76 ]7.97 5.31 103.9 904 These show very good agreement with comparable measurements on synthetic counterparts. There is some indication that the two proposed synthetic polymorphs of glaucophane (E~sT, 1963) are both more disordered than the natural end member. Introduction The chemical and physical properties of the glaucophane-riebeckite-magnesio- riebeckite group of alkali amphiboles are imperfectly known, despite a large pertinent literature and numerous surveys beginning with Mu~GocI (1906). I~]~C]~TLu 1VIIYASHmO (1957) and D]~]~ et al. (1963) have written excellent summaries of the properties of the group from data in the literature. The reason for the imperfect knowledge lies in the incompleteness of most amphibole de- scriptions. Good chemical analyses, e.g., KU~ITZ' (1930) are available over most of the composition span, but optical data are sparse and in some instances un- reliable. Contributing factors to the paucity of optical data are the difficulties * Work performed under the auspices of the U.S. Atomic Energy Commission. 5* 68 I.Y. BorG: in making measurements on highly opaque iron-rich minerals which character- istically show extreme dispersion of optic directions as well as optic axes. Fibrosity of some members (Mg-croeidolites and erocidolites proper) has also hindered investigation. There are, for example, forty or more chemical analyses of fibrous types in the literature, but fewer than six are accompanied by even a partial description of their optical properties. The large number of analyses of fibrous varieties is doubtless related to the relatively pure state in which they occur naturally, as well as to their commercial importance as asbestiform minerals. The present study was undertaken to establish the elusive relations between optical, chemical, and x-ray parameters. To this end the author has selected eleven representative samples, which were chosen so as to span the composition range of the glancophane-riebeckite series, and she has supplied a complete set of data for each. Included in the group are the type crossite, first described by PALAC~E (1894) from Berkeley, California; the type osannite from pegmatites from Alter Pedroso, Portugal, named by HLAWATSCH (1906) for its distinctive optic orientation; and the riebeckite from St. Peters Dome, Colorado, early descriptions of which [optic plane parallel to (010), LAc~oIx, 1889; Mu~GocI, 1906; JOHNSE~, 1910], strongly influenced optical criteria for recognition of the species. By current chemical and structural criteria, the latter two amphiboles are riebeckite-arfvedsonites. True riebeckites, meaning amphiboles near to the composition of the end member Na2Fe'3'Fe'~"SisO~2(OH)2 are rare; however, one was located and included in the study. Finally, the new data have been combined with selected values chosen from previous investigations to provide graphical descriptions of minerals representing solid solutions between either glaucophane and riebeckite or magnesioriebeckite and ferroglaucophane. The optical indicatrix can have one of four orientations with respect to the crystallography of these alkali amphiboles. Very small variations in refractive indices within grains of a single thin section can produce a dramatic variation in optical properties (orientation). Since the different orientations have been a source of confusion, an indication of which compositions they are associated with is given, as well as an explanation of how they come about. Although some new optical data are presented here for the fibrous varieties, they have not been the subject of systematic study. No attempt has been made to explain their fibrous habit except to note that both Mg-crocidolites and croeidolites proper are included within very limited ranges of composition. The clearly merit more study, as do the members of the magnesioriebeckite-crossite-ferroglaucophane series, whose properties are only generally outlined here. Nomenclature Substitutions into the general amphibole formula AX2Y5ZsO22(OH)2 are the basis of the nomenclature proposed by MIYASHIaO (1957) and adopted here with minor modifications. The formula units (X, Y and Z) for one-half the unit cell content and corresponding cation sites (A, M1, M2, M s, M 4 and Si) (WAR~]~, 1930; PmLLn)S, 1963; GHOSE, 1965): X----Na and Ca in M 4 cation site (6--8 fold coordination) with K and excess Na when (Na~-Ca)~ 2.00 accommodated at A (10 fold coordination). Optical Properties and Cell Parameters in the Glaucophane-Riebeckite Series 69 Y=R'"§ where I~"'=A1 w, Fe'", Ti in the M S site and R"=Mg, Fe", Li, Zn and Cu in the M 1 and M 3 sites (all six-fold coordination). Z = Si and tetrahedrally coordinated A1Iv. (oH) = OH§247 el. FERROGLAUCOPHAN E RIEBECKITE NQ 2 Fe3"Ar2 Si8022 (OH)2 Na2 Fe 5"Fe 2"'Si8 ~)2z (OH) z I0 , ~.--..-| ,o~ o o! .8 Fe"+ Mn /e a P CROSSITE " 4 Fe" +Mn 4-Mg ~ i./ 6 " .4 *" o8 g 5. " " .. I 'L 4 e''//~ _'-. ~ . ' o ~ .2 .4 .6 .8 1.0 No 2 Mg3 AI2Si8022 (OH)2 Fe'" + Ti No 2 Mg 3 Fe 2'" Si8022 (OH)2 GLAUCOPHANE Fe"' + Ti +AI 3~'- MAGNESIORIEBECKITE Fig. 1. Chemical variation in the glaucophane-riebeckite-magnesioriebeckite-ferroglaucophane group. Formulas of 109 amphiboles are calculated and plotted from chemical analyses on the basis of 24 (O, OH, F, CI) per half-unit cell. Open circles numbered 1--11 are amphiboles for which new chemical, optical and/or x-ray data are presented; filled circles lettered a--o are from the literature and are identified in the captions to Figs. 3, 4, 5 and 6. Crosses indicate fibrous varieties The end members in the series (Fig. 1) are represented by the following formulae : Glaucophane Na~Mg3A12SisO22(OH)2 l~iebeckite Na2Fe~'Fe'~"SisO~2(OH)2 Magnesioriebeekite I~a2Mg3Fe'~" SisO~2(OH)2 Ferroglaueophane Na2Fe~'AlzSisO22(OH)2 The formula for the related arfvedsonite end member is (Na~.~Ca0.5)3.o(Fe",Mg , Fe'",A1)5(Si, A1)sOe2(OH, F)2 after DEER et al. (1963), where R" --~3.5 and 1~'" _~ 1.5. t~iebeckite-arfvedsonites, (Na, Ca)2.0-3.oFe~:5-3.0Fe~:~-2.0Sis022(O H , F)2, are far more common among natural amphiboles than either pure end member. Riebeckite- arfvedsonite typically contains appreciable amounts of 1~ and unusual elements such as Zn, Cu and Pb, which suggests that they behave as "scavengers" during crystallization [see analyses No. 9, riebeckite and No. 10, No. 11, riebeckite- arfvedsonite, Table 1 this work; Bo~L~Y (1963)]. Table 1. Chemical analyses o/glaucopahne-riebeckites 1 2 3 4 5 6 SiO2 57.73 57.48 57.93 56.72 56.38 50.41 Ti02 n.d. 0.15 0.26 0.08 0.11 1.66 AlcOa 12.04 12.39 11.92 9.52 8.45 7.82 Fe20 a 1.16 2.25 1.31 4.17 4.98 8.73 FeO 5.41 4.91 10.78 8.61 9.40 10.81 MnO n.d. 0.02 0.11 0.19 0.19 0.14 MgO 13.02 12.95 8.05 10.56 9.89 7.39 Li20 n.d. n.d. trace n.d. trace n.d. CaO 1.04 0.43 0.29 1.16 1.29 3.99 1Wa20 6.98 6.66 6.70 6.55 6.77 7.04 K20 0.68 0.05 0.11 0.06 0.08 0.57 I-I~O + 2.27 2.17 2.24 2.19 1.86 1.17 H20- -- 0.00 0.00 0.03 0.04 0.10 F n.d. 0.02 0.05 0.03 0.01 n.d. C1 n.d. 0.04 n.d. 0.02 0.01 n.d. Less 100.33 99.52 99.75 99.89 99.46 99.83 0 = F and C1 0.02 0.02 0.02 0.01 99.50 99.73 99.87 99.45 Analyst W. Ku~Tz D. TItAEMLITZ C.O. INGAMELLS C.O. INGANIELLS C.O. INGAMELLS •. HOLGATE Si 0.211/8.0007.789 0.220t7.780 8.000 7.975~ 7.861~ 7.937 7.443 AIIV ~ . 0.025J 8.000 0.139 8.000 0.063}8.000 0.557} 8.000 Alvi --}1"7~ s23 1.757/ 1.909/ 1.416/ 1.340/ 0.805/ Ti 1 / MnFe"Fe'" --}0"610/3"2270"118/" J| 5.050 0.55610"22910"015}2"001|[] 5.1710.13610"027}2"0721.241/] 4.978 0.997/0"43510"008}1"859|~] 5.058 0.528/0"012}1"8801.107/[[I 5.084 0.969j0"184}1"9581.335/|~] 4.938 0.002}3.170 J 0.013}2.906 J 0.022}3.199 J 0.023}3.204 ) 0.018}2.979 J Mg 2.617J 2.612J 1.652J 2.180J 0.075J 1.626J Li -- -- 0.000 -- 0.000] -- Ca 0.150) 0.062) 0.043) 0.172 / 0.195~ 0.631 / Na 1.825} 2.092 1.748} 1.819 1.788~ 1.851 1.759~ 1.942 1.848[ 2.057 2.016~ 2.754 K 0.117/ 0.009J 0.0201 o.010l 0.014/ 0.107/ 0It 2.042 2.042 1.959] 2.056[ 2.021 / 1.746 / 1.153 1.153 O F -- 0.009} 1.977 0.022/2.078 0.013} 2.039 0.004} 1.753 C1 -- 0.009J o.005J 0.003J 0 21.958 22.023 21.922 21.961 22.247 22.847 7 8 9 10 11 SiO~ 55.38 55.10 51.17 49.87 48.30 TiO~ 0.36 0.68 0.63 0.34 0.38 A120 a 5.29 4.27 1.11 1.04 2.66 F%0 a 9.74 10.61 15.18 14.25 13.41 FeO 13.07 9.78 18.48 20.19 20.40 MnO 0.18 0.48 2.85 1.22 1.33 MgO 6.31 8.86 0.32 0.03 1.17 .r Li20 n.d.