Raw gypsum minerals – gypsum anhydrates and evaporite minerals
Natural gypsum sources
Gypsum, Anhydrate, and Gypsum Containing Minerals
Natural gypsum and anhydrates are close to being monomineral rocks consisting mostly of one specific mineral either gypsum or anhydrate usually with an insignificant addition of quartz, carbonates, clay, bitumen, pyrite, and others. Generally, deposits of gypsum and anhydrates are situated close to each other.
Gypsum is a sulfate which is a dihydrate form of sulfate. Aside of crystallized water gypsum contains other forms of moisture which occupies both surface and voids of rocks. Crystallized gypsum is a monoclinic crystal system formed with flat, needle, and fibrous crystals. Flat crystals perfectly bond when aligned with the symmetry line yet less prone to bonding and forming perfect forms in other planes.
Pure gypsum is a colorless transparent crystal that may have gray, yellow, pink, muddy, and even black shade depending on the amount of additives. It has a glassy surface and spiky splintery fracture.
Depending on the structure of the crystal gypsum sources can be categorized into:
- Granulated robust gypsum with an earthy fracture which is often referred to as alabaster;
- Plane gypsum forming large transparent crystals which is often referred to as satin spar;
- Fibrous splintery gypsum with silky shine made of symmetrical crystals which is often called selenite.
Anhydrates are also sulfates but they do not have any water. Orthorhombic dehydrated crystals often create small cubic and prismatic formations. May look both earthy and fibrous depending on additives. Anhydrates usually have a white color, but may look grayish, blueish, pinkish, and blackish. It has a glassy surface and uneven fracture. When exposed to moisture, anhydrates adsorb water and turn into gypsum.
Physical properties of Gypsum and Anhydrates
|True density, g/Sm³||2,32||2,89|
|Mohs scale of minerals hardness||1,5…2,0||3,0…3,5|
|Ultimate tensile strength MPa |
|Coefficient of brittleness||8,5||11|
Gypsum containing minerals are various small crystals of gypsum combined with clay or carbonate materials. These rocks may be referred to as earthy gypsum, clay gypsum, and many other names. Structurally, all these rocks are weakly dispersed mixtures that have crumbly texture with a grayish, yellowish, or darkish shade. Over 80% of such materials have particles with a size less than 0.01mm. True density is roughly 2g/sm3. Mohs scale of minerals hardness is less than 1.