THE OPTICAL-PROPERTIES OF CUMMINGTONITE AND THEIR DEPENDENCE ON FE-MGORDER-DISORDER

The optical properties (refractive indices, 2V, and Z(sic)(c)) of 51 s amples of unheated and heat-treated natural FeMg-cummingtonite have be en measured with a microrefractometer spindle-stage. Maximum refractiv e indices (n(z)) of unheated, metamorphic cummingtonites show a straig ht-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-lin e 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 d egrees at X-Fe = 0.30, and (optically negative) maxima at end-member c ompositions. The variation in 2V(z) from X-Fe = 0.0 to 0.30 (magnesio- cummingtonite) is basically the same as that of anthophyllite. Cumming tonites re-equilibrated at high temperature by hydrothermal treatment and quenching possess values of 2V(z) that are smooth functions of tem perature (600 degrees, 700 degrees, 750 degrees, 800 degrees C) and de fine curved isotherms on the 2V(z) versus X-Fe diagram. The shape of t he optical indicatrix (2V(z)) of cummingtonite can be expressed as a f unction 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 o f 2V(z) are contained in a triangular field on the 2V(z) vs. X-Fe diag ram, 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 experimental ly M-site ordered cummingtonite (750 degrees --> 600 degrees C) is wit hin error limits identical to that of experimentally disordered cummin gtonite (RT --> 600 degrees C), confirming reversibility of the orderi ng process on a laboratory time scale. Phenocrysts of cummingtonite in rhyolites and dacites have values of 2V(z) and X-Fe, hence s(123), th at are consistent with quenching at temperatures in the range 640 degr ees to 775 degrees C, the higher values for tephra-fall deposits. The measurement of 2V of ferromagnesian amphibole with the spindle-stage p rovides, together with composition X-Fe, a measure of its state of M-s ite order which, with advantages of speed, cost, and precision, is com petitive with alternative techniques such as Mossbauer spectroscopy an d X-ray diffraction.