An experimental determination of calcic amphibole solid solution along the join tremolite-tschermakite

Calcic amphiboles along the tremolite (Tr)-tschermakite (Ts) join in the system CaO-MgO-Al₂O₃-SiO₂-H₂O were synthesized at P_fluid = 8–21 kbar and at T = 750–900 °C. Reactions were reversed at seven P-T locations within the tschermakitic amphibole field of stability. In some experiments, natural stoichiometric end-member tremolite, for which analyses showed F and Cl to be absent, was employed as seed crystals to facilitate nucleation and growth of tschermakitic amphiboles. Synthetic amphiboles occur in various corundum-bearing assemblages that define the maximum Al content in Tr-Ts solid solutions at a given P and T. At 750–850 °C and 12–21 kbar, amphibole + corundum coexist with zoisite + H₂O (± talc, chlorite, and magnesium staurolite); at lower P, amphibole coexists with anorthite + H₂O (± chlorite, corundum, spinel, pyroxenes, and sapphirine). Amphibole + corundum + clinopyroxene + H₂O (± anorthite, forsterite, sapphirine, and garnet) are stable at 900 °C over the P_fluid range 12–18 kbar. These amphibole-bearing assemblages are replaced at higher P by clinopyroxene + talc + chlorite + zoisite + H₂O at 650–750 °C, and at higher temperatures by garnet + clinopyroxene + H₂O (± zoisite, orthopyroxene, and magnesium staurolite).

All condensed phases were analyzed with an electron microprobe. Pyroxenes vary significantly in total Al content (~0.1–0.5 cations per formula unit, pfu, based on 6 O atoms), whereas garnet is nearly constant in its composition, Py₇₉Gr₂₁. Composition of magnesium staurolite varies from 7.80 to 7.94 Si and 3.65 to 3.78 Mg cations pfu based on 46 O atoms (anhydrous formula). Synthetic amphiboles are at least ternary solid solutions and contain 4–11 mol% Mg₇Si₈O₂₂(OH)₂. Calcic amphiboles with Ts > about 45 mol% contain as much as 0.15 excess cations pfu based on 23 O atoms (anhydrous formula), whereas less tschermakitic ones are deficient in cation occupancy by up to 0.18 pfu. This trend is attributed to an Al₂☐Mg₋₃ substitution in octahedral sites of calcium amphiboles, as is found in the 2:1 layer of clinochlore-sudoite solid solutions. Variable dioctahedral occupancy in the amphibole structure further complicates amphibole site assignment based on electron microprobe analyses.

Compositions of synthetic calcic amphiboles change systematically with P and T governed by coexisting mineral assemblages. The Ts content [= (8-Si-Na)/2] increases with T (∂Ts/∂T = ~0.1 mol% K⁻¹) in the range 750–850 °C, but remains nearly constant at 850–900 °C. Pressure dramatically affects the Ts content of calcic amphiboles: it increases with P_fluid at 8–12 kbar (∂Ts/∂P = 2–3 mol% kbar⁻¹), but significantly decreases at 12–21 kbar (∂Ts/∂P = −2.5 mol% kbar⁻¹). Hence, the most tschermakitic amphibole, containing 60 ± 5 mol% Ts, i.e., 1.2 ± 0.1 ^[4]Al, occurs at 12 kbar and 850–900 °C. This Al value is identical to that of synthetic magnesium chlorite and may represent a fundamental limit to Ts solubility in the 2:1 layer of biopyriboles. The Al content of Tr-Ts amphibole is a function of mineral assemblages as well as P and T and cannot be used to deduce physical conditions without identifying appropriate reaction assemblages.