MINERALOGY/MICROMOUNT
November 8, 2003
Meeting was called to order by Chairman, Kathy Dedina.
The December program will be the traditional identification contest. Members are requested to bring 3 unlabelled specimens and 4 pieces of paper with names. All minerals must be listed in Audubon’s Field Guide to North American Rocks and Minerals.
Jim Daly reported further on proposed acquisitions for a study group specialized library.
This month’s program was on gypsum.
Jim Daly distributed copies of a reprint from the latest issue of Rocks and Minerals. In it, John S. White makes the point that the species name is gypsum, and selenite is a varietal name referring to transparent varieties, and is totally unnecessary.
Kathy Dedina discussed the history of gypsum. The name is derived from the Greek gypsos, meaning plaster. Selenite is from the Greek name for the moon. Gypsum appears to have been known and used since ancient times. Although the first plaster used, about 9000 BC, was made from calcined limestone, gypsum plaster was used in ancient India and China to produce smooth surfaces over rough stone or brick walls. The ancient Egyptians 5000 years ago burnt gypsum in open fires, crushed it to a powder, and mixed the powder with water to make jointing compound for the blocks of their monuments, such as the pyramids. They also used plaster to make molds directly from the human body. The Romans used a plaster made from gypsum, lime, sand and marble dust. In addition to construction uses, they made thousands of copies of Greek statues. After the fall of the Roman Empire, plaster wasn’t used again in Europe until the mid-13th century. In the 1700’s Paris was the capital of plaster use. All wooden houses were coated with plaster as fire protection. This was decreed by the king after the London fire of 1666 Clear varieties of gypsum (“selenite”) were used by the ancient Greeks as glazing in their temples. Around 1500 BC in Crete, the world’s oldest known flushing toilet was enclosed by gypsum partitions. Alabaster (which can be either gypsum or marble) has been used for statues and decorations since ancient times.
Sheila Bergmann described the physical properties of gypsum: Hardness 1.5-2, Specific Gravity 2.3-2.4, color white, colorless, gray, yellow red, brown or orange.Streak white. One perfect cleavage, two distinct. Breaks easily into thin flexible flakes. Soluble in hot water. Chemically it is calcium sulfate, with two molecules of water of hydration. Crystals are monoclinic. Untwinned single crystals are often rhombic in form.
Dick Ade provided an explanation of the crystallography of gypsum. Like many other hydrous sulfates, gypsum crystallizes in the monoclinic system. Anhydrous sulfates, such as anhydrite, barite and cerussite are orthorhombic. Basanite, which is a calcium sulfate with ½ molecule of water is also orthorhombic. Gypsum cleaves easily because the water molecules line up opposite each other in the crystal lattice. This was illustrated with models. Many examples of single crystals and twins were shown.
John Good reported on localities for gypsum: Gypsum is a very common mineral, sometimes found in beds over 1000 ft. thick. The White Sands in New Mexico is a desert composed of gypsum sand rather than silica. Exceptional crystals are known from Tyrol, Austria; Bologna, Pavia & Sicily, Italy; Whyalla & Pernatty Lagoon, Australia; Naica, Mexico; Nova Scotia, Canada; Ellsworth, Ohio; South Wash, Utah and Lockport, New York. “Desert Roses” are found in the Sahara Desert in Africa and Alfalfa Co., Oklahoma.
Jim Daly described the uses of gypsum:
About 95% of all gypsum is used in the construction industry. The biggest single use is in wallboard or drywall. The second largest use is in plaster of paris, also known as spackle or drywall “mud”.
For both of these applications, the gypsum is calcined. This means that it is heated until ¾ of its water of hydration has been driven off. It then becomes gypsum hemihydrate, or plaster of paris. After mixing with water and exposure to air, plaster hardens. If the slurry is poured between two layers of paper and allowed to harden, it becomes wallboard.
Plaster, and more recently wallboard, are used for interior construction because it is cheap, fireproof, relatively strong, and sound-deadening.
Gypsum is also used for agricultural soil treatment and athletic field marking. In these applications it competes with lime. Much of the gypsum used in these areas is recycled from scrap wallboard, etc.
Other uses for gypsum are in Portland cement as a retarder for hardening, for grease absorption, sludge drying, water treatment, and as an additive in paint.
Kathy & Judy Dedina provided refreshments.
Submitted by Jim Daly, Recording Secretary
Last Updated 11/25/2003
Number Of Visitors Since 2/5/2005