Gypsum

• One common use for gypsum is to make the wallboard that the walls in houses are made from
• white streak
• soft
• waxy luster
• comes in a variety of colours
• Gypsum comes in different varieties.
  • When it is chalky or opaque, it goes by the name of gypsum.
  • When it is clear & transparent, it is known as selenite.
  • When it is granular and comes in massive chunks it is called alabaster.
  • When it forms in petals and looks like a flower, it is called a "gypsum rose".

Gypsum Rose

• Gypsum crystals often form blades or petals. These specimens are called "gypsum roses"
• the frosted look at the edge of this specimen was artificially created by heating the edge of the crystal blades with a torch which dehydrated that part of the crystal

SPECIMEN ORIGIN: MEXICO
 

•  selenite is a variety of gypsum
• What sets selinite apart is that it is clear like glass whereas gypsum is opaque

Alabaster is a variety of gypsum. It is opaque. When gypsum looks like it is granular (made from compacted little grains) and is found in compact masses, it is called alabaster. It takes a soft shine and is often used to carve sculptures.

Unless we’ve found it in wildly unusual places, most of us consider gypsum to be a pretty dull substance. Perhaps, but it has an ancient history, huge commercial benefits and several exotic cousins.

Gypsum has a simple chemical composition, in words, calcium sulfate with some water hooked on – we’ll see how important that water is later. The simple formula is CaSO4.2H20 . A cousin to gypsum is the mineral anhydrite – no water. These minerals are deposited in vast amounts from bodies of water, usually seawater, which have become trapped in lakes or bays, where the water evaporates as incoming flow brings in a constant supply of various salts in solution. For not only does gypsum drop out, but so, eventually do all the other salts in seawater. These deposits are called evaporite rocks for that reason. Included are halite or common (table) salt and finally salts of potassium and others.

These deposits may be of great thickness. In Western Canada’s deeper oil and gas wells, hundreds of metres of Palaeozoic anhydrite are commonly found. Outcrops of anhydrite soon pick up water to become gypsum and wash away.

The Egyptians used gypsum as building material and so do we moderns in stucco, wallboard, paint fillers, glass and soil conditioners. Nova Scotia produces over 80% of Canadian gypsum, mostly for export. Some comes from Ontario, Manitoba and B.C.

In an unusual process the sulfur in gypsum over salt domes along the Gulf of Mexico separates to yield valuable sulfur deposits. A major use is in plaster of Paris, made by heating gypsum to about 150° C. Later, at the hospital where the doctor is putting a cast on that ski-fractured leg, he adds water to the powder and presto! It hardens – to gypsum again.

As collectors we are more interested in the crystalline varieties, and some are strange and beautiful:

• Selenite – some up to 10 cm in length, the clear crystals have distinct arrowhead shapes and may be intricately twinned (combined). They may be found in some clay outcrops as isolated pieces. Named after "selene", the moon.

• Satin spar – a fibrous silky variety sometimes used in cheap jewelry

• Rock gypsum – massive, scaly, often impure, ground up for land plaster on acid soils.

• Alabaster – this is the fine-grained variety prized by sculptors new and old for ornamental objects and vessels. Many have been found in Egyptian tombs. But the alabaster of the ancients was really marble. The modern centre of trade is Florence, Italy. In cheap souvenirs too!

• Look-alike-travertine (H3) – this fizzes with dilute HCl. Gypsums are H2 and don’t react to dilute acid.

• Weirdoes – needle forms curved and curly, rosettes and crystalline "balls", in caves and desert sands. Many colours too.