Violet-colored diopside from southern Baffin island, Nunavut, Canada

Abstract

Diopside with an unusual violet color from southern Baffin Island, Nunavut, Arctic Canada, is found in calc-silicate lenses, associated with marialite, pargasite, phlogopite, calcite, apatite, titanite, talc, chlorite, plagioclase and quartz. It occurs as massive aggregates of roughly equant grains. Basal parting is evident, and pyroxene cleavage is subtle. Indices of refraction are n_α 1.670(1), n_β 1.675(1), and n_γ 1.695(1), and 2V_Z is equal to 57.6(5)° at 589 nm. Pleochroism is nonexistent. D_(calc) = 3.30 g/cm^3. Cell dimensions determined from powder X-ray diffraction are ɑ 9.730(4), b 8.873(3), c 5.275(2) Å, β 105.95(3)°. A single-crystal X-ray structural refinement was performed to determine bond lengths and angles. The empirical formula, based on results of electron-microprobe analyses and absorption spectroscopy indicating 0.30 wt.% H_2O in the structure, is (Ca_(0.96)Na_(0.04))(Mg_(0.86)Al_(0.06) Fe^(2+)_(0.05)Ti^(4+)_(0.02))(Si_(1.89)Al_(0.11))O_(5.93)OH_(0.07). The unusual color is due to intervalence charge-transfer between Fe^(2+) and Ti^(4+) at the M1 site, and is observed owing to the low overall concentration of Fe in the diopside. Cathodoluminescence indicates that Mn^(2+) is present in the M1 site. Absorption spectroscopy demonstrates that Mn^(3+) does not contribute to the violet color.

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