"Optically" Quotes from Famous Books
... dark-green colour. They vary in specific gravity between 2.6 and 3.0, according to the amount of iron present. Well-developed crystals are met with only in the species clinochlore and penninite; those of the former are six-sided plates and are optically biaxial, whilst those of the latter have the form of acute rhombohedra and are usually optically uniaxial. The species prochlorite and corundophilite also occur as more or less distinct six-sided plates. These four better crystallized species are grouped together by G. Tschermak ... — Encyclopaedia Britannica, 11th Edition, Volume 6, Slice 3 - "Chitral" to "Cincinnati" • Various |