🌊 Formation in brief

Selenite is a transparent, well-formed gypsum variety with the composition CaSO₄·2H₂O (calcium sulfate dihydrate). It typically forms where waters saturated with calcium and sulfates slowly evaporate or gently circulate through cavities, allowing large, clear crystals to grow over long periods under stable conditions. Imagine a calm geological greenhouse: warm, mineral-rich fluids, minimal disturbances, and plenty of time.

1. Source: Calcium may come from limestone dissolution; sulfate—from sulfide oxidation or dissolution of older sulfate salts.
2. Concentration: Evaporation or slow flow increases ion concentration until gypsum becomes supersaturated.
3. Crystallization: Small seed crystals form on walls, deposits, or previous minerals.
4. Growth: With stable chemistry and temperature, crystals grow into blades/plates—this is classic selenite.
5. Textural evolution: Changes in water chemistry, impurities, or flow can direct growth into fibrous satin spar or fine-grained alabaster.

🗺️ Geological environments where selenite thrives

1) Evaporite basins and sabkhas

Classic gypsum homes—evaporite sequences, layers of salts (gypsum, anhydrite, halite) deposited as sea or lake water evaporates. In sabkhas (coastal salty flats) and salars (closed basin salt flats), capillary rise and evaporation repeatedly drive solutions through sediments. This can form transparent selenite blades, fibrous deposits, or impressive rosette aggregates with trapped sand (“desert roses”).

2) Caves and karst cavities

In cave environments, slowly moving sulfate-rich waters and stable temperatures favor giant selenite crystals. Large crystals require minimal disturbances, stable chemistry, and continuous ion supply—conditions caves can provide for thousands to hundreds of thousands of years.

3) Salt domes and cap rock

When deep salt masses plastically rise, interaction with groundwater can convert anhydrite (CaSO₄) into gypsum. In cavities within the cap rock, excellent selenite crystals often grow, frequently alongside halite and calcite.

4) Hydrothermal and volcanic area margins (secondary gypsum)

Sulfate-bearing fluids from hot springs or fumarolic environments, mixing or cooling, can precipitate gypsum. In such settings, selenite crusts and veins form, although crystals are usually smaller and less perfect than cave giants.

5) Soil and desert cup

In dry soil areas, rising groundwater evaporates forming gypsum veins and nodules. Over time, repeated wet–dry cycles restructure these deposits into rosettes or fibrous masses. These are the “garden” selenite forms that gardeners dig up and treasure.

⚗️ Chemistry, phase changes, and crystal growth

In the gypsum structure, two water molecules attach to each calcium sulfate unit. Gentle heating or very dry conditions can partially dehydrate gypsum to bassanite (CaSO₄·½H₂O), and further dehydration—to anhydrite (CaSO₄). Upon reintroduction of water, frequent rehydration occurs. This hydration–dehydration cycle explains why gypsum is both industrially useful (construction gypsum, “plaster of Paris”) and environmentally sensitive (don’t “bake” your crystals!).

Why is one gypsum transparent (selenite), and the other silky (satin spar)?

• Supersaturation and growth rate: Slow, steady growth at low supersaturation usually forms large, transparent blades.
• Impurities and inclusions: Clay, iron oxides, or air channels promote fibrous/parallel-oriented growth and silky luster.
• Space and disturbances: Wide, calm cavities allow growth of large transparent crystals; tight pores promote fibrous bundles.

Structure, cleavage, and twins

Gypsum is a monoclinic system mineral with perfect cleavage on {010} planes, giving selenite its plate-like cleavage and pearly luster. Classic "swallowtail" twins arise from twinning on common planes, forming dramatic V-shaped crystals. Growth striations along the c axis (length) are common on blades.

🧩 Gypsum varieties and habits (selenite "family")

"Selenite" is often used broadly in trade, but geologically it means transparent, well-formed crystals. Other gypsum varieties have distinctive textures and appearances:

Selenite (in the strict sense)

• Appearance: Transparent, colorless plates and blades; sometimes honey or smoky colored due to inclusions.
• Habit: Tabular, bladed, prismatic; frequent "swallowtail" twins; distinct cleavage.
• Environment: Cavities in evaporites, caves, cap rock voids; require long, stable growth periods.

Satin spar

• Appearance: Fibrous bundles with a silky luster and frequent cat's eye (chatoyancy) effect.
• Habit: Parallel-arranged fibers; often cut into "rods", towers, and palm stones.
• Environment: Veins and layers in sediments where directional growth and impurities promote fiber formation.

Alabaster

• Appearance: Fine-grained, massive gypsum; softly glowing when backlit; white or soft shades.
• Habit: Microcrystalline aggregates; excellent for carving and sculpture.
• Environment: Low-energy settings with abundant nucleation, forming small intergrown crystals.

Desert rose (gypsum rosettes)

• Appearance: Rose-shaped plate clusters; "petals" often covered with sand; yellowish-brown shades.
• Habitus: Radially arranged plates forming flower-shaped aggregates; sometimes called "sand roses."
• Environment: Dry sabkhas and dunes where capillary brines evaporate, trapping sand grains during growth.

Cave "flowers" and needles

• Appearance: Curved "rings", brushes or needle-like forms on cave walls and vaults.
• Habitus: Fibrous/curvilinear growth influenced by air flows, moisture gradients, and capillary films.
• Environment: Caves with stable humidity and slow supersaturation changes.

📊 Varieties–environments matrix (who grows where?)

🧭 Field notes: how to "read" a selenite outcrop

1. Bedding: Alternating gypsum/halite layers loudly proclaim an "evaporite basin." Transparent selenite seams inside indicate stable brine periods.
2. Textures: Rosettes and satin spar veins along fractures suggest capillary flow and repeated wet–dry cycles.
3. Geochemistry: Nearby carbonates? Calcium supply likely from limestone. Oxidized sulfides above? Sulfate source identified.
4. Diagenesis: Gypsum pseudomorphs after anhydrite (or vice versa) record hydration fluctuations during burial/uplift.
5. Paleoenvironment: Desert roses and dune cross-bedding? Dry shore or continental sabkha conditions.

🕵️ Similar minerals and common confusions

• Glass: Heavier, harder, lacks perfect cleavage planes; no silky cat's eye effect.
• Calcite: Harder (3), strongly fizzes in acid, rhombohedral cleavage, more pronounced double refraction.
• Halite: Cubic cleavage and salty taste (please don't lick your minerals).
• Ulexite ("TV stone"): True fiber optic effect projecting images onto the surface; satin spar can't do that trick.

🧼 Care, storage, and display of geological specimens

• Keep dry: Slightly soluble; high humidity dulls the surface.
• Avoid heat: Can dehydrate and crack; do not heat in sun or lamps.
• Protect flat surfaces: Keep on soft foam or felt; support long blades along their entire length.
• Dust: Use a gentle air blower or a very soft, dry brush; no water spraying.
• Lighting: Side lighting highlights pearly cleavage; backlighting makes alabaster glow.

❓ FAQ

✨ The essentials

The story of selenite — a dance of water, salt, and time. In quiet basins and hidden cave cavities, gypsum arranges into sparkling blades, silky fibers, glowing masses, and sand-petaled roses. Each variety records its birth conditions: chemistry, flow, temperature, and space. Learn to "read" those textures — and you'll read Earth's diary, one glowing page at a time.

Last blink: if geology had mood lighting, it would be called "selenite." Gentle, calm, soothing — and scientifically fascinating. 😄