Satin spar

Satin spar, less commonly atlas spar, is a trivial, partly obsolete name for fibrous varieties of two minerals, gypsum and calcite,^: 40 possessing a characteristic silky (satin) luster. Despite their very low hardness (2 or 3 on the Mohs scale), both atlas (satin) spars (especially selenite) are widely used as ornamental stones.

It should be understood that both of the listed minerals, despite their name of “spars”, do not strictly belong to the number of the most well-known under this name feldspars — both from the point of view of chemical composition and from the standpoint of classical mineralogy.

Main minerals and varieties

Selenite is a fibrous variety of gypsum,^: 527 the most common form of satin spars;
Gypsum, satin spar^: 429 in the form of a lustrous fibrous variety, not, however, defined as selenite;^: 217
Calcite or calcite spar is a fibrous variety^: 429 superficially resembling selenite,^: 169 but much less common in nature;
Aragonite is one of the polymorphic natural varieties of calcite,^: 429 sometimes having the same satin effect.