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Okenite is a soft, white, fibrous silicate mineral known for its distinctive 'cotton ball' or 'snowball' aggregates. These aggregates are composed of numerous delicate, acicular (needle-like) crystals radiating outwards from a central point. It is a hydrous calcium silicate, meaning it contains both water and calcium within its crystal structure. Its extreme fragility makes it a challenging mineral to handle and preserve. It is often found lining cavities within basaltic rocks, frequently alongside other zeolites like apophyllite, stilbite, and heulandite.
How to Identify
- Color
- Typically white, sometimes with a very faint yellowish or grayish tint.
- Luster
- Silky to pearly when individual fibers are visible; aggregate masses appear dull to silky.
- Texture
- Extremely soft and fibrous, often described as feeling like cotton or wool. The aggregates are very delicate and easily crushed.
- Crystal Form
- Usually found as spherical or hemispherical aggregates of radiating acicular (needle-like) crystals, often referred to as 'cotton balls' or 'snowballs'. Individual crystals are monoclinic but rarely seen distinctly.
- Cleavage
- Perfect on {100} and {001}, but rarely observed due to the fibrous habit and extreme fragility.
- Geological Environment
- Occurs as a secondary mineral in amygdaloidal cavities and geodes within basaltic and other volcanic rocks, formed by low-temperature hydrothermal alteration. It is commonly associated with zeolites (e.g., apophyllite, stilbite, heulandite), prehnite, and calcite.
Key Facts
- Hardness: 1 on the Mohs scale (extremely soft)
- Specific Gravity: 2.33 - 2.36 g/cm³
- Crystal System: Triclinic (often described as monoclinic in older literature, but modern analysis confirms triclinic)
- Color: White, sometimes with a faint yellowish or grayish tint
- Luster: Silky to pearly
- Transparency: Translucent to opaque
- Fracture: Uneven, but rarely observed due to fibrous nature
- Cleavage: Perfect on {100} and {001}, but rarely seen due to fibrous habit
- Composition: Hydrous Calcium Silicate
Quick Check
- Color: White
- Luster: Silky to pearly (fibrous aggregates appear dull to silky)
- Streak: White
Physical Characteristics
- Crystal Habit: Typically forms spherical or hemispherical aggregates of radiating acicular (needle-like) crystals, resembling cotton balls or snowballs. Individual crystals are rarely distinct.
- Cleavage Type: Perfect in two directions ({100} and {001}), but difficult to observe due to the fibrous nature and extreme softness.
- Fracture Type: Uneven, but not typically observed due to the mineral's fibrous and fragile nature.
- Tenacity: Brittle, but more accurately described as extremely fragile and easily crushed due to its delicate fibrous structure.
- Luster Type: Silky to pearly on individual fibers; aggregate masses appear dull to silky.
Formation
Okenite is a hydrous calcium silicate mineral that forms as a secondary mineral in basaltic geodes and amygdales. It precipitates from low-temperature hydrothermal solutions rich in calcium and silica, often in the late stages of volcanic rock alteration. It is commonly associated with other zeolites and related minerals.
Usage
Okenite has no significant industrial or commercial uses due to its softness, fragility, and rarity. It is primarily a collector's mineral, prized for its unique aesthetic appearance, particularly its 'cotton ball' or 'snowball' formations. It is also of scientific interest for mineralogical studies.
Age Distribution
Typically found in Cenozoic to Mesozoic volcanic rocks, but its formation is tied to hydrothermal alteration rather than specific geological ages of the host rock.
Where to Find
Deccan Traps, India
The most famous and prolific locality for high-quality okenite specimens, particularly in the states of Maharashtra and Gujarat. Found in basaltic geodes.
Greenland
Found in basaltic rocks, though specimens are less common and typically smaller than those from India.
Ireland
Reported from basaltic occurrences, often in association with other zeolites.
Chile
Occurrences in volcanic rocks have been noted.
USA (Oregon, Washington)
Minor occurrences in basaltic flows, often as small, less aesthetic specimens.
Finding Tips
Look in Volcanic Geodes
Okenite is almost exclusively found lining the interiors of geodes and amygdales within basaltic lava flows. Look for these rounded cavities in volcanic rock outcrops.
Associated Minerals
It is often found with other secondary minerals such as apophyllite, stilbite, heulandite, prehnite, and calcite. The presence of these minerals can indicate a favorable environment for okenite.
Handle with Extreme Care
Okenite is exceptionally fragile. Even slight pressure can crush its delicate fibrous aggregates. Use soft padding and protective containers for transport and storage. Avoid touching the 'cotton ball' formations directly.
Moisture Sensitivity
While a hydrous mineral, prolonged exposure to high humidity or direct water can potentially degrade its delicate structure over time. Store in a dry environment.
Similar Rocks
Mesolite
Na2Ca2(Al2Si3O10)3·8H2O
Also known as: Needle Zeolite
Scolecite
CaAl2Si3O10·3H2O
Also known as: Fibrous Zeolite
Natrolite
Na2Al2Si3O10·2H2O
Also known as: Needle Zeolite
Scientific Classification
- Mineral Class
- Silicates
- Group
- Phyllosilicates (though sometimes classified as a tectosilicate due to its complex framework structure, it exhibits characteristics of both)
- Crystal System
- Triclinic
- Chemical Formula
- Ca10Si18O46·18H2O
- Composition
- Calcium (Ca), Silicon (Si), Oxygen (O), Hydrogen (H) in the form of water (H2O)
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