Selenite, satin spar, desert rose, and gypsum flower are the four crystalline varieties of gypsum.
Gypsum is a very soft mineral (hardness: 2 on Mohs Scale) composed of calcium sulfate dihydrate (meaning has 2 molecules of water), with the chemical formula CaSO4·2H2O. Gypsum in all its varieties can be scratched with a fingernail. Alabaster is a massive form of gypsum, meaning that its crystal habit is not obvious to the naked eye.
Crystal habit and properties
Crystal habit refers to the shapes that crystals exhibit.
Selenite crystals commonly occur as tabular, reticular, and columnar crystals, often with no imperfections or inclusions; and thereby, appear water or glass-like. Nevertheless, there are many collectible selenite crystals that have interesting inclusions such as, accompanying related minerals, interior druse, dendrites, and fossils. In some rare instances, water was encased as a fluid inclusion when the crystal formed. Selenite sometimes forms in thin tabular or mica-like sheets; and has been used as glass panes. Selenite crystals sometimes will also exhibit bladed rosette habit (usually transparent) often with accompanying transparent, columnar crystals. Selenite can be found both attached to a matrix or base rock; but can commonly be found as entire free-floating crystals, often in clay beds.
Satin spar is almost always prismatic and fibrous in a parallel crystal habit. Satin spar often occurs in seams, some of them quite long; and is often attached to a matrix or base rock.
Desert roses are most often bladed, exhibiting the familiar shape of a rose, and almost always have an exterior druse. Desert roses are almost always unattached to a matrix or base rock; most often found lying around.
Gypsum flowers are most often acicular, scaly, stellate, and lenticular. Gypsum flowers most often exhibit simple twinning (known as contact twins); where that parallel, long, needle-like crystals, sometimes having severe curves and bends, will frequently form “ram’s horns”, "fishtail", "spear-head", and "swallowtail" twins. Gypsum flowers sometimes form quite densely in acicular mats; and can be quite fragile. Gypsum flowers often are attached to a matrix or base rock.
Occurance
Gypsum occurs on every continent and is the commonest of all the sulfate minerals.
Gypsum is formed as an evaporative mineral, frequently found in alkaline lake muds, clay beds, evaporated seas, salt flats, and caves. Gypsum, also, is frequently found in conjunction with other minerals such as, copper ores, sulfur and sulfides, silver, iron ores, coal, calcite, dolomite, limestone, and opal. Gypsum has been dated to almost every geologic age.[3]
In dry, desert conditions and arid areas, sand may become trapped both on the inside and the outside of gypsum crystals as they form. Interior inclusion of sand can take on shapes such as, an interior hourglass shape common to selenite crystals of the ancient Great Salt Plain Lake bed, Oklahoma, USA.[4] Exterior inclusion occurs as embedded sand grains on the surface such as, commonly seen in the familiar desert rose.
When gypsum dehydrates severely, anhydrite is formed. If water is reintroduced, gypsum can and will reform - including as the four crystalline varieties. An example of gypsum crystals reforming in modern times is found at Philips Copper Mine (closed and abandoned), Putnam County, New York, USA where selenite micro crystal coatings are commonly found on numerous surfaces (rock and otherwise) in the cave and in the dump.[5]
Whereas geology, mineralogy, and rockhounding groups, clubs, and societies as well as museums usually date, photograph, and note location of minerals, much of the retail mineral and jewellery trade can be somewhat casual about locations and descriptive claims.
Source: Wikipedia.Org
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