Got a photo? Identify rocks instantly with the Rockby app
Open the appDescription
Satin Spar is a fibrous variety of the mineral gypsum (CaSO4·2H2O). It is characterized by its distinctive silky or pearly luster, which is a result of light reflecting off the parallel, elongated crystal fibers. These fibers are typically oriented perpendicular to the vein walls in which they grow, giving the mineral a characteristic 'chatoyant' effect when polished or viewed from different angles. It is a soft mineral, easily scratched with a fingernail, and is translucent to opaque.
How to Identify
- Color
- Typically white, colorless, or pale shades of yellow, pink, or brown.
- Luster
- Distinctive silky or pearly luster, often chatoyant.
- Texture
- Fibrous, with parallel, elongated crystals. Can feel smooth to slightly rough along the fibers.
- Crystal Form
- Aggregates of parallel, acicular (needle-like) to columnar crystals, forming veins or layers.
- Cleavage
- Perfect cleavage in one direction (parallel to the fiber length), and good cleavage in two other directions, though less apparent due to the fibrous habit.
- Geological Environment
- Evaporitic sedimentary environments, often found in association with other evaporite minerals like halite and anhydrite. Occurs in shales, limestones, and sandstones as vein fillings or bedded deposits.
Key Facts
- Hardness: 2 on the Mohs scale
- Specific Gravity: 2.31-2.33
- Crystal System: Monoclinic
- Color: White, colorless, pale yellow, pink, or brown
- Luster: Silky to pearly
- Transparency: Translucent to opaque
- Fracture: Uneven to splintery (across fibers)
- Cleavage: Perfect in one direction {010}, good in two others {100} and {111}
- Composition: Hydrous calcium sulfate (CaSO4·2H2O)
Quick Check
- Color: White to colorless, pale yellow, pink, or brown
- Luster: Silky to pearly
- Streak: White
Physical Characteristics
- Crystal Habit: Fibrous aggregates, often forming veins with fibers oriented perpendicular to the vein walls; acicular to columnar crystals.
- Cleavage Type: Perfect basal cleavage {010}, good prismatic cleavage {100} and {111}. The fibrous nature often makes the perfect cleavage less obvious than in selenite.
- Fracture Type: Uneven to splintery, especially when broken across the fibers.
- Tenacity: Sectile (can be cut with a knife), flexible but inelastic (bends but does not return to original shape).
- Luster Type: Silky to pearly, due to reflection from parallel fibers.
Formation
Satin Spar is a variety of gypsum that forms in evaporitic environments, typically from the precipitation of calcium sulfate from saline waters. It often forms in veins or layers within existing gypsum beds, or as a secondary infilling of fractures and cavities. The fibrous habit is a result of crystal growth under specific conditions, often involving unidirectional growth within confined spaces, leading to elongated, parallel crystals.
Usage
While not as widely used as massive gypsum (alabaster or selenite), satin spar is sometimes used for ornamental purposes due to its attractive silky luster. It is carved into small decorative objects, spheres, and polished specimens. Historically, it has been used in some folk remedies, though this has no scientific basis. Its primary significance is geological, indicating past evaporitic conditions.
Age Distribution
Found in sedimentary deposits of various ages, from Precambrian to Cenozoic, wherever evaporitic conditions have occurred.
Where to Find
United States
Significant deposits are found in states such as Oklahoma, Texas, New Mexico, and Utah, often in association with Permian-age evaporite sequences.
Canada
Nova Scotia and New Brunswick have notable gypsum deposits, including fibrous varieties.
Mexico
Extensive gypsum deposits, including satin spar, are found in various regions.
Europe
Spain, Italy, and the UK (e.g., Cumbria, Nottinghamshire) have historical and active gypsum mining operations that yield satin spar.
Australia
Found in evaporitic basins across the continent, particularly in arid and semi-arid regions.
Finding Tips
Look for Evaporite Deposits
Satin spar is typically found in sedimentary rocks that formed in ancient seas or lakes that underwent significant evaporation. Look for areas with shales, limestones, and sandstones that might contain gypsum beds.
Examine Veins and Fractures
Satin spar often occurs as vein fillings within other rocks. Look for white to translucent fibrous material filling cracks or fissures.
Test Hardness
Gypsum is very soft (Mohs hardness 2). If you can scratch it easily with your fingernail, it's a good indicator. Be careful not to confuse it with harder fibrous minerals.
Observe Luster
The characteristic silky or pearly luster is a key identifier. Rotate the specimen under light to observe the chatoyancy.
Check for Cleavage
While fibrous, gypsum still exhibits perfect cleavage. You might be able to gently separate fibers or observe the cleavage planes on broken surfaces.
Similar Rocks
Selenite
Gypsum (variety: Selenite)
Also known as: Clear Gypsum
Alabaster
Gypsum (variety: Alabaster)
Also known as: Massive Gypsum
Ulexite
Ulexite
Also known as: TV Rock
Scientific Classification
- Mineral Class
- Sulfates
- Group
- Gypsum Group
- Crystal System
- Monoclinic
- Chemical Formula
- CaSO4·2H2O
- Composition
- Calcium sulfate dihydrate
Explore Satin Spar
Identify rocks anywhere
Free on iOS, Android, and Web. Photo identification, full database, and your personal collection.