greasy lustre

mineralogy

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In mineral: Lustre
[Mg₃Si₄O₁₀(OH)₂] may show pearly lustre); greasy, having the appearance of being covered with a thin layer of oil (such lustre results from the scattering of light by a microscopically rough surface; some nepheline [(Na, K)AlSiO₄] and milky quartz may exhibit this); silky, descriptive of the lustre of a skein of…
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