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: Al-T= 0.10(11), Al-C= 0.06(6), (B)(Fe, Mn, Mg) = 0.09(7), Na-B = 0.04(5), Na-B = 0.09(9), and Cr, Ti, and K congruent to 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 Angstrom, 10.021 +/- 0.011 Angstrom; b = 18.055 +/- 0.004 Angstrom, 18.353 +/- 0.018 Angstrom; c = 5.278 +/- 0.001 Angstrom, 5.315 +/- 0.003 Angstrom; and cell volume = 906.6 +/- 0.5 Angstrom(3), 944 +/- 2 Angstrom(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(gamma) and n(beta) are linear with respect to ferro-actinolite substitution, but n(alpha) 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(alpha) = 1.602 +/- 0.001, 1.661 +/- 0.005; n(beta) = 1.621 +/- 0.001, 1.692 +/- 0.004; and n(gamma) = 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.
