Sulfates: Anhydrite – Gypsum Group
|Chemical Composition||(CaSO4-2H2O) – Hydrated Calcium Sulfate|
|Color||Usually white, colorless or gray in massive form, crystals are clear, transparent to translucent. If impurities are present, gypsum may also appear to be red, brown or yellow.|
|Cleavage||Good cleavage in one direction, only fair in two others to produce thin rhombic fragments.|
|Hardness||2 (very soft)|
|Specific Gravity||2.3 (feels rather light)|
|Luster||Crystals are vitreous (glass-like) to pearly, massive form is dull|
Gypsum occurs in two main varieties as white earthy masses or as clear crystals. In crystal form, its softness and single perfect cleavage with two less well-developed cleavage directions usually serve to distinguish gypsum from other similar-looking minerals.
When gypsum loses its water component, it forms the mineral anhydrite (CaSO4). When anhydrite absorbs water it becomes gypsum. This ease of exchange is the basis of gypsum’s use in plaster. Consequently, the two minerals are very similar to one another and are only easily distinguished by heating a sample in a closed tube. As it is heated, gypsum samples will produce fine water droplets, while anhydrite does not. So technically there is an important difference between the two minerals, but for non-specialists interested in collecting or appreciating earth materials, the two can almost be considered as different forms of the same material.
Halite and gypsum both form clear crystals that are very soft and are easily scratched. Licking a sample will quickly distinguish the two, as halite has the same composition as table salt and a very distinctive ‘salty’ taste. For those who dislike the idea of licking mineral samples, the two minerals can also be distinguished by their cleavage patterns. Halite has three perfect cleavages that form at right angles to form cubes. Gypsum crystals only have one perfect cleavage direction. The other two cleavage directions are not as pronounced, so broken gypsum crystals tend to form rhomb-shaped fragments, rather than cubes.
Calcite crystals can be distinguished from clear gypsum crystals by both their cleavage and the ease with which they react with dilute acid to effervesce or bubble. Calcite has three perfect cleavages that form rhombic fragments, while gypsum only has one perfect cleavage and two less distinct cleavage directions so that, when broken, it tends to form less well-developed rhombs than calcite does.
Dolomite crystals can be distinguished from clear gypsum crystals by their cleavage. Dolomite has three perfect cleavages that form well-developed rhombic fragments when broken. In contrast, gypsum has only one perfect cleavage direction and two less distinct cleavage directions. Broken gypsum crystals tend to form less well-developed rhombs than dolomite does. If part of a sample is crushed into a powder, the powdered dolomite will also react with acid to produce small bubbles, while gypsum does not.
Did you know...
From ancient art to walls and modern dentistry, gypsum in the form of plaster has been widely used throughout human history. Its versatility is due to the ease with which gypsum can lose or gain water, an ability tied to its origin. Gypsum, a soft non-metallic mineral, almost exclusively forms in sedimentary settings, as seawater is evaporated or as dissolved ions precipitate from groundwater. Its mineral structure still contains some water though. Heating gypsum will drive off this remaining water to produce a dry powder that can later be combined with water to again produce gypsum. Although gypsum can form clear transparent crystals, it usually occurs as massive white chalky deposits. It is the most common sulfate mineral and one of the most widely used non-metallic minerals in the world.
Description and Identifying Characteristics
Gypsum is a very soft mineral that is easily identified by its hardness, cleavage, and solubility in water. Typically clear to white, gypsum may be colored reddish to brown or yellow if impurities are present. Most gypsum occurs in its massive form, as layers of rock that often intercalate layers of shale, limestone, or dolostone. Alabaster is a common name used for particularly pure deposits of massive gypsum. Gypsum also occurs in crystal form, with the two most common varieties being large clear crystals that are often termed selenite, and fibrous crystals, known as satin spar. ‘Selenite’ is the Greek word for moon, and refers to the crystals well-developed pearl-like luster that can reflect a soft moon-like glow. In arid sandy areas, growing gypsum crystals may bind sand grains together into complex clusters known as ‘desert roses’.
In Our Earth: The Geologic Importance of Gypsum
Gypsum almost exclusively occurs in settings where seawater is evaporating or where groundwater containing dissolved ions from evaporite rocks reforms these ions as gypsum. As a consequence, gypsum typically occurs as layers associated with limestone, dolostone, shale, and rock salt. As seawater evaporates, gypsum is the first ‘salt’ to be precipitated, followed by anhydrite, halite, and finally sylvite. Usually found with other evaporite and carbonate minerals, such as anhydrite, calcite, dolomite, borax, and halite, in altered carbonate rocks, gypsum may also be found with sulfur and pyrite.
In ground water systems, gypsum can also form at hot springs or as cave deposits, especially in areas where groundwater has passed through evaporite sedimentary rocks. To a lesser extent, gypsum may be precipitated around volcanic vents called ‘fumeroles’, where it is often associated with sulfur.
In Our Society: The Economic Importance of Gypsum
Gypsum has been widely used since prehistoric times. The name comes from the Greek word ‘Gupsos’, which means ‘to cook’ or ‘burned’, a reference to how gypsum is commonly prepared for use as plaster. For centuries, gypsum has been used for plaster and pigments in both constructing and decorating buildings. Although plaster may have been its most important use, Assyrians also sculpted the alabaster variety of massive gypsum into ornaments and figurines, while Greeks used transparent selenite gypsum crystals for their temple windows, long before glass was invented.
When gypsum is heated, water is driven from its crystal structure to produce anhydrite. Anhydrite is commonly known as Plaster of Paris, because thick deposits of gypsum occur in the Paris Basin. When Plaster of Paris is mixed with water, it forms a paste that can be easily shaped and that hardens into solid gypsum. This is the source of our plaster walls and many cast objects. Gypsum has a very low thermal conductivity, so it is an excellent, low cost, insulating material for buildings. The water within gypsum’s crystalline structure also actually helps to minimize fire damage. If a fire breaks out, heat drives the water out of the gypsum walls to cool and protect the wood or steel supporting the walls. Consequently, one of the main modern uses of gypsum is the manufacture of plasterboard (also called drywall or sheetrock) commonly used in construction. The average American home may contain as much as seven tons of gypsum, this mass of plaster translates into one and a half tons, or roughly 350 gallons, of water built into your home’s structure. While this is not enough to quench a major fire, it is enough to reduce the damage of a small blaze.
The ease with which gypsum loses and regains its water of crystallization also makes it ideally suited for sculpture and casts. Gypsum has many applications in art and pottery, but is also used in medicine as casts for broken bones or as dental molds for making artificial teeth. Its many other uses include the ‘paste’ component of toothpaste, modern chalk used in classrooms, and as filler for paper and paints. It is also an important component added to cement, to slow the rate at which concrete settles or hardens. Large amounts of gypsum are used as fertilizer or as a conditioner for soil, displacing sodium in the soil and allowing the soil to hold more moisture.
Gypsum in the Upper Midwest:
Few concentrated gypsum deposits occur in the Upper Midwest, but gypsum crystals, precipitated from groundwater, are disseminated through some local carbonate rocks and can be found in old mine shafts in Wisconsin. Selenite gypsum crystals also occur in Cretaceous-aged clays in central and western Minnesota.