oMg2(Mgs)SisO2z(OH,F)2. Sodium-magnesio-cummingtonite (SMC). NaNaMg(Mg5)SisOz2(OH,F)2. Sodium-anthophyllite (SAn). NaMgz(Mgs)SivA1Ozz(OH,F) ...
Contrib Mineral Petrol (1992) 111:248 259
Contributions to Mineralogy and Petrology 9 Springer-Verlag 1992
An experimental study of glaucophanic amphiboles in the system Na20-MgO-AI203-SiO2-SiF 4 (NMASF): some implications for glaucophane stability in natural and synthetic systems at high temperatures and pressures Mark D. Welch* and Colin M. Graham
Department of Geology and Geophysics, Edinburgh University, West Mains Road, Edinburgh EH9 3JW, UK Received September 21, 1988/AcceptedDecember 19, 1991 Abstract. The phase relations of glaucophanic amphiboles have been studied at 18-31kbar/680-950~ in the synthetic system Na20-MgO-A12Oa-SiOz-SiF 4 (NMASF) using the bulk composition of fluor-glaucophane, NazMg3A12SisO22F2 . Previous experimental studies of glaucophane in the water-bearing system (NMASH) have been hampered by problems of fine grain size (electron microprobe analyses with low oxide totals and contamination by other phases), and consequently good compositional data are lacking. Fluor-amphiboles, on the other hand, generally have much higher thermal stabilities than their hydrous counterparts. By using the fluorine-analogue system NMASF, amphibole crystals sufficiently coarse for electron microprobe analysis have been obtained. Furthermore, NMASH amphibole phase relations are directly analogous to those of the NMASF system because SiF4 fills the role of H 2 0 as the fluid species. High-pressure NMASF amphibole parageneses are comparable to those obtained for NMASH amphiboles under similar pressure-temperature conditions, except that the NMASF solidus was not encountered. In the pressure-temperature range of the NMASF experiments, fluor-glaucophane is unstable relative to glaucophanenyb6ite-Mg-magnesio-katophorite amphiboles. Variations in synthetic fluor-amphibole composition with P and T are discussed in terms of changes in the thermodynamic activities of the principal amphibole end-members, such as glaucophane (aGp) and nyb6ite (aNy) using an ideal-mixing-on-sites model. The most glaucophanic amphiboles analysed have acp = 0.50-0.60 and coexist with jadeite and coesite at 30 kbar/800~ Amphiboles become increasingly nyb6itic with decreasing pressure through the NaAlSi_ 1 exchange, which is the principal variation observed. The most nyb6itic amphiboles have any = 0.65-0.70 and coexist with fluor-sodium-phlogopite and quartz at 21-24 kbar/800-850~ At 800~ amphiboles are essentially glaucophane-nyb6ite solid solutions.
*Present address: Department of Earth Sciences, Cambridge University, Downing Street, Cambridge, CB2 3EQ, UK Offprint requests to: C.M. Graham
At 850~ there is some minor displacement along MgMgSi_ 1, but Mg-magnesio-katophorite activities are very low ( < 0.06). Activities of the eight other NMASF amphibole end-members are < 0.001, except for eckermannite activity which varies from 0.01-0.11. Our results indicate that: (a) synthetic amphiboles mimic the essential stoichiometries observed in blueschist amphiboles; (b) synthetic studies should be relevant to petrologically important high-pressure parageneses and reactions involving glaucophanic amphiboles, sodic pyroxenes, albite and talc; (c) the high-pressure stability limit of fluorglaucophane lies at pressures higher than those reached in this study (31 kbar); (d) in natural systems an approach to glaucophane stoichiometry should be favoured by high water activities as well as high pressures.
Abbreviations and formulae used in this paper:
Glaucophane ( G p ) oNa2(Mg3A12)Si8022(OH,F)2 Nyb6ite ( N y ) NaNaz(Mg3AIz)SiTA1022(OH,F)2 Eckermannite ( E k ) NaNa2(Mg4AI)SisO 22(OH,F)2 Magnesio-cummingtonite ( M C ) oMg2(Mgs)SisO2z(OH,F)2 Sodium-magnesio-cummingtonite (SMC) NaNaMg(Mg5)SisOz2(OH,F)2 Sodium-anthophyllite (SAn) NaMgz(Mgs)SivA1Ozz(OH,F) 2 Gedrite ( G d ) oMgz(Mg3A12)Si6A12022(OH,F)2 Sodium-gedrite ( S G d ) NaMg2(Mg4AI)SioAIzO22(OH,F)2 Mg-magnesio-aluminotaramite (MAT) NaNaMg(Mg3AI2)Si6AI2022(OH,F)2 Mg-magnesio-katophorite (MKt) NaNaMg(Mg4A1)SiTA1022(OH,F)2 Mg-magnesio-barroisite (MBa) oNaMg(Mg4A1)SiTAIO22(OH,F)2 Jadeite (Jd) NaAISi206 Enstatite (En) Mg2Si206 Mg2SiO4 Forsterite (Fo) NaAISiO4 Nepheline (Ne) Albite (Ab) NaAlSi30s SiO2 Quartz/Coesite (Qz/Co) Sodium-phlogopite (Sphl) NaMg3Si3AIOlo(OH,F)2 Talc (Tc) oMg3Si40lo(OH,F}2 o = vacant A-site in amphiboles and interlayer site in talc. Octahedral cations in amphiboles are bracketted.
249
Introduction The stability relations of glaucophane have been the subject of vigorous debate for over 25 years following Ernst's pioneering experimental studies in which he proposed that two glaucophane p o l y m o r p h s existed (Ernst 1961, 1963). Subsequent studies (Iiyama 1963; Maresch 1973, 1977; K o o n s 1982) have shown that the proposed p o l y m o r p h i s m is spurious and that synthetic "glaucophanes" are significantly displaced from glaucophane stoichiometry towards other end-members such as nyb6ite, eckermannite and sodium-magnesio-cummingtonite. Maresch (1977) demonstrated convincingly that Ernst's low-pressure "polymorph", G l a u c o p h a n e 1, was close to sodium-magnesio-cummingtonite. He also suggested that the high-pressure "polymorph", G l a u c o p h a n e 2, could have a p p r o a c h e d glaucophane in composition. In experiments on the glaucophane composition, K o o n s (1982) noted that "glaucophane" b r e a k d o w n reactions were divariant. His alkali amphiboles always coexisted with other phases such as jadeite, sheet-silicate and quartz. He also proposed that glaucophane-rich amphiboles are only stable under H 2 0 - u n d e r s a t u r a t e d conditions. C a r m a n and Gilbert (1983) published the results of more than 200 runs from which they claimed to have synthesized end-member glaucophane and defined its stability relations. They proposed that glaucophane breaks d o w n univariantly at its upper and lower stability limits to jadeite + talc and albite + sheet-silicate (a sodium-phlogopite - talc solid solution) respectively. This contrasts with K o o n s ' observations which indicate that a "complex amphibole" is stable above and below the limits defined by C a r m a n and Gilbert's study. With this chequered history in mind, the present study of glaucophane-type amphiboles in the fluorine-analogue system N a 2 0 - M g O - A 1 2 0 3 - S i O 2 - S i F 4 ( N M A S F ) was undertaken with a view to synthesizing fluor-glaucophane, oNazMg3A12SisOz2F 2, and deriving thermodynamic data from its phase equilibria. Application of an F - O H exchange free-energy (cf. Westrich 1982) would allow a g o o d estimate of thermochemical data for hyd r o x y - g l a u c o p h a n e to be made. Little is k n o w n in detail about compositional trends in synthetic glaucophanic amphiboles, due to their finegrained nature. By using the fluorine-analogue a p p r o a c h it is possible to synthesise crystals which are coarse enough to be analysed by the electron microprobe (EMP), and which have a high degree of polysomatic order (see G r a h a m and N a v r o t s k y 1986). Furthermore, Westrich and N a v r o t s k y (1981) and G r a h a m and N a v r o t s k y (1986) have shown that fluor-amphiboles are amenable to calorimetric investigation, and we originally intended to compare phase-equilibrium data with enthalpy and entropy data obtained calorimetrically. However, the instability of end-member fluor-glaucophane, as demonstrated in this study, precluded calorimetric investigation. Despite the instability of fluor-glaucophane under all the experimental conditions of this study, it has been possible to use the results of synthesis and reversal experiments on the fluor-glaucophane bulk composition to resolve some ambiguities of the glaucophane story, and to gain some petrologically relevant insights into the factors
governing the stability of glaucophanic amphiboles in blueschists.
Experimental procedures Syntheses A gel of composition Mg3A12SisO22was prepared using the method of Biggar and O'Hara (1969). NaF was added to this composition by mixing in an agate mortar under acetone for 20 minutes to give a starting material of fluor-glaucophane composition. An XRF analysis of the MAS gel is given in Table 2. A small sample of the gelNaF mixture was glassed at 1750~ (5 mins) and the glass analysed by electron microprobe (WDS). The average microprobe analysis is given in Table 1. Both gel and mix have ideal compositions within analytical error. Fluorine was not determined, but was assumed to be present in amounts equivalent to Na:F = 1: l, as fluorine was added in the form of NaF. After preparation the fluor-glaucophane mix (gel-NaF) was dried at 300~ in an oven and stored in a desicator over phosphoric acid. Approximately 0.015 g of the mix was loaded into a pre-dried Pt capsule (2 mm O.D.) which was then welded shut. The pressure-cell, containing the charge, was dried in an oven at 150~ for an hour before running. Details of synthesis conditions of starting materials used in the crystal-mix experiments are given later in the section on these experiments. In view of the ambiguous results provided by the crystal-mix experiments, most experiments used the gel-NaF mix as the starting material.
Solid-media apparatus All experiments on the fluor-glaucophane composition were performed using a conventional piston-cylinder apparatus of the Boyd and England (1960) design. A 0.5" pressure-cell was used, comprising an outer talc sheath surrounding a pyrex-glass sheath surrounding a graphite heater. Crushable alumina inner pieces surrounded the 2 mm O.D platinum charge capsule. Below 750~ the pyrex sheath was replaced by a boron-nitride sheath in order to reduce underpressures at lower temperatures. Temperature was controlled and measured by a single axial Pt PtsvRh13 thermocouple. Pressures were read from an oil-filled Heise gauge. The sample was pressurised to a pressure 5 kbar above that desired, then heated steadily to the run temperature over 10 min during which time the pyrex glass softened and the oil pressure dropped to a value within 0.5 kbar of the run pressure. To achieve lower friction corrections by running under conditions of retreating piston (Johannes et al. 1971) the cell was repressurised to a 5 kbar overpressure and the excess oil pressure bled offmanually. When a BN sheath was used, the cell was
Table 1. XRF and electron microprobe analyses of MAS gel and gel + NaF starting materials MAS gel (XRF) Ideal Oxide Na20 MgO A120 3 SiOz
Microprobe
wt %
Atoms
wt % (2~)a wt % (20") b
Atoms
17.21 14.38 66.38 97.97
3.07 2.03 7.94 13.04
7.82 (0.20) 15.37 (0.22) 13.01 (0.27) 61.77(0.80) 97.97
2.04 3.05 2.02 7.95 15.06
8.10 (0.25) 15.75 (0.34) 23.19 (0.40) 61.17 (0.47) 98.21
a Analytical precision b Two standard deviations (n - 1) of mean (n = 24)
250 pressurised to a cold pressure equal to the run pressure, then heated to the run temperature. A small rise in pressure occurred upon heating (0.5-1.0 kbar) and this excess was bled off manually, again giving conditions of retreating piston. The talc-pyrex/BN cell was calibrated for pressure in the range 600-1000~ using two pressuresensitive reactions: (a) high-albite = jadeite + s-quartz, and (b) ~quartz = coesite. For the first reaction a 1 : 1 : 1 molar crystal mix of synthetic high-albite, jadeite and natural s-quartz was used. The reaction was bracketed to within 1 kbar at several temperatures and the results were compared with the NaCl-cell determination of Holland (1980). Underpressures of less than 1 kbar were found. The second reaction was located using the results of synthesis experiments on the fluor-glaucophane bulk composition, and these data compared with the NaCl-cell determination of Bohlen and Boettcher (1982). At these higher pressures (25 35 kbar) underpressures of 1 5 kbar were recognised, these increasing with increasing temperature. For the purpose of correcting nominal "raw" pressures, underpressures were assumed to vary linearly at constant temperature between the two calibration curves, with a small extrapolation to cover all the experiments on fluor-glaucophane. No correction was applied for the effect of pressure upon the thermocouple EMF. Temperatures are believed to be accurate to _+ 10~ (C.E. Ford, oral comm.). Pressures and temperatures were recorded twice a day.
which it might be expected to occur by analogy with relations in NMASH. Attempts to synthesise fluor-talc from MgzSi4010 gel + MgF 2, reagent mixes and glasses failed to produce 100% fluor-talc. Instead, assemblages such as sellaite (MgF2)+ quartz _+ fluor-norbergite (MgzSiO4.MgFz) _+ enstatie + "fluor-talc" were produced. As the starting materials were dried thoroughly before and after loading into Pt capsules prior to welding shut, the presence of water is unlikely, and therefore a reaction such as 3MgF 2 + 5SIO2 (in gel) = "Mg3Si4OloF2" + SiF 4 (gas) probably occurs. Furthermore, Van Valkenburg (1955) has shown that the talc structure can accommodate a maximum of one fluorine atom per formula unit. His bulk compositions with F > OH produced fluor-hydroxy talcs + sellaite + quartz. The status of "fluor-talc" remains enigmatic. The apparent instability of fluor-talc in experiments on the fluorglaucophane bulk composition leads to the assemblage fluorsodium-phlogopite + quartz being stable at much higher temperatures (up to 950~ than in NMASH. In all runs in which a sheet-silicate was produced this had the composition of end-member fluor-sodium-phlogopite, NaMgaSiaAIO10F 2, as determined by microprobe analysis.
The r6le of SiF4 in NMASF phase equilibria Product identification Run products were usually recovered as sintered crystal aggregates. They were disaggregated by grinding and examined by opticalmicroscopy, powder X-ray diffraction and the electron microprobe (Camebax system with a back-scattered-electron imaging facility). Seven phases were encountered in experiments on the fluor-glaucophane bulk composition: fluor-amphibole, fluor-sodium-phlogopite, jadeite, enstatite, albite, quartz and coesite. Optical identification and distinction of these phases was made using crystal habit, extinction-angle and refractive index. Amphibole occurs as lengthslow elongate/acicular crystals (7 ^z = 8 11 ~ with refractive indices of 1.585-1.605 and sizes ranging from sub-micrometre to 35 gm long by 6 gm in diameter. The small amounts of jadeite present could only be detected by optical microscopy.
Electron microprobe analysis Samples were mounted as uncrushed sintered aggregates or as crushed disaggregated grain mounts. The latter presented problems in preparation, namely severe plucking of crystals from the epoxy during polishing, resulting in very few good-sized grains remaining on the slide surface. Analyses were obtained using a beam current of 5.5 nA, an accelerating potential of 15 kV and a 2 gm x 2gm beam raster (rather than a finely focussed "spot" analysis). An on-peak counting time of 20 s was used. These operating conditions miniraised beam damage to the sample and loss of Na from the analysis volume, while retaining acceptable counting statistics. The lower voltage reduces the depth of beam penetration from the usual 5 lam (20-30 kV) to about 3 gm (P.G. Hill, oral comm.), so improving the chances of the whole X-ray excitation volume lying inside a single amphibole grain. The identification and handling of problems arising from contamination of electron-microprobe analyses of finegrained synthetic amphiboles by adjacent non-amphibole phases or diluted by epoxy are discussed in detail by Graham et al. (1989). Element standards used were jadeite (Na), periclase (Mg), corundum (A1) and wollastonite (Si). Under the operating conditions given above, the analytical precision (2~) was calculated to be about 2.5% Na20, 1.3% MgO, 2.1% A1203 and 1.3% SiO2 (% relative).
Chemical relations in NMASF A major difference between phase relations in NMASH and NMASF is the absence of fluor-talc from NMASF assemblages in
The amphibole compositional data and the solid parageneses obtained in this study require the presence of an additional fluorinebearing species. A thorough survey of the chemical literature on simple and compound NMAS fluorides (e.g. Eitel, 1954; Cotton and Wilkinson, 1980 and references therein; Weast 1975 revealed SiF4 as the only plausible fluoride in NMASF that satisfies the paragenetic and compositional constraints referred to above. SiF 4 is a gas at STP (b.p. - 86~ it has a high thermal stability (e.g. 1150~ at 1 bar); it is the dominant and most stable fluorsilane, and it is a well known fluoride. Calculation shows that for the sample sizes used here, the largest quantities of SiF 4 that could be produced are < 20 gg. Such minuscule quantities cannot be determined routinely. We propose that the r61e of SiF 4 in N M A S F is analogous to that of H20 in NMASH. For example, consider the NMASH end-member reaction: 5Na2Mg3A12SisOzz(OH)2 = 3Na2MgsA1SiTA102z(OH)2 Gp MKt + 4NaAISizO 6 + llSiO2 + 2H20. Jd Co The analogous NMASF reaction is: 5NazMg3A12SisOzzF 2 = 3Na2MgsAISiTA1OzzF z + 4NaA1Si20 6 Gp Mkt Jd + 10SiO~ + SiF4. Co Similarly: 9Gp = 4Ny + 3MKt + 23Qz + 2HzO(NMASH) 9Gp = 4Ny + 3MKt + 22Qz + SiF4(NMASF).
Chemical variations in amphiboles: calculation of glaucophane and nyb6ite activities In this paper we discuss chemical trends in our synthetic fluoramphiboles in terms of changes in the thermodynamic activities of the important end-members such as glaucophane and nyb6ite. This approach provides a simple and systematic quantitative basis for the inter-comparison of the compositional variations in the NMASF amphiboles as a function of pressure and temperature. Because the activity-composition relations of complex multi-component NMASF and NMASH amphiboles are unknown, we have assumed ideal mixing on sites (e.g. Powell 1978). The resulting activities thus provide a relative rather than an absolute basis for inter-comparison. The basis for calculating end-member activities is now described.
251 ordered model leads to the following activity expressions (eg. Powell 1978):
Studies of atomic site occupancies of natural sodic amphiboles indicate that AI is strongly partitioned into T(1) and M(2) sites. The data of Raudsepp et al. (1987) indicate that this may also hold for synthetic analogues, in which strong partitioning of M 3+ (Sc 3+, In 3+) into M(2) was observed. However, this still needs to be demonstrated for A1, which is much closer in size to Mg than are Sc 3 + and In 3 +. A-site splitting is also common (e.g. Hawthorne and Grundy 1972), but the actual preferences between A(2) and A(m) are not well known and so A-site splitting will not be considered here. In natural C2/m alkali amphiboles Fe 2 + is incorporated preferentially to Mg into the M(4) site (e.g. Ungaretti et al. 1981). However, in the synthetic N M A S F amphiboles of this study, the amount of Mg on M(4) sites ranges from 0.01 to 0.19 atoms per formula unit, as the absence of Fe from the system permits the entry of Mg into M(4). In the absence of evidence to the contrary, it will be assumed that Na and Mg mix randomly on M(4) sites. In order to calculate activities for "ordered" end-members in "ordered" solid solutions, we define the "ordered" state as having the following characteristics: (a) all octahedral A1 on M(2) sites; (b) all tetrahedral A1 on T(1) sites; (c) random mixing of A1 and Mg on M(2) sites; (d) random mixing of A1 and Si on T(1) sites; (e) random mixing of Na and Mg on M(4) sites. Mixing of Na and vacancies on A-sites have been ignored, because of the lack of constraints on A-site occupancies in sodic amphiboles. Microprobe errors were propagated through the uncertainties in activities using the standard equation for error propagation (Powell and Holland 1985). The
aGp = Xo(A).X2a(M4).X3g(M 1,3).X21(M2).X4i(T1).XS4i(T2) aNy = 9.48 XNa(A).X2Ta(Mg).X3g(M1,3).X21(M2).X3i(Ti).XAI(Ti).Xs4i(T2) For comparison, we have also calculated activities for a "disordered" model in which there is also complete Mg-A1 disorder on octahedral sites. For example: a
-2 2 3 2 4 4 Gp - 8.94 Xo(A),XNa(M4).XMg(M1,2,3),XAI(MI,2,3).Xsi(T1).Xsi(T2) __ 2 3 2 [ a N y -- 274.32 XNa(A).XNa(M4).XMg(M1,2,a).XAI(M1,2,3),
X3i~T~).XAJtT~-Xs~tT2) Activities for the disordered model are 10% higher than those of the ordered model. As the two models give similar activities (well within propagated errors), we will only use the ordered model as a basis for illustrating and discussing variations in amphibole chemistry. Results
Synthesis experiments T h e r u n c o n d i t i o n s a n d results of synthesis e x p e r i m e n t s o n the f l u o r - g l a u c o p h a n e c o m p o s i t i o n are g i v e n in T a b le 2. T h i s s h o w s the r a w p r e s s u r e s a n d the m e a n c o r r e c t e d
Table 2. Run conditions and results of synthesis experiments on the fluor-glaucophane bulk composition Run #
T(~
Pr.w (kbar)
Mean P r a w
corr.(-%)
P ....
t(h)
27 26 25 24
680 700 700 702
30.0-30.3 31.6-32.1 33.6 34.0 34.9-35.4
30.2 31.9 33.8 35.2
7.5 9.0 10.0 10.8
27.9 29.0 30.4 31.4
37 65 22 43
Am,Co Am,Co Am,Jd,Co Am,Jd,Co
19 18 17
752 755 752
21.4-22.0 24.9-25.5 29.7-30.0
21.7 25.2 29.9
3.5 6.0 9.0
20.9 23.7 27.2
98 72 96
Am,Sphl,Qz Am,Qz Am,Qz
14 15 13" 16 12" 11 1a 20a 21"
800 800 800 799 801 809 800 804 803
18,2-19.2 19.7-20.6 21.7-22.2 22.6-23.5 24.6-25.l 27.5 28.2 29.6 30.4 32.2-32.5 33.9 35.0
18.7 20.2 22.0 23.1 24.9 27.9 30.0 32.4 34.5
1.0 2.0 3.5 4.0 5.5 8.0 9.8 11.8 13.5
18.5 19.7 21.2 22.1 23.5 25.6 27.1 28.5 29.8
60 77 66 54 32 26 139 86 85
Ab,Sphl,Qz Ab,Sphl,Qz Am,Sphl,Qz Am,Sphl,Qz Am,Sphl,Qz Am.Qz Am,Qz Am,Co Am,Jd,Co
9 7 8" 6 2a 3 10 23 22 30 29 31 28 34 32 33 5 4 35 36
850 850 853 853 854 881 855 860 852 900 902 930 904 920 950 910 950 925 935 900
19.6 20.0 22.9 23.3 24.9 25.6 26.7 27.7 29.3 29.7 29.8 30.1 31.6 33.2 32.7 33.1 34.4 35.2 21.7 22.1 24.5-25.2 25.5 26.4 26.8 27.1 27.6-28.4 27.7 28.2 28.5-29.3 29.4-30.1 29.8-30.3 31.8 32.0 32.0-32.3
19.8 23.1 25.3 27.2 29.5 30.0 32.4 32.9 34.8 21.9 21.9 24.9 26.0 28.0 28.0 28.9 29.8 30.1 31.9 32.2
0.0 3.0 5.0 7.0 9.5 9.8 12.5 13.0 14.8 0.0 3.5 3.5 5.8 6.5 5.5 8.0 8.0 9.0 11.5 12.0
19.8 22.4 24.0 25.3 26.7 27.0 28.4 28.6 29.7 21.9 24.0 25.0 25.4 26.2 26.4 26.6 27.4 27.3 28.2 28.3
40 72 112 38 84 90 72 80 106 50 88 40 64 96 57 78 72 42 107 130
Ab,Sphl,Qz Am,Sphl,Qz Am, Sphl,Qz Am,Qz Am,Qz Am,Qz Am,Oz Am,Co Am,Jd,Co Ab,Sphl,Qz Am,Sphl,Qz Am,Sphl,Qz,En Am,Qz Am,Sphl,Qz,En Am,Sphl,Qz, En Am,Oz Am,Sphl,Qz,En Am,Sphl,Qz,En Am, Sphl,Qz,En Am,Qz
a Electron microprobe data
Result
252 pressures. The pressure-corrected data are plotted in Fig. 1. The data define five distinct assemblages, four of which contain amphibole: (1) Am + Jd + Co, (2) Am + Q z / C o , (3) A m + S p h l + Q z , (4) A m + S p h l + Q z + En, (5) Ab + Sphl + Qz. Electron microprobe analyses of amphiboles from assemblages 1 3 are given in Table 3. Site occupancies used in the calculation of end-member activities are given in Table 4. Amphibole formulae were recalculated to 46 negative charges (22 O 2 - + 2F-). Fluorine was not determined, but assumed to be present in stoichiometric amounts. Ideal fluorine-free oxide totals should be 97.2-97.4%. However, the totals ranged from Table 3(a). Electron microprobe analyses of amphiboles from synthesis experiments. 3(a) From Am + Jd + Co at 30 kbar/803~ (AJC 1-3) 1
NazO MgO A1203 SiO2 Total
2
3
8.07 8.14 15.65 15.73 12.99 12.86 58.28 58.61 95.29 95.34 Cations per formula 2.10 2.12 3.13 3.15 2.06 2.04 7.86 7.87 15.15 15.18
Na Mg AI Si Total
8.56 16.58 13.07 60.11 98.32 2.16 3.22 2.01 7.84 15.23
90-101.5%, implying in some cases significant losses of counts to interstitial epoxy or contamination by adjacent non-amphibole phases (see discussion in Graham et al. 1989). Totals of 94.0-98.5% were taken to be "acceptable". Over 100 amphiboles were analysed of which only 23 were "acceptable". Such a low success rate is common when dealing with such fine-grained acicular products (F.C. Hawthorne, J.R. Holloway, W.V. Maresch - oral comm.). Theoretically, the N M A S H / F systems both contain nine independent limiting amphibole end-members, and two intermediates (Mg-magnesio-barroisite and Mg-magnes~o-katophorite). Within analytical error all the fluoramphibole analyses lie in or very close to the plane defined by glaucophane, nyb6ite and Mg-magnesio-katophorite together with magnesio-curnmingtonite. However, of the eleven end-members only glaucophane, nyb6ite and eckermannite have non-trivial activities. Eckermannite activities range from 0.01 _ 0.01 to 0.11 ___0.04. Mg-magnesio-katophorite activities are < 0.06. The other endmembers have activities of < 0.001. The G p - N y - M K t MC plane is also coplanar with a plane defined by glaucophane and two exchange vectors, NaA1Si_I and M g M g N a _ 1AI_ 1. All the amphiboles are primarily displaced from fluor-glaucophane along the NaA1Si_ 1 vector towards nyb6ite, with minor displacement along M g M g N a _ IAI_ 1 towards magnesio-cummingtonite and along NaSiMg_ IAI_ ~ towards eckermannite.
Table 3(b). From Am + Qz/Co (AQ 1-5 : 27 kbar/800~ AQ6-8 : 27 kbar/850~ AQ 9,10:28.5 kbar/800~ I Na20 MgO A1103 SiO2 Total Na Mg A1 Si Total
8.99 16.57 14.54 58.21 98.31 Cations per 2.28 3.24 2.25 7.63 15.40
2 9.12 16.65 14.74 57.11 97.62 formula 2.34 3.28 2.30 7.55 15.47
3
4
5
6
7
8
9
10
9.25 16.21 14.18 58.08 97.72
9.22 16.25 14.93 57.83 98.23
9.31 16.36 14.21 56,93 96,81
9.34 16.85 14.75 55.47 96.41
9.20 16.58 14.22 55.31 95.31
9.11 17.37 14.49 57.51 98.48
8.51 15.50 13.92 59.00 96.93
8.48 16.10 13.92 59.06 97.56
2.36 3.19 2.20 7.66 15.41
2.35 3.18 2.31 7.59 15.43
2.41 3.25 2.23 7.60 15.49
2.43 3.37 2.34 7.45 15.59
2.42 3.36 2.28 7.51 15.57
2.32 3.40 2.24 7.54 15.50
2.18 3.05 2.17 7.80 15.20
2.16 3.16 2.16 7.76 15.24
Table 3(c). From Am + Sphl + Qz (APQ 1 3:21 kbar/800~ APQ 4-8:23.5 kbar/800~ APQ 9,10:24 kbar/850~
Na20 MgO A1203 SiOz Total Na Mg A1 Si Total
1
2
3
4
5
6
7
8
9
10
10.17 15.63 16.55 52.81 95.16 Cations per 2.70 3.19 2.67 7.23 15.79
10.42 15.99 16.83 53.29 96.53 formula 2.73 3.22 2.68 7.20 15.83
10.34 15.48 16.70 51.97 94.39
9.84 15.96 15.57 55.38 96.65
9.36 15.33 15.54 53.89 94.12
9.84 16.37 15.34 56.32 97.87
10.04 15.67 15.44 56.54 97.69
10.22 15.46 15.88 55.31 96.87
10.06 16.67 15.60 54.77 97.10
9.86 17.06 16.08 54.89 97.99
2.77 3.19 2.72 7.17 15.85
2.56 3.19 2.45 7.43 15.63
2.50 3.14 2.52 7.41 15.57
2.53 3.23 2.39 7.46 15.61
2.58 3.10 2.41 7.50 15.59
2.65 3.09 2.51 7.41 15.66
2.61 3.33 2.46 7.34 15.74
2.54 3.38 2.52 7.29 15.73
253 Table 4. Amphibole site occupancies APQ = Amph + Sphl + Qz
used
in
the
activity
calculations
AJC = Amph + Jd + Co;
AQ = Amph + Qz/Co;
Analysis
Run
Na [A]
Na [M4]
Mg [M4]
Mg [M2]
A1 [M2]
Si [T1]
A1 [T1]
acp
ayy
aEk
AJC 1 AJC 2 AJC 3
21 21 21
0.17 0.16 0.23
1.93 1.94 1.93
0.07 0.06 0.07
0.08 0.09 0.15
1.92 1.91 1.85
3.86 3.87 3.84
0.14 0.13 0.16
0.64 0.62 0.51
0.04 0.04 0.06
0.02 0.02 0.05
AQ AQ AQ AQ AQ AQ AQ AQ AQ AQ
1 1 1 1 1 2 2 2 20 20
0.40 0.47 0.41 0.43 0.49 0.59 0.57 0.50 0.20 0.24
1.88 1.87 1.95 1.92 1.92 1.84 1.85 1.82 1.98 1.92
0.12 0.13 0.05 0.08 0.08 0.16 0.15 0.18 0.02 0.08
0.12 0.15 0.14 0.10 0.17 0.21 0.21 0.22 0.03 0.08
1.88 1.85 1.86 1.90 1.83 1.79 1.79 1.78 1.97 1.92
3.63 3.55 3.66 3.59 3.60 3.45 3.51 3.54 3.80 3.76
0.37 0.45 0.34 0.41 0.40 0.55 0.49 0.46 0.20 0.24
0.32 0.25 0.34 0.31 0.26 0.15 0.18 0.20 0.61 0.51
0.20 0.26 0.21 0.25 0.26 0.34 0.30 0.25 0.08 0.09
0.05 0.07 0.07 0.05 0.09 0.10 0.03 0.10 0.01 0.03
13 13 13 12 12 12 12 12 12 8
0.79 0.83 0.85 0.63 0.57 0.61 0.59 0.66 0.74 0.73
1.91 1.90 1.92 1.93 1.93 1.92 1.99 1.99 1.87 1.81
0.09 0.10 0.08 0.07 0.07 0.08 0.01 0.01 0.13 0.19
0.10 0.12 0.11 0.12 0.07 0.15 0.09 0.08 0.20 0.19
1.90 1.88 1.89 1.88 1.93 1.85 1.91 1.92 1.80 1.81
3.23 3.20 3.17 3.43 3.41 3.46 3.50 3.41 3.34 3.29
0.77 0.80 0.83 0.57 0.59 0.54 0.50 0.59 0.66 0.71
0.07 0.06 0.05 0.16 0.20 0.18 0.22 0.16 0.09 0.09
0.62 0.64 0.69 0.44 0.43 0.39 0.42 0.52 0.48 0.46
0.06 0.07 0.06 0.07 0.04 0.09 0.06 0.05 0.11 0.09
1 2 3 4 5 6 7 8 9 10
APQ APQ APQ APQ APQ APQ APQ APQ APQ APQ
1 2 3 4 5 6 7 8 9 10
E x p e r i m e n t a l p a r a g e n e s e s a n d c o m p o s i t i o n a l trends in the f l u o r - a m p h i b o l e s are n o w described with reference to Tables 2 4 a n d Figs. 1-3. The m o s t g l a u c o p h a n i c amphiboles coexist with coesite 4- m i n o r j a d e i t e at pressures a b o v e 28 kbar; those synthesized at 800~ characteristically have low A-site occupancies (0.2 Na) a n d > 1.9 N a on M(4). These a m p h i b o l e s have a~p = 0.5 0.60, aNy < 0.1, aEk < 0.05. A m p h i b o l e s b e c o m e increasingly n y b 6 i t i c with decreasing pressure at the expense of j a d e i t e which is eventually lost at a r o u n d 28 kbar. At temperatures below 925~ a field in which g l a u c o p h a n e - n y b 6 i t e a m p h i b o l e s coexist with q u a r t z o r coesite occurs at 2 3 - 3 0 k b a r at 750~ a n d 2 5 - 2 9 k b a r at 900~ Above 925~ this a s s e m b l a g e is replaced by one c o m p r i s i n g a m p h i b o l e + s o d i u m p h l o g o p i t e + q u a r t z + enstatite. At 27 k b a r / 8 0 0 - 8 5 0 ~ a m p h i b o l e s are essentially glaucop h a n e - n y b 6 i t e solid solutions with c o m p a r a b l e a~p a n d any of a r o u n d 0.2-0.35. E c k e r m a n n i t e activities are < 0.05. The d a t a for 800~ (Fig. 2) suggest that a large c h a n g e in c o m p o s i t i o n occurs over a small pressure interval (27-30 kbar). A m p h i b o l e s from e x p e r i m e n t s at 850~ (Table 2: runs 2 a n d 8) are m o r e k a t o p h o r i t i c a n d nyb6itic t h a n those at 800~ at c o m p a r a b l e pressures (24-27 kbar). M g - m a g n e s i o - k a t o p h o r i t e activities increase from 0.01 to 0.06. N y b6ite activities increase from a r o u n d 0.35 to 0.45, whereas g l a u c o p h a n e activities decrease from a r o u n d 0.25 to 0.15. At t e m p e r a t u r e s below 920~ a n a r r o w field occurs at 19-23 k b a r / 7 5 0 ~ in which nyb6itic a m p h i b o l e s coexist with e n d - m e m b e r s o d i u m - p h l o g o p i t e a n d quartz. N y b 6 i t e activities increase from a r o u n d 0.5 at 24 k b a r to 0.65 at 21 kbar. The c o r r e s p o n d i n g range for g l a u c o p h a n e activities is 0.2-0.05. At 21 k b a r 80% of A-sites are full. Finalty,
at 19 k b a r / 7 5 0 ~ to 23 kbar/900~ nyb6itic a m p h i b o l e s b r e a k d o w n u n i v a r i a n t l y to give an albite + s o d i u m p h l o g o p i t e + q u a r t z assemblage. Hence, the principal crystal-chemical change in a m p h i b o l e is a progressive filling of the A-site with decreasing pressure, p r i m a r i l y t h r o u g h the N a A 1 S i _ I substitution which also involves A1-Si exchange on t e t r a h e d r a l sites. In terms of activities,
Key: e A r n Jd Co
O A m Co/Qz
(])Am Sphl Qz En I
321
i
G @
i
e 0
OO
2B
0
0 O
.~
(~Am Sphl Qz ~ A b Sphl Qz
i
e
~ - ' ~ (D@ 0 O (~@ 0.~
~)
24
13_
~|
20
| |
16
I
600
I
700
I
I
800
i
I
900
I
1000
T (~
Fig. 1. Summary of the results of synthesis experiments on the fluorglaucophane bulk composition. Each experiment is shown as a mean corrected pressure. The sizes of product symbols are comparable to uncertainties in pressure and temperature. Boundaries separating parageneses are shown as solid lines
254
amphiboles are close to being binary glaucophane-nyb6ite solid solutions, although they plot in the Gp-NyMKt-MC plane. At 22-30 kbar and temperatures above 920~ the Am + Qz/Co _+ Jd _+ Sphl fields are sharply truncated by a field comprising Am + Qz/Co + Sphl + En. Amphibole is a relatively minor constituent of these run products, and no satisfactory microprobe analyses were obtained (poor totals and contamination by other phases). The systematic variations in amphibole composition and paragenesis as functions of pressure and temperature are summarised in Fig. 3. With decreasing pressure, amphiboles sweep across the Gp-Ny-MKt(-MC) plane away from glaucophanic compositions towards nyb6ite, with a
0.8
I
I
I
I
[
24
25
I
I
I
I
I
26
27
28
29
30
0.7 Ny
ll.6 0.5 .J .~ 0.4 .
0.7) and low glaucophane activities ( < 0.1). The very high-temperature ( > 920~ enstatite-bearing assemblage remains largely unexplored in detail. Enstatite, a phase absent from the other amphibole (Gp-Ny) parageneses, arises from Mg-saturation in amphibole. In the previous section we mentioned the increase in Mgmagnesio-katophorite activities in Gp-Ny amphiboles on going from 800~ to 850~ at 24-27 kbar. Values of 13 (moles of MgMgNa_ 1AI_ 1 or MgMgSi_ 1) increase from around 0.1 to 0.2, This suggests that the high-temperature Mg-saturated amphiboles coexisting with enstatite, quartz and sodium-phlogopite may have significantly higher aMK~ values than their lower temperature Gp-Ny counterparts. The variance of this assemblage is two when SiF 4 is absent and one when it is present. Hence, if the Phase Rule is to be obeyed and the assemblage is truly divariant the absence of SiF 4 is required. This has not
Am S p h l ~ , ~ 1 . --~ / Ab Sphl Qz
Am Qz 20
18
- ~ Am Ab Tc Qz !
700
28
800
i
i
700
i
800
I
900
20 i
16
T (~
Fig. 4. A comparison of the results of Koons (1982) for hydroxyglaucophane, based largely upon syntheses, and those of this study. The dash-dot line is the water-saturated NASH solidus
been demonstrated. Further work on the stability relations of these refractory amphiboles is needed to clarify this problem.
Comparison with recent NMASH studies In this section we compare the N M A S F results with those obtained in two recent and extensive studies of glaucophanic amphiboles in NMASH, namely Koons (1982) and Carman and Gilbert (1983). The N M A S F parageneses are essentially analogous to those obtained by Koons (1982) under nominally water-undersaturated conditions. The correspondence between the two paragenetic sequences is striking (see Fig. 4). The differences are: (a) the sheetsilicate is talc in N M A S H and sodium-phlogopite in NMASF, due to the instability of fluor-talc; (b) analogous field boundaries in N M A S F are displaced to pressures about 4 kbar higher than in NMASH; (c) no melting occurred in the N M A S F products. Koons obtained two distinct types of assemblage depending upon whether or not H 2 0 was in excess. According to Koons, waterundersaturated conditions were obtained by using unsealed capsules. It is unclear as to whether or not H 2 0 is eventually lost altogether from the charge, given that amphibole grew in unsealed runs. Presumably, amphibole growth is rapid relative to H 2 0 loss. However, are the specific amphiboles and amphibole-bearing assemblages
256 stable or metastable under water-undersaturated conditions? The absence of melt from most (but not all) products from unsealed runs performed at conditions where melting should have occurred (e.g. Jd + Qz + H 2 0 = melt) suggests water-undersaturation. Koons noted that " . . . no dehydration of amphibole was observed even in unsealed capsules with run times of up to three weeks". At any rate, Koons' unsealed experiments produced assemblages such as Am + Qz/Co _+ Jd/Ab +_ Tc. In contrast, his water-saturated (sealed) experiments produced assemblages containing kulkeite + sodium-phlogopite. Carman and Gilbert (1983) used a wide range of starting materials, including gels, gel mixtures, oxidecarbonate mixtures and crystalline mixtures of albite + talc and jadeite + talc to define the stability relations of glaucophanic amphiboles in NMASH at high pressure. We compare our results with theirs in Fig. 5. The major difference between the studies is that Carman and Gilbert claim that their experiments define the stability field of an amphibole closely approaching end-member glaucophane in composition, whereas we emphasize the complex solid solution behaviour of the amphiboles and the continuous reactions governing their stability. We also have reason to believe that amphiboles at the lower-pressure univariant breakdown reaction Am + Qz = Ab + Sphl are nyb6itic rather than glaucophanic. However, there is some agreement regarding the high-temperature amphibole-bearing paragenesis Am + Sphl + Qz + En, in that Carman and Gilbert recognised that a refractory amphibole was stable
3~ L
i
,
I A m Jd
]o
I
28 ,.Q D.
24
20
16 \ 600
,
I
i
I 800
700
T
i
l 900
i 1000
(~
Fig. 5. A comparison of the results of Carman and Gilbert (1983) and those of this study. The water-saturated NASH solidus is shown as a dash-dot line. The position of Jd + c~-Qz= Ab is taken from Holland (1980). According to Carman and Gilbert the upper- and lower-pressure stability limits of stoichiometric glaucophane are defined by the water-conserving, pressure-sensitive reactions Gp = 2Jd + Tc and Gp = Ab + [Sphl - Ts]~, respectively. The high-temperature stability limit of glaucophane lies about 80~lower than the thermal limit of Gp-Ny-MKt amphibole stability in NMASF. In both NMASF and NMASH, an amphibole of unknown stoichiometry is stable above this boundary
at temperatures above the thermal limit of glaucophane stability up to 980~ On the basis of X-ray data they suggested that this amphibole was probably nyb6itic. Only one of Carman and Gilbert's experiments was performed under conditions which allow a direct comparison with Koons' results to be made: run 4070 at 25 kbar/800~ (using Gp gel with 2.8% water). The product was Gp + Qz + Jd ( + En?). This experiment is inside Koons' Am + Jd + Qz field (Koons, 1982, Fig. 5). It is interesting to note that Carman and Gilbert used sealed capsules in all of their experiments, and so presumably this product grew under water-saturated conditions. Similar products were obtained by Carman and Gilbert from syntheses using oxide-carbonate mixtures on the glaucophane bulk composition. For example, Am + Qz + Jd + magnesite (MgCOa) is typical. Annealing experiments on these products performed at 25-40 kbar/750-800~ produced Gp + Qz/Co + magnesite _ Jd ___Tc, which are comparable to those obtained by Koons under similar conditions. Using the experimental details given by Carman and Gilbert (1983) and the H 2 0 - - C O 2 solution model of Poweli and Holland (1985), it is possible to estimate water activities of Carman and Gilbert's oxidecarbonate experiments. Calculation indicates that in the range 25-35 kbar/700-800~ and XH,o = 0.8, water activities are 0.8-0.9 (aco2 are 0.2 0.8). Hence, although the experiments were water-undersaturated, water activities were probably high. In our experiments SiF 4 is inferred to be the only fluid species. Charges were opened after each experiment, but no weight losses were observed. As mentioned previously, the minute masses of SiF 4 likely to be present in products could not be reliably detected. Consequently, we must concede that the integrity of welds after runs may be open to question, although considerable care was taken in sample preparation and recovery. We note in passing that Pawley (1992) has studied nyb6itic amphiboles in NMASH by using sealed capsules at temperatures above 650~ (above Koons' kulkeite-bearing field), and found parageneses similar to those obtained by Koons in his unsealed experiments. A NaC1 pressure cell was used, which is much less likely to cause capsule leakage than the conventional Boyd and England (1960) cell. Again, assemblages which should have produced melt failed to do so. Pawley (pers. comm.) agrees that the question of H20saturated-undersaturated conditions for the high-temperature amphibole-bearing assemblages remains unresolved in the face of conflicting data. The apparently systematic compositional trends of the NMASF amphiboles suggests that all charges leaked or, more likely, all remained closed; that is, the observed trend is due only to pressure. In his experiments using the bulk composition of glaucophane, Koons (1982) rationalised the compositions of glaucophanic amphiboles coexisting with quartz in terms of displacement from ideal glaucophane along the compound exchanBe vector MgMgSi_ 1 (MgMgNa_ 1AI_ ~ + NaA1Si_ 1) towards an amphibole having the composition of Mg-magnesio-katophorite. However, glaucophane does not lie on the quartz/Mgmagnesio-katophorite join. Furthermore, in Koons' Fig. 3 the MgMgNa_ ~AI_ 1 vector is mislabelled and should be MgMgSi_ v The values of ~ and 13 discussed in the
257
Sa Atomic % AI
/~
Ekl0
20
30
40
Mg
Mg MC
90
80
70
60 Gd
Atomic % Mg
previous section indicate that amphiboles coexisting with quartz alone (Am + Qz + fluid) should be much more nyb6itic than Koons proposes. Koons presents analyses for amphiboles from his Am + Qz assemblage which on inspection are clearly contaminated by quartz (YSi = 8.1) and have low oxide totals. As such these analyses do not shed light on amphibole stoichiometry. Figure 6 is a Na-Mg-A1 atomic % plot, projected from Si, of amphibole analyses from this study and that of Carman and Gilbert (1983). This plot is comparable to that used by the latter workers, and is of limited use as it does not give a representative picture of the true variations in amphibole composition that clearly occur. In particular, the characteristic out-of-paper displacement along NaA1Si_ 1 and MgMgSi_ 1 vectors, which are out of the plane of the diagram, is confused. We use it here merely to allow a comparison to be made between our study and theirs, given that they only presented their amphibole analyses in this form. The polyhedral compositional space is defined by nine independent limiting amphibole end-members. The positions of two important intermediate compositions, Mg-magnesio-barroisite and Mg-magnesio-katophorite, are also shown. Within the small analytical uncertainties all our amphibole analyses plot on or very close to the Gp-Ny-Kt plane, which is seen in Fig. 6 as a line extending from Mg-magnesio-katophorite through glaucophane to nyb6ite. An envelope enclosing the few microprobe analyses of Carman and Gilbert (labelled CG in the figure) is also shown. From this it is evident that the CG amphiboles are at least binary. The size of their envelope is comparable to ours, and we have demonstrated the wide variation in amphibole composition to which this corresponds. Their projection can result in a misleading clustering of analyses and a deceptive quality. We cannot comment further on the compositions of Carman and Gilbert's amphiboles, except to say that a close approach to glaucophane stoichiometry has not been conclusively demonstrated. This is not unexpected given the considerable analytical difficulties frequently encountered when dealing with such fine-grained materials. Carman and Gilbert also state that high amphibole
50
40
Fig. 6. An atomic % plot, projected from Si onto NaMg-A1, showing the compositions of amphiboles from this study and those of Carman and Gilbert (1983). The eleven NMASH/F amphibole end-members are shown. Most fluor-amphibole analyses plot on or close to the Gp-Ny-MKt-MC plane, whereas those of Carman and Gilbert lie outside this plane, being displaced towards Mg-magnesio-barroisite. Our amphibole field is sub-divided into three fields labelled 1, 2 and 3 which correspond to amphiboles from AJC, AQ and APQ parageneses, respectively. This plot indicates that the amphiboles of Carman and Gilbert probably did not have glaucophane stoichiometry
yields (90-95%) in some experiments imply a close approach to glaucophane stoichiometry. We calculate that, for the experimental products concerned, glaucophane activities are probably < 0.5. Comparison with natural systems
A comparison of our experimental results on the compositions and parageneses of sodic fluor-amphiboles with those of natural blueschist amphiboles has to be approached with caution. The major effect of fluorine substitution in amphiboles is to increase their thermal stability (e.g. Holloway and Ford 1975). It appears that this is due primarily to the much more exothermic values for AH~' of fluor-end-members. The difference can amount to 150 200 kJ/mol (Westrich and Navrotsky 1981; Westrich and Holloway 1981; Graham and Navrotsky 1986). "Fe-F avoidance" in micas (Munoz 1984) and amphiboles may also lead to ordering effects, with consequences for their stabilities. The effect of fluorine versus hydroxyl is more likely to influence the thermal rather than baric stability of amphibole, because of the small volume differences (0.2 kJ/kbar) between fluor- and hydroxy-amphiboles. Furthermore, most blueschist amphiboles have significant amounts of Ca, Fe z + and Fe 3 + which are not modelled directly by the NMASH/F systems. For these reasons a rigorous application of our results to natural systems is precluded. However, when some minor substitution of Ca for Mg and Fe z+ is considered, it does appear that the essential stoichiometries of low-Ca high-pressure amphiboles are represented by the synthetic amphiboles. We have searched the literature for analyses of low-Ca glaucophanic amphiboles from high pressure ( > 10 kbar) and high-temperature ( > 600~ blueschists and found that most have compositions lying within a plane defined by glaucophane, nyb6ite and katophorite end-members. For the purposes of detailed comparison, we have calculated the activities of all eleven end-members, considering Ca substitution for Mg and Fe z + on M(4), for glaucophanes taken from Brown and Forbes (1986) and Holland (1988).
258 Their rocks crystallised at 1 5 - 2 0 k b a r and about 600~ and the amphiboles have M g / [ M g + F e 2+ + Mn 2+] > 0.86 and A1/[A1 + Fe 3+] > 0.9. These amphiboles have aGp = 0.27-0.49, and all other end-members have trivial activities. The essentially glaucophanic stoichiometry is more apparent when Mg and Fe z+ are combined: aap = 0.68-0.79. Holland (1988) obtained heat capacity data for a natural low-Ca glaucophane (aap = 0.49, or 0.79 when Mg and Fe z + are combined), and derived new values for the enthalpy and third-law entropy of end-member glaucophane. He then used these values to calculate the positions of key glaucophane equilibria and found that there was reasonable agreement between these and experimental N M A S H studies. Although the maximum stability limits of glaucophane (i.e. on its own composition) are not directly relevant to rocks, experimental parageneses such as Amphibole + Jadeite + Quartz and derived glaucophane and nyb6ite activities are relevant to natural systems in which glaucophane (s.l.) coexists with sodic pyroxene and quartz. These experimental studies are encouraging, but there is a pressing need to extend the scope to include natural low-Ca samples, because Hoffmann (1972) and Maresch (1973) have shown that Fe z+ has a substantial effect upon the baric and thermal stabilities of glaucophanic amphiboles.
compositional evolution of natural magnesian glaucophanes as a function of fluid composition, Our results indicate that pressure has a marked effect on amphibole composition. (5) We have found that N M A S F high-pressure amphiboles, and by implication those of NMASH, mimic the essential stoichiometries of low-Ca high-pressure sodic amphiboles. Furthermore, petrologically important highpressure blueschist parageneses and reactions have been encountered in the N M A S H / F systems. These observations suggest that synthetic studies are relevant to natural systems, and that further high-pressure studies of glaucophanic amphiboles, particularly in N C M A S H and N M F A S H , will be valuable.
Acknowledgements. M.D.W. acknowledges support from a NERC research studentship. We gratefully acknowledge help from Peter Hill and Douglas Russell in electron microprobe analysis. Thorough reviews by Tim Holland and Walter Maresch substantially improved an earlier version of this paper. They and Alison Pawley are thanked for many helpful discussions concerning the glaucophane story. The Experimental Petrology Laboratories at Edinburgh are supported by NERC.
Conclusions
References
(1) At 15-30 kbar/700-920~ amphiboles are essentially binary glaucophane-nyb6ite solid solutions. Fluor-glaucophane is unstable under these conditions. The baric stability of glaucophane-nyb6ite amphiboles is dominated by continuous reactions in which the glaucophane component breaks down progressively by dehydration with decreasing pressure. The key reactions are: G p + NaA1Si_ 1 = Ams~ + Qz + SiF 4 _+ Jd. The most glaucophanic fluor-amphibole encountered (within analytical uncertainty) has aGp = 0.72. (2) We have found a parallel between amphibole stability relations in N M A S F and those obtained by Koons (1982) in NMASH, which suggests that high water activities are important in stabilising glaucophane stoichiometry. This conclusion supports the petrological observations on natural glaucophanes made by Holland (1979). (3) The lower pressure stability limit of N M A S F amphibole at 10-17 kbar/650-900~ involves the univariant fluid-conserving breakdown of highly nyb6itic amphiboles to albite and sheet-silicate. The high pressure disproportionation reaction G p = Jd + Tc does not occur in NMASF, due to the instability of fluor-talc on the glaucophane bulk composition. The analogous N M A S F assemblage Amphibole + Jadeite + Sodium-phlogophite + Quartz was also not found. (4) There remains some ambiguity concerning whether or not HzO-saturated conditions obtained in the experimental results of Koons (1982), Pawley (1992) and this study. In view of the likely importance of fluid composition (an2o) on amphibole composition and stability (see conclusion 2 above), it would be worthwhile studying the
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