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Gypsum (CaSO4·2H2O) is a soft sulfate mineral composed of calcium sulfate dihydrate. It is a common evaporite mineral, often found in extensive beds in sedimentary rock formations. It is characterized by its low hardness, perfect cleavage, and often translucent to transparent appearance. Different varieties exhibit distinct physical characteristics and uses.
How to Identify
- Color
- Colorless, white, gray, yellow, red, brown. Often transparent to translucent.
- Luster
- Vitreous (glassy) to pearly, silky (satin spar).
- Texture
- Can be massive, granular, fibrous, or crystalline. Selenite is typically clear and crystalline, alabaster is fine-grained and massive, and satin spar is fibrous.
- Crystal Form
- Monoclinic crystals, often tabular, prismatic, or acicular. Can form 'swallowtail' twins. Also occurs as massive, granular, fibrous, or rosette-like aggregates ('desert rose').
- Cleavage
- Perfect in one direction (pinacoidal), good in another, and distinct in a third, resulting in rhombic fragments. Selenite often shows thin, flexible sheets.
- Geological Environment
- Evaporite deposits in arid or semi-arid regions, often associated with halite, anhydrite, and dolomite. Also found in hydrothermal veins, volcanic fumaroles, and as a weathering product of sulfide ores.
Key Facts
- Hardness: 2 on the Mohs scale (can be scratched by a fingernail).
- Specific Gravity: 2.31-2.33 g/cm³.
- Crystal System: Monoclinic.
- Color: Colorless, white, gray, yellow, red, brown.
- Luster: Vitreous, pearly, silky.
- Transparency: Transparent to translucent.
- Fracture: Conchoidal to uneven.
- Cleavage: Perfect on {010}, good on {100}, distinct on {011}.
- Composition: Calcium sulfate dihydrate (CaSO4·2H2O).
Quick Check
- Color: Colorless to white (most common), but can be tinted by impurities.
- Luster: Vitreous, pearly, or silky.
- Streak: White
Physical Characteristics
- Crystal Habit: Tabular, prismatic, acicular, massive, granular, fibrous, rosette-like (desert rose), swallowtail twins.
- Cleavage Type: Perfect in one direction, good in another, distinct in a third.
- Fracture Type: Conchoidal to uneven.
- Tenacity: Sectile (can be cut with a knife), flexible in thin sheets (selenite).
- Luster Type: Vitreous (glassy), pearly (on cleavage surfaces), silky (satin spar).
Formation
Gypsum forms primarily through the evaporation of saline water (seawater, lake water) in restricted basins. As water evaporates, dissolved ions (Ca2+ and SO42-) become concentrated and precipitate as gypsum. It can also form through the hydration of anhydrite (CaSO4) in the presence of groundwater, or as a byproduct of volcanic activity (fumarolic deposits) and hydrothermal alteration.
Usage
Gypsum is a widely used industrial mineral. Its primary use is in the manufacture of plaster of Paris, wallboard (drywall), and cement. It is also used as a soil conditioner in agriculture, a filler in paints and paper, and in various dental and medical applications (casts). High-purity gypsum is used in food and pharmaceutical industries.
Age Distribution
Found in sedimentary sequences of all ages, particularly abundant in Permian, Triassic, and Tertiary evaporite deposits.
Where to Find
Naica Mine, Chihuahua, Mexico
Famous for its giant selenite crystals, some of the largest natural crystals ever found, in the 'Cave of Crystals'.
White Sands National Park, New Mexico, USA
Vast dunes composed of gypsum sand, formed from the erosion of gypsum deposits in the Tularosa Basin.
Utah, USA
Significant deposits of gypsum, particularly in the Great Salt Lake region and other evaporite basins.
Nova Scotia, Canada
Large gypsum deposits, particularly in the Windsor Group, are extensively mined.
Spain
Historically significant alabaster quarries and various gypsum occurrences.
Australia
Extensive gypsum deposits, including large 'desert rose' formations.
Finding Tips
Look for Evaporite Environments
Gypsum is common in sedimentary basins where ancient seas or lakes evaporated, often alongside other evaporite minerals like halite and anhydrite.
Check for Softness
Gypsum is very soft (Mohs hardness 2). It can be easily scratched with a fingernail, which helps distinguish it from harder minerals like calcite.
Observe Cleavage
Look for its perfect cleavage, especially in selenite varieties, which can be peeled into thin, flexible sheets.
Identify Crystal Habits
Recognize the different forms: clear, tabular selenite; fibrous, silky satin spar; or massive, fine-grained alabaster.
Consider Hydration
In some areas, gypsum forms from the hydration of anhydrite, so look for it in proximity to anhydrite deposits, especially where groundwater is present.
Similar Rocks
Anhydrite
CaSO4
Also known as: Anhydrous Calcium Sulfate
Calcite
CaCO3
Also known as: Calcium Carbonate
Halite
NaCl
Also known as: Rock Salt
Scientific Classification
- Mineral Class
- Sulfate Mineral
- Group
- Gypsum Group
- Crystal System
- Monoclinic
- Chemical Formula
- CaSO4·2H2O
- Composition
- Calcium (Ca), Sulfur (S), Oxygen (O), Hydrogen (H). Specifically, 32.57% CaO, 46.50% SO3, 20.93% H2O.
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