Transcript
SONDERDRUCK Neues Jahrbudl fur Mineralogie . Abhandlungen, Bd. 114, 246-280
Andradite Stability Relations in the CaSi03-Fe203 Join up to 30 Kb By
H. G. Huckenholz and H. S. Yoder, Jr. Geophysical Laboratory, Carnegie Institution of Washington, Washington, D. C. 20008, U.S.A.
With 11 Figures and 6 Tables in the text
Papers from the GEOPHYSICAL
LABORATORY
Carnegie Institution of Washington No. 1583
N. ]b. Miner. Abh.
3
I 246 - 280 I
Stuttgart, Marz 1971
Andradite Stability Relations in the CaSi03-Fe203 Join up to 30 Kb By
H. G. Huckenholz1 and H. S. Yoder, Jr. Geophysical Laboratory, Carnegie Institution of Washington, Washington, D . C. 20008, U.S.A. With 11 Figures and 6 Tables in the text
1 Present address : Mineralogisch-Petrographisches Institut der Universitat Munchen, 8000 Munchen 2, West Germany.
Abstract The maximum stability limit of andradite, ea 3 Fe23+5i30'2' has been determined in atmosphere of air, in excess O 2 up to 30 kb, and in the presence of excess H 20 and O 2 up to 20 kb. A new technique of general use involving the breakdown of Pt0 2 has been devised for studying systems at high oxygen pressu re. Andradite is stable up to 1137° ± 5° e at 1 atmosphere air pressure, where it breaks down to pseudowollastonite and hematite. The garnet has a = 12.056 ± 0.003 A and n = 1.887 ± 0.002. Under anhydrous conditions and excess oxygen the upper stability limit of andradite increases with total pressure at various rates to 1510° e at 30 kb. The maxi mum stability curve of andradite passes through the following invariant (I) and singular (5) points: 1155°
± 10° e,
1310°
± 10° e, 11 ± 0.5 kb
51420°
1.3
± 0.1 kb
± 100 e, 19 ± lkb
Andradite, pseudowollastonite, wollastonite, hematite Andradite, wollastonite, hematite, liquid Andradite, wollastonite, liquid
The nature of the melting behavior suggests that the solubility of oxygen in the liquid is negligible. Under hydrous conditions and excess oxygen the upper stability limit increases to 1163° ± 10° e, 1.6 ± 0.5 kb, where andradite, pseudowollastonite, hematite, liquid, and gas are in equilibrium. From that invariant point the maximum stability curve is essentially isothermal to 20 kb tota l pressure. The average a and n for andradite under anhydrous conditions are, respectively, 12.059 ± 0.003 A and 1.887 ± 0.002, and under hydrous conditions they are 12.061 ± 0.003 A and 1.886 ± 0.004. No FeO was detected in the andradite by analysis. One garnet having a = 12.065 may contain OH as hydrogarnet solid solution, based on infrared data; however, that sample (n = 1.886) did not exhibit the lowest refracti ve index measured (n = 1.883). The garnet formed on quenching has a > 12.067 A eve n under anhydrous conditions. Preliminary analytical data are presented for natural coexisting andradite and hedenbergite. The apparent anomalous coexistence may be attributed to the relatively
Andradite Stability Relations in the CaSiOs-Fe 20 s Join up to 30 Kb
247
high manganese content of the garnet and clinopyroxene as well as their possible respective solid solutions with skiagite and ferri-Tschermak's molecules. The wide range of stability of andradite is emphasized in relation to its common occurrences and its role as a component in common garnet. The higher the oxidation state of skarn rocks, alpine serpentinites, ijolites, and related rocks, the larger the andradite component in the garnet. The small amount of andradite component in garnets from gneisses, mica schists, hornfelses, amphibolites, glaucophane schists, granulites, charnockites, eclogites, and peridotites is relatively insensitive to the oxidation state of those rocks.
Introduction Andradite (CasFezs+Sis012) occurs mainly in calcium- and iron-bearing metamorphic and igneous rocks formed under oxidizing conditions. Enrichment of ferric iron owing to oxidation in skarns and in rocks of the nepheline syenite and ijolite family favors the formation of andradite, which may contain titanium as an abundant constituent. Additional occurrences of andradite and of andraditic garnet are alpine serpentinites, ijolitic pegmatites, calc-silicate rocks, and marbles (Table 1). Table 1. Amount of andradite component (mol %) present environments. Environment of garnet 1. Skarn rocks (55)a 2. Nepheline syenites, ijolites, pyroxenites, carbonatites, and related rocks (44) 3. Alpine serpentinites (15) 4. Ijolitic pegmatites and related rocks (12) 5. Calc-silicate rocks (35) 6. Marbles (18) 7. Amphibolites and glaucophane schists (21) 8. 9. 10. 11. 12. 13. 14.
Eclogites Peridotites Inclusions in diamonds (9) Granodiorites, andesites, and dacites (20) Metagabbros, charnockites, and granulites (23) Mica schists and gneisses (104) Granites and related granitic pegmatites (62)
tn
garnets of various
Percentage of andradite in garnet Average Range 82 b
36-99
57.5 b 53 b 48 b 25.5 9.5 6 5.3 5.2 3.8 4 3.5 3 2
7-71 0-98 18-72 0-64 0-58 0-13
1.7-7.1 0-10 0-13 0-19 0-10
Data for Nos. 1,3,5,6,7,11,12, 13, 14 were taken from TnOGEn, 1959; Nos. 2 and 4 from HUCKENHOLZ, 1969 b, and HOWIE & WOOLLEY, 1968; Nos. 8 and 9 from RICKWOOD et al., 1968; No. 10 from MEYER & BOYD, 1970. a Number of samples is given in parentheses. b Containing Ti-andradite components. The CaaFe2s+Sis012 component in garnets of different composltlon and origin is ubiquitous and presumably reflects primarily the oxidation state under which the garnet has crystallized. Paragenetic relationships between
H. G. Huckenholz and H. S. Yoder, Jr.
248
garnets and other iron-magnesium silicates are complicated, however, and not well understood at present. A crude correlation between the oxidation state of garnet-bearing rocks (expressed by the atomic ratio of Fe3+/[Fe3+ + Fe!+]) and the amount of the Ca3Fe23+Sia012 component present in their associated garnet (Fig. 1) is evident. In general, the higher the oxidation state in skarn rocks, alpine serpentinite, ijolite, and related rocks, the larger the amount of CasFe23+Si3012 component in the garnet. For gneiss, mica
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.2 60 u 0
en en 50 0 c
,
~ 40 "0 0
-0c
30
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20
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Fe 3 +/Fe3 ++Fi+atamic ratio in garnet-bearing rack Fig. 1. Correlation of oxidation state of garnet-bearing rocks (expressed as the atomic ratio of Fe 3+j [Fe3++ Fe!+]) with the amount of andradite component (Ca3Fe2s+SisOl!) present in their garnets. Three skarn rocks and two alpine serpentinites (solid square), six ijolites and related rocks (solid circle), ten mica schists and gneisses, ten amphibolites and glaucophane schists, and five hornfelses (open circle), ten granulites and charnockites (open triangle); five eclogites and twenty-five peridotites (open square). Data were taken from ATHERTON, 1964; BRAUNS, 1922; BURRI & PARGA-PONDAL, 1936; CHINNER, 1960, 1962; Cocco & GARAVELLI, 1954; COLEMAN & LEE, 1963; HACKMAN, 1900; HERITSCH & LIEIl, 1924; HIETANEN, 1959; HOWIE & SUBRAMANIAM, 1957; KNORRlNG & KENNEDY, 1958; KORNPROBST, 1969; LARSEN, 1942; LEE et ai., 1963; MAcHA TSCHlCI & VON GAERTNER, 1927; MULLIGAN, 1968; OlCRUSCH, 1968; PABST, 1931; VON PHILIPSEORN, 1930; PINERS, 1894; RICKWOOD et al., 1968; ROST & GRIGEL, 1969; SCHUMANN, 1930; SCHURMANN, 1938; SOBOLEV et ai., 1968; STEWART, 1952; WASHINGTON, 1920; WATTERS, 1958; WIMMENAUER, 1962; WYN E-EDWARDS & HAY, 1963; YODER & TILLEY, 1962; ZEDLITZ, 1933.
Andradite Stability Relations in the CaSiOs-Fe 20 a Join up
to
30 Kb
249
schist, hornfels, amphibolite, glaucophane schist, granulite, charnockite, eclogite, and peridotite, the small amount of andradite component in garnet is relatively insensitive to the oxidation state. On the other hand, analytical error, reduction of the chemical analysis to garnet end members, and compositional variations in other elements in the rock may obscure the dependence. Although andradite occurs mainly in mineral assemblages formed under low pressure and high temperature, it is omnipresent as a component in other garnets. In addition to temperature and total pressure, the influence of water and the partial pressure of oxygen play an important role in the formation of andradite. It was the purpose of this study to investigate the thermal behavior of pure andradite up to 30 kb under oxidizing conditions with and without water. Related studies on compositions in the join CaSiOs-Fe 2 0a, in which andradite, hematite, and the polymorphs of CaSiOa are phases, bear directly on the formation of rocks in which these minerals form stable assemblages.
Previous syntheses of andradite JAGITSCH (1956) tried unsuccessfully to synthesize andradite at temperatures below 1280 0 C and 1 atmosphere pressu re from oxides of appropriate composition. Andradite was obtained by GELLER et al. (1964) under anhydrous conditions with the use of a lithium molybdate flux. ho & FRONDEL (1967) reported the synthesis of andradite from a gel of Ca3Fe23+Sia012 bulk composition at a temperature of 1050 0 C in air. Detailed information on the stability of andradite at 1 atmosphere pressure was given by HUCKENHOLZ, SCHAIRER & YODER (1969) and HUCKENHOLZ (1969 b) as a result of their study of the joins CaSi0 3 -Fe 20 3 and CaSi0 3-CaTi0 3 -Fe 2 0s. Hydrothermal syntheses of andradite in cold-seal pressure vessels were carried out by FLINT et al. (1941), JAGITSCH (1956), SWANSON et al. (1960), and ERNST (1966). Starting with different materials they crystallized andradite at pressures of 150 to 3010 bars and temperatures of 480 0 to 850 0 C. GUSTAFSON & ERNST (1970) also report the synthesis of andradite under hydrothermal conditions. High-pressure synthesis of andradite was conducted by COES (1955) at 20,000 atmospheres and 900 0 C with wollastonite, Fe 203, and FeCls as starting materials. It should be emphasized that the garnets synthesized in all previous studies are most likely n ear andradite in composition. Most of them contain FeO, and no systematic data are available on the stability of andradite at higher pressures and temperatures under highly oxidizing condi tions.
Starting materials The experiments were carried out with the use of several kinds of starting material. They may be characterized briefly as follows.
250
H. G. Huckenholz and H. S. Yoder, Jr.
Homogeneous Glass Near the CasFe2s+Sis012 Bulk Composition The initial mixture, prepared in two different batches (10 g each), contained 33.11 wt % CaO, 31.43 wt Ofo Fe 2 0s, and 35.46 wt % Si02 (based on atomic weights as given by the American Chemical Society, 1929). The two batches were fused in air five times at temperatures of 1450 0 to 1500 0 C with intermediate quenching and crushing to obtain a homogeneous product. The homogeneous glasses were ground under acetone in an automatic agate mortar for about 1 hour. The finely powdered material of the two batches contained 3.95 and 5.00 % FeO, respectively, after the fusion treatment. Their refractive indices are 1.797 and 1.794, respectively. The different contents of ferrous iron are due to differences in time and temperature (which are known to affect Fes+j Fe2+) during the period of fusion. The densities of the two glasses, calculated by means of the Gladstone-Dale equation, are 3.334 and 3.322, respectively. Chemical analysis of the two glasses revealed only minor changes in bulk composition. The values obtained are 33.01 and 33.13010 for CaO; 31.45 and 31.57010 for Fe 2 0s (total iron); and 35.39 and 35.38 Ofo for Si0 2 , respectively. Wollastonite + Hematite Mixture Near the CasFe2s+SisO'2 Bulk Composition This material was obtained by crystallizing andradite glass in air at a temperature of about 900 0 C for 1 day. No andradite was formed at this temperature, its formation being very sluggish, and the crystalline mixture consisted entirely of wollastonite and hematite. This mixture was used to ascertain the reversibility of certain equilibria. Nearly Pure Andradite Andradite was crystallized from one batch of the finely powdered glass near the CaaFez3' SisO'2 bulk composition at 1135 0 C and 1 atmosphere pressure. At least 56 days were necessary with ten thorough grindings during the crystallization period for a complete solution of all metastably formed wollastonite in andradite (HUCKENHOLZ, 1969 b). Andradite prepared in this way contains 0.25 Ofo FeO by chemical analysis. Its cell parameter is 12.0S4(2r A; index of refraction, 1.886; and calculated density, 3.854. grinding at various intervals during a period of 100 days. The mixture contained 0.10 % FeO. The unit-cell parameter of the garnet and its refracAndradite + Hematite Mixture Near the CaFe23+SiOo (Ferri-Tschermak's Molecule) Bulk Composition The mixture, corresponding to a composition of wollastonite42.1ohematite57.9o (wt 010), was crystallized from glass at 1050 0 C with quenching and tive index are 12.055(4) A and 1.889, respectively. 2
The plus-or-minus error in the last place is given in parentheses.
Andradite Stability Relations in the CaSiOs-Fe2 0s Join up to 30 Kb
251
Glass and Crystalline Material of a Woliastonite7sHematite27 Bulk Composition The homogeneous glass (n = 1.778) contained 3.06 % FeO after the fusion treatment. A portion of it was crystallized at 1050 0 C with quenching and grinding at various intervals during a period of 130 days. The stable mineral assemblage is andradite + wollastonite and con tains 0.40 Ofo FeO. The unitcell parameter of the garnet and its refractive index are 12.057(5) A and 1.885, respectively.
Apparatus and Experimental Techniques Results at 1 atmosphere pressure were obtained by the quenching method, first employed by SHEPHERD & RANKIN (1909). A slightly modified, internally heated, gas-media apparatus of YODER (1950 b) was used for experiments at pressures up to 10 kb. Runs above 10 kb were made in a single-stage, solid-media apparatus simi lar to that of BoYD & ENGLAND (1960). Anhydrous Experiments In order to avoid reduction of ferric iron, anhydrous runs at elevated pressures were initially "buffered" with hematite. Hematite alone was not entirely satisfactory as a source of oxygen, however, and the iron present in runs could not be kept entirely in the ferric state. Magnetite appeared in the runs even though the buffer remained unchanged. All the experiments described herein were conducted in the presence of platinum dioxide. Between 400 0 and 500 0 C the reaction Pt02 ->- Pt + O 2 takes place at 1 atmosphere. The pressure effect on this reaction is not yet known. The released oxygen is retained by the sealed platinum tube and prevents reduction of ferric iron to ferrous iron. Several techniques were employed in using platinum dioxide 3 as a source of oxygen. First, the sample was placed in a small inner tube of platinum whose open end was slightly crimped. The outer tube contained the oxygen source material and was sealed. Two more efficient techniques involved placing the platinum dioxide at the bottom of the sample or miXJing it directly with the sample; no inner tube was required . The last twO techniques were used exclusively in the runs carried out in the single-stage, solid-media apparatus. The platinum dioxide supplied was very fine grained and when in contact with air, absorbed moisture quickly. Continuous drying at 200 0 C and immediate sealing of tubes after loading with hot Pt02 and sample reduced to a negligible amount the absorption of water, which would drastically change the thermal behavior of the sample. A test for possible oxygen loss during drying was made by decomposing a known amount of the dried 3
Material obtained from Engelhard Industries, Newark, New Jersey.
SONDERDRUCK Neues Jahrbuch fiir Mineralogie • Abhandlungen, Bd. 114, 246-280
Andradite Stability Relations in the CaSi03-Fe203 Join up to 30 Kb By
H. G. Huckenholz and H. S. Yoder, Jr. Geophysical Laboratory, Carnegie Institution of Washington, Washington, D. C. 20008, U.S.A.
With 11 Figures and 6 Tables in the text
Papers from the GEOPHYSICAL
LABORATORY
Carnegie Institution of Washington No. 1583
