Gypsum and Alabaster

Gypsum, one of the more common sedimentary rocks in the earth s crust, occurs in varieties that differ from each other in texture rather than in composition. In most of its varieties gypsum is very soft and can be scratched even with a fingernail. One massive and fine-grained variety of gypsum, known as alabaster, much appreciated for designing delicate decorative objects, has been used particularly for carving and ornamentation. 

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The oxide, or lime, occurs in great abundance in,nature chiefly as carbonate, in the forms of marble, limestone, chalk and calcareous spar also as sulphate, in the shape of gypsum and alabaster. Carbonate of lime is the chief ingredient of shells, and phosphate of lime is the earthy part of bones. Lime is found in the ashes of many plants. 

Out of these, the first two, rock gypsum and alabaster, are of the most significant fi om an economic point of view and are most widely mined and used. Usually, people do not differentiate between these two and refer to them by the generalized term gypsum. But all the types are chemically alike and exhibit the same properties. 

Limestone and travertine have a number of synonyms, including Mexican onyx and Egyptian alabaster. True alabaster is massive gypsum, and onyx is banded quartz. 

Occurs only in combination, as limestone, marble, chalk (GaCO,) gypsum, selenite, alabaster (CaSO,), aud many other minerals. In bones, egg-shells, oyster-shells, etc., as Ca,(PO,), and CaCO and in many vegetable structures... 

Figure 1. Pictures of Desert rose, Satin Spar, Selenite, and Alabaster. These pictures were obtained from http //gwydir.demon.co.uk/jo/minerals/gypsum.htm.

Alabaster is a variety of gypsum, and is known as a decorative stone. It is white but sometimes beautifully shaded with light-colored tones of yellow or red. Its usefulness is limited by its softness (hardness 2 on Mohs scale). A gypsum curiosity is the yellow Desert rose, found in the Sahara. 

Gypsum (Calcium Sulfate Dihydrate, Alabaster), CaSO4.2H20 raw 172.17, sp gr 2.31-2.33, white substance, mp loses 3/2HaO by 128° and rest by 167° hardness 1.5 to 2. Occurs naturally in the USA, Canada, England, France, Sicily, Switzerland etc si sol in w sol in acids, glycerin and aqueous Na3S2Oa and ammonium salts... 

ALABASTER. A fine-grained variety of the mineral gypsum, formerly much used for vases and statuary. It is usually white in color or may be of other light, pleasing tints. 

Sulfates contain S04, which combines with metals and other elements. Gypsum, the most common sulfate, is used to make plaster and wall board. It is often used as gesso or filler in paints, and its massive form (alabaster) has... 

Gypsum sometimes occurs in finely crystalline masses, which is called alabaster. This material resembles marble or fine limestone in texture and color, and has been used for similar purposes. Gypsum, however, is soluble in water, so most objects made of this material do not last if exposed to moisture in any form. Most objects identified as alabaster are actually made of some form of calcite. 

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