Gypsum ("gypsum roses" or "desert roses" or "selenite roses") from the upper Cenozoic of Mexico.

A mineral is a naturally-occurring, solid, inorganic, crystalline substance having a fairly definite chemical composition and having fairly definite physical properties. At its simplest, a mineral is a naturally-occurring solid chemical. Currently, there are over 5500 named and described minerals - about 200 of them are common and about 20 of them are very common. Mineral classification is based on anion chemistry. Major categories of minerals are: elements, sulfides, oxides, halides, carbonates, sulfates, phosphates, and silicates.

The sulfate minerals all have one or more sulfate anions (SO4 -2).

Gypsum is a moderately common hydrous calcium sulfate mineral (CaSO4·2H2O). Gypsum has a nonmetallic luster, is usually clearish to whitish, is soft (H≡2), and rather fragile/brittle. Gypsum has 3 cleavage planes, one of which is well developed (“one good cleavage), and the other two are not well developed (“two poor cleavages”). Broken gypsum specimens are frequently thin plates - the consequence of the one good cleavage plane.

Gypsum has economic value as a mineral. It is the starting ingredient for making plaster and wallboard.

The Mexican specimens shown above represent a distinctive form of intergrown gypsum plates called "gypsum roses" or "desert roses". This material is sometimes called "selenite". "Selenite" is often perceived to be a macrocrystalline "variety" of gypsum, but it is not. Selenite is gypsum - it differs in no way from gypsum - it is gypsum. Use of the term "selenite" should be discouraged and is here considered an unnecessary objective junior synonym.

The brown-colored material between the gypsum plates is siliciclastic sediment - mostly sand. The gypsum formed by evaporation of water in a desert lake. The whitish-colored edges of the gypsum plates are not natural. Specimens from this locality (see details below) are "flamed" with a torch to whiten the edges and to hide damage from mining. The original gypsum is clearish/colorless. Intense heat dehydrates the gypsum, turning it white.

Locality: Ahumada Playa - old playa lake deposit north of the town of Ahumada, Samalayuca Desert, northern Chihuahua State, far-northern Mexico

More info. on this locality: <a href="http://www.mindat.org/loc-180811.html" rel="noreferrer nofollow">www.mindat.org/loc-180811.html</a>

Photo gallery of gypsum: