A Carboniferous Continental Andesite--Rhyolite Association from the 'Nychum' Area, North Queensland, Australia

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Abstract
Hydrothermal investigation of the bulk composition 2CaO·Al2O3·l/2Fe2O2·3SiO2+excess H2O (Ps 33 +excess H2O) has been conducted using conventional apparatus and solid oxygen buffer techniques. Coarse-grained epidotes (over 150 microns in some cases) were readily synthesized from oxide mixtures with a 98 per cent yield as well as from their high temperature equivalents at 600–700 °C and 5 kb Pfluid and over a range of oxygen fugacities. Electron microprobe analyses show that maximum Fe+3 content of synthetic epidotes varies as a function of fo2. Epidote is most iron-rich (Ps 33 ± 2) at high (HM and CCO) oxygen buffers and becomes progressively more aluminous (Ps 25 ± 3) with decreasing fo2 values and temperatures. Such variation is consistent with the change of refractive indices and cell dimensions. The mean refractive indices and cell dimensions for synthetic epidote (Ps 33) are Nα = 1.745 ± 0.005, Nγ = l.786±0.005, a = 8.920±0.005 Å, b = 5.645±0.004 Å, c = 10.190 ű0.006 Å, and β = 115° 31′±4′ and for epidote (Ps 25) are Nα = 1.735±0.005, Nγ = 1.775±0.005, a = 8.891±0.005 Å, b = 5.625±0.004 Å, c = 10.177±0.006 Å, and β = 115° 30′±3′. Mössbauer spectra indicate synthetic epidotes are relatively disordered.

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