Phyllosilicates with a monoclinic crystal system, a single sheet of mica is comprised of two layers of silicon, each of which exhibits a relative negative charge. These layers are cross-linked together by one of several cations, such as potassium or sodium.

Margarite is a mica that features calcium as the interlayer of cations that holds each sheet of the mineral together. The substitution of this element for those more commonly included in micas results in a less flexible mineral. Thus, although margarite still exhibits the perfect cleavage characteristic to most micas, the thin sheets it produces when cleaved are not as bendable as those of many other varieties. Margarite is, therefore, classified as one of the brittle micas. White, gray, yellowish, or pink in color, margarite typically displays a pearly luster, its name stemming from the Greek word for “pearl.” The mineral generally occurs in low and medium-grade metamorphic rocks, such as mica schists, and often in association the aluminum oxide mineral corundum, which readily weathers to margarite and other aluminous minerals.

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