I have already covered the exotic Desert Roses that are crystals of Gypsum that have incorporated sand grains into their flower like structure, see issue 59, Spring 2013.
The mineral Gypsum is both a practical mineral and a beautiful one, it often forms as slender wonderful crystals in deserts and mines, we all use this mineral in our every day lives in everything from toothpaste to plaster on the walls of our homes and offices.
Gypsum and related minerals
This is an evaporite class of mineral that is mined the world over on a vast scale, often in massive underground mines in ancient evaporite deposits. It is frequently in these mines that we find the wonderful crystal clusters, forming amongst other massive evaporate beds in which Gypsum can be found. Sometimes the crystals form as very beautiful, delicate, slender needles, they grow this way due to the ease with which Selenite dissolves in water. The word Gypsum actually comes from the Greek gypsos which means plaster and because it was well known from the Quarries of Montmatre in Paris it became known as Plaster of Paris. After mining, the material would be dehydrated, heated, crushed and the resulting powder would be mixed with water and after rehydration could be used for casting and carving. The first recorded use of plaster in this way was by a 4th century Greek sculptor called Lysippus of Sicyon who made the first plaster cast of a human face.
On a practical level Gypsum CaSO4 2H2O is a Calcium Sulphate dihydrate, meaning it has two molecules of water in its structure, it crystallises in the Monoclinic system. It is very soft at only 2.5 on the Mohs scale and it can be scratched with the fingernail.
The name Selenite comes from the Greek selene (moon) and lithos (stone) not to be confused with the moonstone that is a variety of feldspar used in jewellery. The mineral can form as Curved Rams Horn crystals, as fishtail or swallowtail twins, duck bill twins or exhibiting spear or arrow heads in the crystals.
Amongst the earliest recoded uses of Gypsum were as large cleaved fragments of clear Gypsum used in many homes to protect pictures of the Virgin Mary when glass was far to expensive and difficult to form as sheets. At that time the glass industry was in its early days and sheets of clear Gypsum were used to protect pictures of the sacred Virgin Mary and thus it gained the name of Marienglass, protecting the sacred images of the mother of Jesus thathung in most devout Catholic homes.
The massive variety of layered Gypsum can exhibit on cleaved surfaces a distinctive sheen of a Satin like material, the very silky sheen originates from the fine parallel fibres and gives the cleaved material a wonderful inner glow on the fibrous surface, this can be seen in one of the pictures associated with this article, this variety is given the apt name of Satin Spa.
This is another name given to the massive form of Gypsum when no layering can be seen, this is anhydrous Calcium Sulphate, CaSO4, and this mineral belongs to the orthorhombic class of crystal system and it has a hardness of 3.5. When this massive form is exposed to circulating ground waters then the Anhydrite absorbs the water and rehydrates to turn into fibrous Gypsum, as this happens, it also expands in the process and this distorts and crumples pre existing layering that is typical seen in many associated evaporites.
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Gypsum is the name used when referring to massive layered deposits, the very beautiful crystals of translucent Gypsum are usually given the name Selenite; this mineral comes in a variety of shapes and sizes and can be found amongst the evaporite deposits from mines such as Poland’s famous salt mines.
Good crystals are also frequently found in desert environments in evaporite deposits formed in the vicinity of dried up lakes and pools in the deserts such as in Australia.
A recent beautiful find is the Golden translucent Selenite crystals from the now abandoned mine of Salinas de Otuma, Paracas, Pisco; Ica, Peru, crystals seem to have an inner glow and are highly prized by collectors.
Selenite can be found in a variety of colours from Black, Brown, Orange, White, Yellow and Green depending on small amounts of additional elements, some crystals even have several colours in the same specimen.
Many of you will have seen the images of the spectacular giant crystals found in a cave in the Naica Mine, Chihuahua, Mexico; this is a spectacular large cave found in the mine and preserved in a pristine environment, here the vital temperatures and humidity are kept at consistent levels to preserve this find. One giant crystal weighs 55 tons and at 11 metres long is calculated to be 500,000 years old.
In a totally different occurrence for Gypsum, when deep water clay deposits are found near to the surface, normal weathering leaches out Sulphur from included Iron Pyrite and this then mixes with Calcium from adjacent fossil shells, the combination of elements produces perfectly shaped transparent to translucent Selenite Crystals that grow in the soft clay.
A good example of this is foundin the Cretaceous Gault Clays of the English Coast, exposures of this type of mineral growth can be found near to Dover Harbour, good Ammonite fossils can also be found here with very fine Selenite crystals growing on the fossil surface.
Gypsums practical uses
The main use for the mineral Gypsum is as plaster and plasterboards used in the building industry. Also related evaporite minerals such as Polyhallite are used and this has the addition of Potassium and Magnesium in its structure, it is used in the Agricultural and Chemical industries. This evaporite is referred to as Potassium Alum and has both Potassium and Aluminium in its makeup, this is extracted together with other related evaporite minerals at the Boulby Mine in Yorkshire.
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