Abstract

Serendibite has been recently discovered in the Portage-du-Fort area, Pontiac Regional County Municipality, Quebec. Dark blue, often polysynthetic twinned on {011} and with no cleavage, serendibite crystals occur exclusively in a calc-silicate rock. Fine-grained anhedral serendibite, sometimes poikiloblastic, occurs with aluminous diopside. Less commonly, serendibite may form decussate masses in the same rock type. Serendibite is biaxial negative, with indices of refraction α 1.685(2), β 1.700(2), and γ 1.712(2); 2Vmeas. = 93.6(4)°, 2Vcalc. = 91°. Orientation matrix is X ^ c = +20.5°, Y ^ b = −42.6°, and Z ^ a = −24.6°. Dispersion is strong, r << v, and there is no pleochroism. Serendibite is triclinic, space group P1¯ with cell parameters refined from an X-ray powder-diffraction pattern: a 10.035(2), b 10.423(3), c 8.648 (2) Å, α 106.47(3)°, β 95.91(2)°, γ 124.46(2)°, V 674.3(2) Å3. The seven strongest lines of the X-ray powder diffraction pattern [d in Å(I)(hkl)] are: 3.328(45)( 2¯230), 3.029 (96)(012, 021, 1¯2¯2), 2.854(96)(0 1¯3, 0 3¯1, 120), 2.689(29)(3 1¯1), 2.604(95)(030), 2.469(100)( 2¯1¯3, 0 3¯3), and 2.357(28)(2 2¯3, 23 3¯). The crystal structure determination refined to R = 2.1% for 3877 unique reflections. The borosilicate structure, a member of the aenigmatite-rhönite group, is composed of layers parallel to (011). Layers of tetrahedral chains cross-linked by octahedral polyhedra alternate with layers of octahedral chains cross-linked by square antiprism polyhedral sheets. Tetrahedral and octahedral polyhedra are partially ordered and site assignments were determined. All cation sites are disordered to some degree.

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