🌊 Formation in a Snapshot

Selenite is the clear, well-crystallized variety of gypsum, with the composition CaSO₄·2H₂O (calcium sulfate dihydrate). It typically forms where waters loaded with calcium and sulfate slowly evaporate or gently circulate through cavities, allowing large, transparent crystals to grow over long, stable periods. Think of a quiet geologic greenhouse: warm, mineral-rich fluids, minimal disturbance, and lots of time.

1. Source: Calcium can come from limestone dissolution; sulfate often comes from oxidation of sulfides or dissolution of older sulfate salts.
2. Concentration: Evaporation or slow flow raises ion levels until gypsum becomes saturated.
3. Nucleation: Tiny seed crystals form on walls, sediments, or pre-existing minerals.
4. Growth: With steady chemistry and temperature, crystals enlarge into blades/plates — this is classic selenite.
5. Textural evolution: Changes in water chemistry, impurities, or flow can shift habits toward fibrous satin spar or fine-grained alabaster.

🗺️ Geologic Settings Where Selenite Thrives

1) Evaporite Basins & Sabkhas

The classic home of gypsum is the evaporite sequence — layers of salts (gypsum, anhydrite, halite) deposited as seas or lakes evaporate. In sabkha (coastal flats) and salars (closed-basin salt flats), capillary rise and evaporation repeatedly cycle solutions through the sediment. This can produce clear selenite blades, fibrous masses, or spectacular rosette aggregates with included sand (“desert roses”).

2) Caves & Karst Cavities

In caves, slow-moving, sulfate-rich waters at stable temperatures favor growth of giant selenite crystals. Large crystals require minimal disturbance, constant chemistry, and a steady supply of ions — conditions that caves can provide for thousands to hundreds of thousands of years.

3) Salt Domes & Cap Rock

Where deep salt bodies rise buoyantly, interaction with groundwater can transform anhydrite (CaSO₄) to gypsum. Cavities within the cap rock may host fine selenite crystals, often with associated halite and calcite.

4) Hydrothermal & Volcanic Margins (Secondary Gypsum)

Sulfate-bearing fluids from hot springs or fumarolic environments can precipitate gypsum upon mixing/cooling. These settings may produce selenite crusts and veins, though crystals are typically smaller and less pristine than cave-grown giants.

5) Soil & Desert Caliche

In arid soils, rising groundwater evaporates to form veinlets and nodules of gypsum. Over time, repeated wet–dry cycles reorganize these into rosettes or fibrous masses. These are the “garden variety” selenites gardeners dig up and cherish.

⚗️ Chemistry, Phase Changes & Crystal Growth

Gypsum’s structure binds two water molecules for every calcium sulfate unit. Gentle heating or very dry conditions can partially dehydrate gypsum to bassanite (CaSO₄·½H₂O) and, with further dehydration, to anhydrite (CaSO₄). Rehydration is common when water becomes available again. This hydration–dehydration cycle explains why gypsum is both industrially useful (plaster of Paris) and environmentally sensitive (don’t bake your crystals!).

Why Some Gypsum Is Clear (Selenite) & Some Is Silky (Satin Spar)

• Supersaturation & Growth Rate: Slow, steady growth at low supersaturation tends to produce large, clear blades.
• Impurities & Inclusions: Clay, iron oxides, or air-filled channels promote fibrous/parallel growth and a silky sheen.
• Space & Disturbance: Wide, quiet cavities allow big transparent crystals; confined pores favor fibrous bundles.

Structure, Cleavage & Twinning

Gypsum is monoclinic with perfect cleavage on {010}, giving selenite its sheet-like splits and pearly planes. Classic “swallowtail” twins arise from twinning on common planes, producing dramatic V-shaped crystals. Growth striations along the c-axis (length) are frequent in blades.

🧩 Varieties & Habits of Gypsum (Selenite Family)

“Selenite” is often used broadly in the trade, but geologically it refers to clear, well-formed crystals. Other habits of gypsum have distinct textures and appearances:

Selenite (strict sense)

• Appearance: Transparent to colorless plates and blades; sometimes honey or smoky from inclusions.
• Habit: Tabular, bladed, prismatic; common swallowtail twinning; prominent cleavage.
• Setting: Cavities in evaporites, caves, cap-rock voids; requires long, stable growth windows.

Satin Spar

• Appearance: Fibrous bundles with silky luster and often chatoyancy (moving band of light).
• Habit: Parallel fibers; commonly cut into “wands,” towers, and palm stones.
• Setting: Veins and layers in sediments where directional growth and impurities encourage fiber formation.

Alabaster

• Appearance: Fine-grained, massive gypsum; soft glow when backlit; white to softly colored.
• Habit: Microcrystalline aggregates; excellent for carving and sculpture.
• Setting: Low-energy environments with abundant nucleation producing tiny intergrown crystals.

Desert Rose (Gypsum Rosettes)

• Appearance: Rosette clusters of bladed crystals; petals often dusted with sand; tan to reddish hues.
• Habit: Radiating plates forming flower-like aggregates; sometimes called “sand roses.”
• Setting: Arid sabkhas and dunes where capillary brines evaporate and include sand grains during growth.

Cave Flowers & Needles

• Appearance: Curving “flowers,” sprays, or acicular (needle-like) forms on cave walls and ceilings.
• Habit: Fibrous/curvilinear growth driven by airflow, humidity gradients, and capillary films.
• Setting: Caves with stable humidity and slow supersaturation changes.

📊 Variety–Environment Matrix (What Grows Where?)

🧭 Field Notes: Reading a Selenite Outcrop

1. Layering: Alternating gypsum/halite beds shout “evaporite basin.” Clear selenite seams within point to periods of stable brines.
2. Textures: Rosettes and satin spar along cracks suggest capillary flow and repeated wet–dry cycles.
3. Geochemistry: Carbonates nearby? Calcium supply likely from limestone. Oxidized sulfides upsection? Sulfate source identified.
4. Diagenesis: Pseudomorphs of gypsum after anhydrite (or vice versa) record hydration swings with burial/uplift.
5. Paleoenvironment: Desert roses and dune cross-bedding? Arid shoreline or continental sabkha conditions.

🕵️ Lookalikes & Common Mix-ups

• Glass: Heavier, harder, no perfect cleavage sheets; lacks silky chatoyancy.
• Calcite: Harder (3), strong effervescence in acid, rhombohedral cleavage, stronger double refraction.
• Halite: Cubic cleavage and saline taste (please don’t lick your minerals).
• Ulexite (“TV rock”): True fiber-optic effect that projects images to the surface; satin spar doesn’t do that trick.

🧼 Care, Storage & Display for Geological Specimens

• Keep dry: Slightly soluble; high humidity dulls surfaces.
• Avoid heat: Can dehydrate and craze; do not sun-bake or lamp-bake.
• Protect faces: Store on soft foam or felt; support long blades along their length.
• Dusting: Use a gentle air bulb or very soft, dry brush; no water sprays.
• Lighting: Side-lighting reveals pearly cleavage; backlighting makes alabaster glow.

❓ FAQ

✨ The Takeaway

Selenite’s story is a dance between water, salt, and time. In quiet basins and hidden caves, gypsum assembles into glittering blades, silky fibers, glowing masses, and sand-petaled roses. Each variety records the conditions of its birth: chemistry, flow, temperature, and space. Learn to read those textures and you’re reading Earth’s diary — one luminous page at a time.

Final wink: If geology had a mood lighting setting, it would be “selenite.” Soft, calm, flattering — and scientifically fascinating. 😄