Iron-magnesium amphiboles: synthesis and stability with respect to temperature, pressure, oxygen fugacity, and sulfur frugacity

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Virginia Polytechnic Institute and State University


Standard hydrothermal and gas-buffering techniques have been used to synthesize and investigate phase relations of amphiboles on the join Mg₇Si₈O₂₂(OH)₂-Fe₇Si₈O₂₂(OH)₂. Synthesized amphiboles from Mg₆Fe₁Si₈O₂₂(OH)₂ to Mg₁Fe₆Si₈O₂₂(OH)₂ are optically orthorhombic. Variation of unit cell parameters with composition suggests that they are members of a single, continuous solid solution.

(a(Å) = 18.577(12) + 0.001284(190)XFe,

b(Å) = 17.942(11) + 0.004862(170)XFe,

c(Å) = 5.285(3) + 0.000617(50)XFe,

V(ų) = 1760.8(1.7) + 0.8226(280)XFe ;

XFe = mole % Fe end-member)

Electron diffraction patterns of composition Mg₅Fe₂ are consistent with that of an orthorhombic amphibole with an a unit cell repeat of ~18.6 Å, but are unlike any known amphibole structure type. Even though the structure type is unknown, measured shifts in peak locations on the powder X-ray diffraction patterns allow compositions to be measured to within ±3 mole % Fe end-member. Combining these results with those of Forbes (1971) and Greenwood (1963), it is now clear that the entire range of amphiboles across the join can be synthesized.

No change in unit cell parameters as a function of fO₂ was observed, and thus there is no evidence for solution of oxy-amphibole component.

At 2 kbar and fO₂ defined by the MH buffer the maximum extent of 2 solution of Fe end-member in amphibole is 12 and 22 mole% at 725° and 630°C respectively; amphibole is unstable below 630°C, being replaced by the assemblage + talc + quartz + magnetite + hematite. At fO₂ defined by the NNO buffer the extent of solid solution expands to 54, 62, and 65 mole % Fe end-member at 725°, 625°, and 600°C, respectively.

Results obtained in this study have been combined with previously published data to produce a T-X section of the upper thermal stability of amphibole at 2 kbar and fO₂ defined by the FMQ buffer. Temperatures for 2 the reaction: amphibole -> pyroxene + quartz + vapor decrease from ~765°C for the pure Mg end-member to ~710°C for 62 mole % Fe end-member. The breakdown reaction: amphibole -> olivine + quartz + vapor, was observed for the more iron-rich amphiboles, and takes place at ~675°C for amphibole of 73 mole % Fe end-member.

Reversed tie lines have been determined between Fe-Mg amphiboles and pyrrhotites in the presence of excess magnetite and quartz at 2 kbar, and 650°, 675°, 700°, and 725°C. This assemblage represents the simultaneous equilibria:

amphibole + O₂ -> magnetite + quartz + H₂O (1)

amphibole + S₂ -> pyrrhotite + quartz + H₂O + O₂. (2)

At 700°C, amphiboles of 29, 41, 49, and 57 mole % Fe end-member coexist with pyrrhotites of N = 0.928, 0.934, 0.943, and 0.950, respectively. Compositions of coexisting amphibole-pyrrhotite pairs apparently are not seriously affected by temperature over the range investigated although scatter of the amphibole data does not allow a rigorous analysis. Sulfur fugacity for runs was determined from pyrrhotite compositions while fO₂ was known from an experimentally determined magnetite-pyrrhotite curve. Knowledge of these two fugacities allowed calculation of fugacities of all species, including H2o, assuming an H-0-S vapor, and thus reactions (1) and (2) were located in terms of fO₂and fS₂.

Several models for the amphibole solid solution were used to explain the variation in composition of coexisting amphibole-pyrrhotite pairs at 700°C. The precision of measurement of both the amphibole compositions and the fugacities of volatile species does not justify other than an ideal solution model. A standard state enthalpy of formation (H°298°;1 atm) of Fe₇Si₈O₂₂(OH)₂ amphibole from the elements of -2262 kcal/mole was calculated from a log Keq vs 1/T plot for reaction (1).

The results in the S-free system have been used to estimate temperatures of formation of amphibole-bearing metamorphic and extrusive igneous rocks. Application of the results in the S-containing system is limited by the scarcity of data on natural amphibole-sulfide assemblages.