Lapis Lazuli (天青石):(Na, Ca)8Al6Si6O24(S, SO4)
The mineral lazurite is a complex sodium calcium aluminum silicate sulfate, in which the part of the silicon atoms (SI) are substituted for sulfur atoms (S) in the form of sulfur anions. The rich blue color is due to the sulfur inherent in the structure of lazurite. It is usually described as an intense blue to a gray blue. The greater the sulfur anions, the more intense is the blue color of lazurite. In its vivid dark blue variety the content of sulfur reaches 0.7%. Lazurite, an amply soft and brittle mineral, is easily processed as a pigment. It is a popular but expensive mineral commonly found combined with other minerals in a rock called lapis lazuli. Lapis lazuli or lapis for short is mostly lazurite but commonly contains pyrite, calcite and other minerals. The name lazurite means "blue rock" and is always a brilliant blue with violet or greenish tints. Small crystals of pyrite (FeS2) are always present in lapis and their gold yellow color distinguishes lapis from a similar mineral, sodalite, and the synthetic pigment, ultramarine. The golden impregnations of pyrite create the impression of stars in a dark blue sky. Calcite produces white streaks in lapis and too much calcite lowers the value of the mineral pigment. The carbonate mineral azurite has a very similar color to lazurite but is associated with the green mineral malachite.
Toxicity:Lazurite is not considered toxic but care should be used in handling the dry powder pigment so as not to inhale the dust.
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