The optical properties of cummingtonite and their dependence on Fe-Mg order-disorder
Evans, Bernard W.; Medenbach, Olaf
European Journal of Mineralogy Volume 9 Number 5 (1997), p. 993 - 1004
published: Sep 24, 1997
manuscript accepted: Apr 24, 1997
manuscript received: Nov 8, 1996
ArtNo. ESP147050905008, Price: 29.00 €
Abstract The optical properties (refractive indices, 2V, and Z^c) of 51 samples of unheated and heat-treated natural FeMg-cummingtonite have been measured with a microrefractometer spindle-stage. Maximum refractive indices (nz) of unheated, metamorphic cummingtonites show a straight-line dependence on composition XFe [where XFe = (Fe+Mn)/(Fe+Mn+Mg)]. Intermediate indices (ny) can be fit to XFe in two straight-line segments, whereas minimum indices (nx) are curved and concave down. The optic axial angle 2VZ of unheated cummingtonite is represented on the composition diagram by a V-shaped plot, with a minimum of 76° at XFe = 0.30, and (optically negative) maxima at end-member compositions. The variation in 2VZ from XFe = 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 2VZ that are smooth functions of temperature (600°, 700°, 750°, 800°C) and define curved isotherms on the 2VZ versus XFe diagram. The shape of the optical indicatrix (2VZ) of cummingtonite can be expressed as a function of composition and state of long-range FeMg-order (sm): 2VZ = 130.3 - 77.7XFe + 46.8(XFe)2 - 56.7123 + 46.9XFe.sm, where sm = XFeM4 - average (XFeMiM2M3>. All possible values of 2VZ are contained in a triangular field on the 2VZ vs. XFe diagram, with corners at 130.3° and XFe = 0.0, 68.6° and XFe = 0.286, and 99.4° and XFe = 1.0. The 2VZ of experimentally M-site ordered cummingtonite (750° → 600°C) is within error limits identical to that of experimentally disordered cummingtonite (RT -> 600°C), confirming reversibility of the ordering process on a laboratory time scale. Phenocrysts of cummingtonite in rhyolites and dacites have values of 2VZ and XFe hence S123, that are consistent with quenching at temperatures in the range 640° to 775°C, the higher values for tephra-fall deposits. The measurement of 2V of ferromagnesian amphibole with the spindle-stage provides, together with composition XFe, 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.