Highly aluminous hornblendes; compositions and occurrences from southwestern Massachusetts

A re-evaluation of Al^IVvs. Al^VI occupancy of calcic and subcalcic amphiboles of the literature recalculated to 23(O) suggests that several analyses from specimen 102–1 are more tschermakitic than any amphiboles previously reported regardless of the Fe₂O₃ approximation used. A standard A1^VIvs. A1^IV plot (such as that described by Leake in 1971) is unsatisfactory because it fails to distinguish the A1^VI content due to glaucophane-type [Na(M4),Al^VI → Ca,Mg] substitutions and the A1^IV content due to edenite-type substitutions [Na(A),Al^IV → □,Si].

On an [Al^VI − Na(M4)] vs. [Al^IV + Na(M4)] plot, the alkali-rich end-members of hornblende are shifted away from tschermakite. Such a plot clearly shows the unusual composition of the southwestern Massachusetts amphiboles and of natural tschermakitic amphiboles in general. The approximate substitutional limitation Al^VI − Na(M4) = 0.75[A1^IV + Na(M4)] changes slightly to 0.82[A1^IV + Na(M4)] when Fe³⁺, Ti⁴⁺, and Cr³⁺ are included with A1^VI, suggesting a very strongly defined limitation of maximum octahedral R³⁺ occupancy in calcic amphiboles.