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Unit-cell parameters, optical properties, and chemical compositions have been measured for 103 samples in the tremolite-actinolite-ferro-actinolite series. The average values of the non-essential constituents are:  T Al = 0.10(11),  C Al = 0.06(6),  B (Fe, Mn, Mg) = 0.09(7),  B Na = 0.04(5),  A Na = 0.09(9), and Cr, Ti, and K ≅ 0. Asbestiform actinolite samples have lower Al contents than massive or “byssolitic” actinolite samples. Unit-cell parameters for end members tremolite and ferro-actinolite based on regressions of the data are: a = 9.841 ± 0.003 Å, 10.021 ± 0.011 Å; b = 18.055 ± 0.004 Å, 18.353 ± 0.018 Å; c = 5.278 ± 0.001 Å, 5.315 ± 0.003 Å; and cell volume = 906.6 ± 0.5 Å 3 , 944 ± 2 Å 3 . The changes in a, b, and cell volume with ferro-actinolite substitution are modeled with quadratic functions, and the change in c with ferro-actinolite substitution is modeled with a linear function. There is a positive correlation between c and Al that results in a discrimination between asbestiform and massive or “byssolitic” habits for c and for the refractive indices. The principal refractive indices n γ  and n β  are linear with respect to ferro-actinolite substitution, but n α  is best modeled by two lines with a change in slope between 26 and 32% ferro-actinolite. Birefringence and extinction angle also change between 26 and 32% ferro-actinolite. The predicted end-member values of the principal refractive indices for tremolite and ferro-actinolite are: n α  = 1.602 ± 0.001, 1.661 ± 0.005; n β  = 1.621 ± 0.001, 1.692 ± 0.004; and n γ  = 1.631 ± 0.001, 1.700 ± 0.003. There is a discontinuity in a at approximately 11% ferro-actinolite that is accompanied by a drop in Ca. There are also indications of discontinuities in optical properties and c between 26 and 32% ferro-actinolite that may be due to an increase in tschermakite substitution. Both discontinuities are accompanied by a decrease in the relative frequency of natural samples.

american mineralogist

The tremolite-actinolite-ferro–actinolite series: Systematic relationships among cell parameters, composition, optical properties, and habit, and evidence of discontinuities