Got a photo? Identify rocks instantly with the Rockby app
Open the appDescription
Satin Spar is a fibrous variety of the mineral gypsum, characterized by its distinctive silky, pearly, or satiny luster. It typically occurs as parallel, elongated, needle-like crystals that are often aggregated into compact masses or veins. Its translucent to opaque appearance and characteristic sheen make it easily distinguishable from other gypsum varieties like selenite (clear, tabular crystals) and alabaster (massive, fine-grained). It is very soft, easily scratched with a fingernail.
How to Identify
- Color
- Typically white, colorless, or pale shades of yellow, pink, or brown due to impurities.
- Luster
- Distinctive silky, pearly, or satiny luster, especially on fractured surfaces parallel to the fibers.
- Texture
- Fibrous, with a smooth, almost slippery feel when rubbed parallel to the fibers. Can be splintery when broken across the fibers.
- Crystal Form
- Aggregates of parallel, elongated, acicular (needle-like) crystals, often forming veins or layers.
- Cleavage
- Perfect cleavage in one direction (parallel to the length of the fibers), producing thin, flexible sheets. Good cleavage in two other directions, but less prominent than selenite.
- Geological Environment
- Evaporite deposits, often found in arid regions where ancient seas or lakes have evaporated. Also occurs as secondary mineralization in sedimentary rocks, shales, limestones, and clays, filling fractures and voids.
Key Facts
- Hardness: 2 on the Mohs scale
- Specific Gravity: 2.31 - 2.32
- Crystal System: Monoclinic
- Color: White, colorless, pale yellow, pink, brown
- Luster: Silky, pearly, satiny
- Transparency: Translucent to opaque
- Fracture: Uneven, splintery across fibers
- Cleavage: Perfect in one direction (010), good in two others (100, 111)
- Composition: Hydrous calcium sulfate (CaSO4·2H2O)
Quick Check
- Color: White, colorless, pale yellow, pink, or brown
- Luster: Silky, pearly, satiny
- Streak: White
Physical Characteristics
- Crystal Habit: Fibrous aggregates, acicular (needle-like) crystals, often parallel or radiating.
- Cleavage Type: Perfect basal cleavage (010), allowing it to be split into thin, flexible sheets. Two other good cleavages.
- Fracture Type: Uneven to splintery, especially when broken perpendicular to the fibers.
- Tenacity: Sectile (can be cut with a knife), flexible in thin sheets, but not elastic.
- Luster Type: Silky, pearly, or satiny, due to the reflection of light off the parallel fibrous structure.
Formation
Satin Spar Gypsum forms primarily through the evaporation of saline waters (e.g., ancient seas, salt lakes) where calcium sulfate becomes supersaturated and precipitates. It often forms in veins or layers within other gypsum deposits, or as secondary infillings in fractures. The fibrous habit is a result of crystal growth under specific conditions, often unidirectional, within confined spaces.
Usage
While not as widely used industrially as massive gypsum, Satin Spar is prized by mineral collectors for its aesthetic appeal. It is sometimes carved into ornamental objects, spheres, or used in metaphysical practices. Due to its softness, it is not suitable for construction or industrial applications requiring durability.
Age Distribution
Found in sedimentary evaporite deposits of various ages, from Precambrian to Cenozoic.
Where to Find
Naica Mine, Chihuahua, Mexico
Famous for its giant selenite crystals, but also produces excellent examples of satin spar.
Cave of the Crystals (Cueva de los Cristales), Naica, Mexico
While known for selenite, the broader Naica district yields various gypsum forms.
Utah, USA
Numerous localities, particularly in evaporite basins, produce satin spar.
New Mexico, USA
White Sands National Park is famous for its gypsum dunes, and satin spar can be found in associated deposits.
England
Historically significant localities, particularly in the Midlands, have yielded satin spar.
Canada
Various sedimentary basins across Canada contain gypsum deposits, including satin spar.
Finding Tips
Look in Evaporite Deposits
Focus your search in areas known for ancient salt flats, dried lake beds, or sedimentary rock formations that indicate past evaporitic conditions.
Check for Veins and Seams
Satin spar often forms in veins or seams within other sedimentary rocks, particularly shales and limestones. Look for linear features or infillings.
Test for Softness
Gypsum is very soft (Mohs hardness 2). You should be able to scratch it easily with a fingernail. This helps distinguish it from harder fibrous minerals.
Observe the Luster
The characteristic silky or satiny luster is a key identifier. Rotate the specimen under light to observe the play of light along the fibers.
Handle with Care
Due to its softness and perfect cleavage, satin spar can be fragile. Handle specimens gently to avoid breakage or scratching.
Similar Rocks
Selenite Gypsum
Gypsum (CaSO4·2H2O)
Also known as: Clear Gypsum
Alabaster Gypsum
Gypsum (CaSO4·2H2O)
Also known as: Massive Gypsum
Ulexite
NaCaB5O9·8H2O
Also known as: TV Rock
Scientific Classification
- Mineral Class
- Sulfates
- Group
- Gypsum Group
- Crystal System
- Monoclinic
- Chemical Formula
- CaSO4·2H2O
- Composition
- Calcium sulfate dihydrate (46.5% CaO, 32.6% SO3, 20.9% H2O)
Explore Satin Spar Gypsum
Identify rocks anywhere
Free on iOS, Android, and Web. Photo identification, full database, and your personal collection.