Evans, BW; Medenbach, O; ,
The optical properties (refractive indices, 2V, and Z(sic)(c)) of 51 samples of unheated and heat-treated natural FeMg-cummingtonite have been measured with a microrefractometer spindle-stage. Maximum refractive indices (n(z)) of unheated, metamorphic cummingtonites show a straight-line dependence on composition X-Fe [where X-Fe = (Fe+Mn)/(Fe+Mn+Mg)]. Intermediate indices (n(y)) can be fit to X-Fe in two straight-line segments, whereas minimum indices (n(x)) are curved and concave down. The optic axial angle 2V(z) of unheated cummingtonite is represented on the composition diagram by a V-shaped plot, with a minimum of 76 degrees at X-Fe = 0.30, and (optically negative) maxima at end-member compositions. The variation in 2V(z) from X-Fe = 0.0 to 0.30 (magnesio-cummingtonite) is basically the same as that of anthophyllite. Cummingtonites re-equilibrated at high temperature by hydrothermal treatment and quenching possess values of 2V(z) that are smooth functions of temperature (600 degrees, 700 degrees, 750 degrees, 800 degrees C) and define curved isotherms on the 2V(z) versus X-Fe diagram. The shape of the optical indicatrix (2V(z)) of cummingtonite can be expressed as a function of composition and state of long-range FeMg-order (s(123)): 2V(z) = 130.3 - 77.7X(Fe) + 46.8(X-Fe)(2) - 56.7s(123) + 46.9X(Fe . S123), where s(123) = X-FeM4 - average (X-FeM1M2M3). All possible values of 2V(z) are contained in a triangular field on the 2V(z) vs. X-Fe diagram, with corners at 130.3 degrees and X-Fe = 0.0, 68.6 degrees and X-Fe = 0.286, and 99.4 degrees and X-Fe = 1.0. The 2V(z) of experimentally M-site ordered cummingtonite (750 degrees --> 600 degrees C) is within error limits identical to that of experimentally disordered cummingtonite (RT --> 600 degrees C), confirming reversibility of the ordering process on a laboratory time scale. Phenocrysts of cummingtonite in rhyolites and dacites have values of 2V(z) and X-Fe, hence s(123), that are consistent with quenching at temperatures in the range 640 degrees to 775 degrees C, the higher values for tephra-fall deposits. The measurement of 2V of ferromagnesian amphibole with the spindle-stage provides, together with composition X-Fe, a measure of its state of M-site order which, with advantages of speed, cost, and precision, is competitive with alternative techniques such as Mossbauer spectroscopy and X-ray diffraction.