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); 2 V meas. = 93.6(4)°, 2 V calc. = 91°. Orientation matrix is X ^ c = +20.5°, Y ^ b = −42.6°, and Z ^ a = −24.6°. Dispersion is strong, r with cell parameters refined from an X-ray powder-diffraction pattern: a 10.035(2), b 10.423(3), c 8.648 (2) A, α 106.47(3)°, β 95.91(2)°, γ 124.46(2)°, V 674.3(2) A3. The seven strongest lines of the X-ray powder diffraction pattern [ d in A( I )( hkl )] are: 3.328(45)( ![Formula][2] 230), 3.029 (96)(012, 021, ![Formula][3] ![Formula][4] 2), 2.854(96)(0 ![Formula][5] 3, 0 ![Formula][6] 1, 120), 2.689(29)(3 ![Formula][7] 1), 2.604(95)(030), 2.469(100)( ![Formula][8] ![Formula][9] 3, 0 ![Formula][10] 3), and 2.357(28)(2 ![Formula][11] 3, 23 ![Formula][12] ). The crystal structure determination refined to R = 2.1% for 3877 unique reflections. The borosilicate structure, a member of the aenigmatite-rhonite 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. [1]: /embed/mml-math-1.gif [2]: /embed/mml-math-2.gif [3]: /embed/mml-math-3.gif [4]: /embed/mml-math-4.gif [5]: /embed/mml-math-5.gif [6]: /embed/mml-math-6.gif [7]: /embed/mml-math-7.gif [8]: /embed/mml-math-8.gif [9]: /embed/mml-math-9.gif [10]: /embed/mml-math-10.gif [11]: /embed/mml-math-11.gif [12]: /embed/mml-math-12.gif
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