Gypsum
Gypsum is a soft, widely distributed sulfate mineral best known as the primary source of plaster, drywall, and cement, and is easily recognized by its white to transparent appearance and gentle touch.
Origin Story
Gypsum’s story stretches back to ancient civilizations, with documented use in Egypt and Mesopotamia for construction and sculpture. Its formation occurs naturally through the evaporation of saline waters in arid environments, leaving behind extensive beds in sedimentary rocks. The mineral was formally described in the 18th century and is now recognized as a distinct species by the International Mineralogical Association (IMA).
Classification
Gypsum belongs to the sulfate mineral group, defined by its chemical composition and crystalline structure. It is classified under Dana code 16a.3.1.1 and Strunz code 7.CD.40, reflecting its place among minerals formed by the combination of calcium, sulfur, and water. Its monoclinic crystal system and clear mineralogical identity make it a cornerstone in both academic and industrial mineral classification schemes.
Appearance or Form
Gypsum typically appears as colorless to white crystals, though impurities can tint it gray, yellow, red, or brown. Its forms range from transparent tabular or prismatic crystals (selenite), to silky fibrous masses (satin spar), and fine-grained, massive aggregates (alabaster). The mineral’s luster varies from vitreous to pearly or silky, and it is transparent to translucent. Thin sheets of gypsum are flexible, and the mineral is easily scratched by a fingernail due to its softness.
Behavior or Usage
Gypsum’s softness and solubility make it ideal for shaping and carving, which has driven its use in sculpture and decorative arts for millennia. Industrially, it is indispensable in the manufacture of plaster, drywall, cement, and as a soil conditioner. Its ability to set and harden when mixed with water underpins its role in construction and casting molds, while its non-toxic nature allows for use as a food additive (E516).
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Taxon-Specific Insights
🌍Cultural Significance
Gypsum has played a vital role in human culture since antiquity, especially in the architectural and artistic achievements of ancient Egypt and Mesopotamia. Alabaster, a fine-grained variety, has been prized for ornamental carving and sculpture, gracing temples, tombs, and palaces. Its enduring presence in construction and art connects modern society to ancient traditions.
📌Notable Facts
- Gypsum is so soft it can be scratched with a fingernail, ranking just 2 on the Mohs hardness scale.
- Its clear crystalline form, selenite, can grow into spectacular, transparent crystals up to several meters long.
- Gypsum is the main ingredient in drywall, making it a hidden but essential part of most modern buildings.
- Alabaster, a variety of gypsum, has been used for centuries for intricate carvings and sculptures.
🧪Chemical Composition
Gypsum’s chemical formula is CaSO₄·2H₂O, meaning it is composed of calcium sulfate dihydrate—calcium, sulfur, oxygen, and two molecules of water per formula unit.
Visual Variations
Composition & Context
Taxon-Specific Insights
🔷Crystal Structure
Gypsum crystallizes in the monoclinic crystal system, typically forming tabular, prismatic, or fibrous crystals. Its structure features perfect cleavage in one direction, allowing thin sheets to be flexible and easily split.
Physical Properties
- Hardness: Mohs 2 (very soft)
- Color: Colorless, white, gray, yellow, red, brown
- Luster: Vitreous, silky, pearly
- Transparency: Transparent to translucent
- Cleavage: Perfect in one direction
- Specific Gravity: ~2.32
- Streak: White
- Tenacity: Flexible in thin sheets
🌍Geological Occurrence
Gypsum is most commonly found in extensive beds within sedimentary rocks, formed by the evaporation of saline waters in arid climates. Major deposits are located in the United States, Canada, Mexico, France, and Spain. It often occurs alongside minerals such as halite, anhydrite, and calcite, and is a key indicator of evaporitic geological environments.
Varieties And Synonyms
- Selenite: Clear, crystalline form
- Satin spar: Fibrous, silky variety
- Alabaster: Fine-grained, massive form used for carving
- Other names: Gypse (French), Gips (German), Yeso (Spanish)
Industrial And Technological Uses
- Manufacture of plaster and drywall (sheetrock)
- Essential component in cement production
- Used as a soil conditioner in agriculture
- Material for sculpture and casting molds
- Food additive (E516) for dietary and processing purposes
🔍Identification Tips
- Gypsum is easily scratched with a fingernail due to its low hardness.
- Look for transparent to white tabular or prismatic crystals (selenite), silky fibrous masses (satin spar), or fine-grained white aggregates (alabaster).
- Perfect cleavage in one direction allows thin sheets to flex slightly.
- Gypsum leaves a white streak when rubbed on a porcelain plate.
Care And Handling
- Store gypsum specimens in a dry environment to prevent gradual dissolution or surface damage.
- Handle gently, as the mineral is very soft and can be scratched or broken easily.
- Clean with a soft brush or cloth; avoid water or harsh chemicals, as gypsum is slightly soluble and sensitive to moisture.
Famous Specimens
- The Naica Mine in Mexico is renowned for its gigantic selenite crystals, some reaching lengths of over 10 meters.
- Alabaster carvings from ancient Egypt and Mesopotamia remain iconic examples of gypsum’s artistic legacy.
- Large, transparent selenite crystals from Oklahoma and Spain are prized by collectors and museums worldwide.