Selected Phase Relationships in the System Al-Mn-Fe-Si-O-H: A Model for Garnet Equilibria

The bulk compositions 3FeO_x.Al₂O₃.3SiO₂ $ excess H₂O and 3MnO.Al₂O₃.3SiO₂$ excess H₂O were investigated employing conventional hydrothermaltechniques. Almandine and spessartine were synthesized and stabilityrelationships determined in terms of temperature, fluid pressure,and oxygen fugacity. Synthetic almandine has unit cell edge, a₀ = 11.528 0.001 index of refraction, N_D = 1.829 0.003. No systematic variationsof these values with respect to temperature, fluid pressure,and oxygen fugacity were observed. Spessartine, synthesizedat high temperatures, has average values of a₀ = 11.614 0.001 and N_D = 1.799 0.003. However, below about 600 C a₀ graduallyincreases to 11.635 0.001 and N_D decreases to 1.772 0.003with decreasing temperature, irrespective of fluid pressureand oxygen fugacity. These changes appear to reflect the productionof hydrospessartine below about 600 C. The stability of almandine strongly depends on the oxygen fugacity.It is stable up to the vicinity of oxygen fugacities definedby the fayalite–magnetite$quartz buffer; the low f_o2,range has not been determined, but lies at oxygen fugacitiesless than those defined by the ironquartz–fayalite buffer.The stability field of almandine$fluid is bounded by the followingP_fluid-T values. At low oxidation states, the low temperature hydrous assemblageof equivalent composition consists of quartz$iron chlorite ($magnetite)$fluidand the high temperature equivalent assemblage consists of fayalite$ironcordierite$hercynite₈₈$fluid. Where f_O2 approximates or is inexcess of that defined by the fayalite–magnetite$quartzbuffer the low temperature hydrous assemblages consist of quartz$ironchlorite$magnetite$fluid, iron chlorite$pyrophyllite$magnetite$fluid,magnetite$mullite$pyrophyllite$fluid, and hematite$mullite$pyrophyllite$fluid;the anhydrous equivalent assemblages consist of quartz$hercynite₈₈,$magnetite₈₈$fluid, quartz$mullite$magnetite$fluid, and quartz$mullite$hematite$fluid,both in order of increasing oxygen fugacity. The stability of spessartine, in contrast to that of almandine,is essentially independent of oxygen fugacity at least up tothat defined by the magnetite-hematite buffer. Spessartine isstable up to the highest temperature, 930 C, employed in thisinvestigation at P_fluid = 500 bars. However, it decomposes toa hydrous assemblage consisting of quartz$manganese chlorite$fluidat the following P_fluid-T values: 414 5 C and 3000 bars;405 5C and 2000 bars; 386 10 C and 1000 bars; 3645C and 500 bars. Garnets are rare constituents of igneous rocks; those whichdo occur are predominantly spessartine-rich, and are virtuallyconfined to felsic magmas. Garnets are absent from mafic igneousrocks because the thermal stability ranges of iron-rich membersare below the solidus. The near absence of almandine in contactmetamorphosed pelitic rocks may reflect a relatively high oxidationstate in the aureoles rather than inappropriate P-T conditions.It is argued that the compositions of pyralspite garnets inpelitic schists are subject to various physical and chemicalfactors, including f_O2. With appropriate provisions, the Mn/Feratios of garnet coexisting with chlorite and quartz might beused as a temperature indicator. The rarity of spessartine in igneous and metamorphic rocks apparentlystems from the departure of rock bulk composition from Mn-richvalues rather than from the absence of appropriate physicalconditions.