LUSTRE

in mineralogy, the appearance of a mineral surface in terms of its light-reflective qualities. Lustre depends upon a mineral's refractive power, diaphaneity (degree of transparency), and structure. Variations in these properties produce different kinds of lustre, whereas variations in the quantity of reflected light produce different intensities of the same lustre. The kind and intensity of lustre is the same for crystal faces of like symmetry but may be different on those with different symmetry. The kinds of lustre are usually described as follows (the prefix sub-, as in submetallic, is used to express imperfect lustre of the kind): metallic (the lustre of metalse.g., gold, tin, copper; minerals with a metallic lustre are usually opaque and have refractive indices near 2.5); adamantine (nearly metallic lustre of diamond and other transparent or translucent minerals with high refractive indices [between 1.9 and 2.5] and relatively great density e.g., cerussite and other compounds of lead); vitreous (the lustre of broken glassthe most common lustre in the mineral kingdom; it occurs in translucent and transparent minerals with refractive indices between 1.3 and 1.8, as in quartz); resinous (the lustre of yellow resinse.g., sphalerite); greasy (the lustre of oiled surfacese.g., nepheline, cerargyrite); pearly (like pearl or mother-of-pearle.g., talc; surfaces parallel to a perfect cleavage exhibit this lustre, which results from the repeated reflections from minute cleavage cracks); silky (like silke.g., satin spar; minerals with a fibrous structure have this lustre); dull, or earthy (without lustree.g., chalk).