Triclinic liddicoatite and elbaite in growth sectors of tourmaline from Madagascar

Crystals of liddicoatite-elbaite tourmaline from a pegmatite in Jochy, Mada gascar are composed of o{02 (2) over bar1}, r{10 (1) over bar1}, c{0001}, a {11 (2) over bar0}, and m{10 (1) over bar0} sectors, which correspond to th e prominent crystal faces, respectively. Therefore, the sectors were produc ed during growth, not by strain after growth. The o, m, and r sectors of on e specimen are biaxial between crossed polars [2V(-) = 30 degrees, 20 degre es, and 15 degrees, respectively] and triclinic, as indicated by X-ray diff raction. The a sector is optically biaxial and orthorhombic. The c sector i s optically uniaxial and essentially trigonal as indicated by single-crysta l X-ray diffraction. The o, r, and c sectors are of liddicoatite component, whereas the a sector of the one specimen corresponds to fluor-elbaite. Ano ther crystal specimen comprises a and m sectors, which are polysyntheticall y twinned, resulting in striations parallel to the c axis on the prism face s, and of liddicoatite. All five sectors have vacancies in the X-site (Cai Na, square).