Friday, November 24, 2000

Kelly Snyder and Peter Russell

Gypsum, hydrated calcium sulphate, Ca SO4.2H2O is a common mineral formed mainly by the evaporation of sea water. Known from antiquity, its name comes from the Arabic jips, for "plaster," then to the Greek gypsos, for chalk.

Gypsum can be found as thick layers in shale and as attractive crystals.

Rock Gypsum - soft, granular, white to gray, with 30-40% impurities 
Selenite - pure crystalline gypsum found in transparent monoclinic crystals 
Alabaster - white, compact, fine grained variety used for carving 
Satin Spar - fine, translucent fibrous variety with a silky sheen

No gypsum deposits are 100% pure. It is usually found with deposits of a combination of the following: limestone, sand, shale, anhydrite and sometimes rock salt. To be a commercial deposit, gypsum content should be at least 75%. The first mining of gypsum in North America was in Nova Scotia about the year 1770. Since that time, Nova Scotia has become the most productive gypsum-mining region in the world. The early producers were almost all farmers from the Hants County area who owned land where gypsum occurred. They quarried the rock and took it via horse and wagon to the nearest shipping terminal. There, the gypsum would be sold to local traders and shipped to the Eastern Seaboard of the United States and used for fertilizing the land. The largest gypsum quarry in the world is in Milford, Nova Scotia.

When heated to a temperature of from 110C to 120C gypsum looses more than half its water of crystallization and is converted to the white powder, plaster of Paris or calcined gypsum. It is after conversion to plaster of Paris that gypsum has its main uses. Regular wallboard and type-x wallboard with its fire-resistant characteristics make up over three-quarters of the market for calcined gypsum. Wallboard provides the internal walls of our homes, schools and other buildings. Minor uses for calcined gypsum include dental plaster, modelling casts, moulds, surgical casts and drilling muds. Automotive and aerospace engineers use gypsum to create product molds. A broken arm is held in place while it recovers by wrapping it in gauze bandages dipped in plaster of Paris.

Natural gypsum is used as a filler in paper and cotton, in paints, an additive to cement controlling the setting time. Gypsum treated soils absorb and contain more water than untreated soils, thereby producing healthier plants. The crops also take longer to deteriorate after harvesting. The shelf life of plants treated with gypsum is extended by two or more weeks. Mushrooms sold in super-markets are grown in gypsum-fortified compost beds. Gypsum encourages the growth of the aerobic bacteria mushrooms require. Gypsum is applied to the land to ensure that peanut plants receive enough calcium to develop normal kernels. Gypsum is used as a flux in the smelting of nickel ores.

We all eat 28 pounds (12.68 kg) of gypsum in our lifetime as food additives! Ice cream, beer, spaghetti and vitamins and other pills all contain small amounts of gypsum filler. Gypsum is the coagulant in tofu, a soybean curd. Gypsum is added to enriched flour used to make breads and cereals. Baking powder, yeast foods, bread conditioners, canned vegetables and artificially sweetened jellies and preserves all contain gypsum. Gypsum is classified as a "safe" additive for use in foods and pharmaceuticals.

Winnipeg selenite rosettes

The rosettes occur in the glacial clay layer. Gypsum in the clay may have formed from the retreat of Lake Agassiz or may have originated from underground Jurassic gypsum deposits located just west of Winnipeg. The rosettes are not found uniformly distributed throughout the banks of the floodway but occur in small "pods" of high concentration in certain areas of the floodway. Within the pods there appears to be horizontal banding of rosette forms, which may reflect changes in the permeability of the clays as well as the effect of depth and unfavorable growth conditions nearer to the surface.

How Is Gypsum Used?

  • Fertilizer
  • Drilling muds
  • Crayons
  • Molds used for casting:
    • china dishes
    • dental equipment
    • false teeth
    • silver
    • metal parts of machinery
  • Flux used in nickel smelting
  • Food additive
  • Medical gauze ingredient
  • Match tip ingredient
  • Portland cement (as an additive that helps control the setting time) 
  • Plaster of Paris (used for sculpture, molds, stage and movie sets, theme parks) 
  • Plate glass manufacture (polishing)
  • Toothpaste ingredient
  • Wallboard