Gypsum is a mineral that belongs to the sulfates and is the most abundant, its chemical formula is CaSO4 2H2O (hydrated calcium sulfate). It is generally found in layers of sedimentary deposits as a result of the evaporation of seawater and its best known use is in white plaster.
Physical properties and characteristics
| Cast | Physical properties and characteristics |
|---|---|
| class, group | Sulfate (Hydrated calcium sulfate) |
| Brightness | Vitreous, pearly and silky |
| Color | Colorless, white, gray, yellow, red, brown |
| Stripe | White |
| Chemical formula | SO4Ca2H2O |
| Chemical composition | CaO=32.6%, SO3=46.5%, H2O=20.9% |
| Hardness | two |
| Specific weight or density | 2.32 g/cm3 |
| Cleavage or exfoliation | In four directions, perfect. |
| Fracture | Unequal |
| crystalline system | monoclinic |
| Way of introducing yourself (habit) | Prismatic, tabular. Although also massive, granular, fibrous. |
| Origin or geological environment | Sedimentary deposits due to evaporation of seawater or brine, hydration of anhydrite, oxidation of sulfides, and in volcanic regions. |
| associated rocks | Sedimentary (Evaporites, limestone), Metamorphic (slates) |
| Associated minerals | Halite, dolomite, barite, anhydrite, sulfur, quartz and pyrite. |
| Applications | Manufacture of the homonymous material, cement, agricultural soil treatment |
The cast It is the most common and abundant mineral of the sulfates. It is generally white in color, but it is also found in grey, yellow, red, brown and can even be colorless.
It generally has a vitreous shine, however, it can be pearly or silky, the color of the stripe is white.
It presents a prismatic or tabular habit, however, it can be massive, granular or fibrous. An uneven fracture and perfect exfoliation.
It has a low hardness of 2 on the Mohs scale as well as its specific weight, which is relatively low at 2.32 g/cm3.
In varieties of cast We mentioned selenite, it is a transparent crystalline variety that is used in a test instrument with the polarizing microscope. Alabaster, another variety, with a massive, fine-grained shape, is used as an ornamental stone, and Satin-spar, which has a fibrous habit.
Origin, formation and geological environment.
The cast Found in extensive masses of great thickness, known as an evaporitic mineral, it generally forms in layers of sedimentary deposits associated with anhydrite, halite, calcite, sulfur, and dolomite.
The formation of this mineral is mainly due to direct deposition due to brine evaporation.
Seawater contains approximately 3.5% salts, of which 80% is sodium chloride and 4% calcium sulfate. Experimentally it was shown that the deposition sequence of these sea salts due to evaporation is: calcium carbonates, calcium sulfates, sodium chlorides, sulfates, sodium-magnesium bromide chlorides and potassium chloride.
In the case of calcium sulfate, it can precipitate as cast, anhydrite or these two minerals together.
It is found interbedded in slates, limestones and frequently in layers under salt deposits, because it is one of the first to crystallize by evaporation. This mineral also crystallizes in veins, forming satin spar.
Its origin is also due to anhydrite hydration, sulfide oxidation and rocks with calcium content that are subjected to the action of sulfate solutions.
In addition, it is found in volcanic regions, even more so if the sulfurous vapors have affected the limestone.
desert rose
In some cases the mineral forms in sandy places, its crystals trapping the sand inside during formation, giving off a brown or gray color, and a dull luster. This can form or result in an hourglass in a crystal.
Likewise, in these same conditions, the well-known “Rosa del Desierto” is formed, which presents a gypsum rosette shape with sand inclusions.
Gypsum and Anhydrite
These two minerals have the same chemical composition, the difference is that anhydrites do not contain water in their structure and gypsum does have this characteristic.
Some anhydrite minerals absorb water and become cast, in some cases this evidence is distinguished and growths of wrinkled layers are observed that show expansion due to the addition of water.
Thin sheet plaster
The optical identification properties of plaster are:
- Color: It is colorless in natural light
- Shape: Generally in thick laminated or acicular prismatic crystals.
- Cleavage: Perfect at (001), Good at (010) and (100)
- Birefringence: shows first-order interference colors, gray to white (similar to potassium feldspar)
- refraction: low
How to identify the gypsum?
It is characterized by its white color, just as the stripe has the same color, vitreous luster, and it dissolves in hydrochloric acid.
It is found with a prismatic or tabular habit and has a low hardness of 2 on the Mohs scale, so it can be scratched with a fingernail, as well as having a low density of 2.32 g/cm3.
This mineral is known for its flexibility, thinner minerals can bend more easily (Good crystals have weaker flexibility and can break if flexed too much).
A great variety of crystals are found intact, without broken or distorted parts, they are found in a bed of clay as floating crystals, they are found to be formed without the need to be united by a matrix.
Gypsum Uses
- Production of drywall.
- Paris plasters.
- Treatment for agricultural soils, fertilizer.
- Portland cement hardening retarder.
- “Alabaster” and “Satin Mast” are varieties of Gypsum, it is used in ornamental things, such as ceramics and artistic sculptures.
- Preparation for white plasters used in the manufacture of cement.
- It is used as a flux to make pottery.
Gypsum as a precious and ornamental stone?
Despite the fact that gypsum can be found in caves forming large crystals that even exceed 1 meter in length, it is not appreciated or suitable for the jewelry industry, either for use as a gem or precious stone.
It is not suitable because it has a very low hardness, it is quite reactive and it is not resistant to the environment.
As an ornamental stone it is prized by mineral collectors especially when it appears in large crystals.
Enhydros
This term refers to a rarity of the cast which is highly prized by collectors. The reason for the great interest is due to turning the glass, a bubble of water is observed moving inside down to the lowest point of the channel.
This occurs when water is trapped in a hollow channel in a crystal during mineral formation.