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Okenite on Apophyllite refers to a mineral specimen where delicate, fibrous, often spherical aggregates of Okenite are found growing on or associated with well-formed, typically blocky or pyramidal crystals of Apophyllite. This combination is highly prized by mineral collectors for its contrasting textures and forms. Okenite is a hydrated calcium silicate, while Apophyllite is a hydrated potassium calcium fluorosilicate. Both are zeolitic minerals, often found together in the same geological environments.
How to Identify
- Color
- Okenite is typically white, sometimes with a very faint yellowish or bluish tint. Apophyllite is commonly colorless, white, or pale green, but can also be pink, yellow, or brown.
- Luster
- Okenite has a silky to pearly luster. Apophyllite has a vitreous (glassy) luster, sometimes pearly on cleavage surfaces.
- Texture
- Okenite forms delicate, radiating fibrous aggregates, often described as 'cotton balls' or 'snowballs'. Apophyllite forms distinct, blocky, tabular, or pyramidal crystals with smooth faces.
- Crystal Form
- Okenite: Spherical or hemispherical aggregates of acicular (needle-like) crystals. Apophyllite: Tetragonal or orthorhombic crystals, often pseudo-cubic, tabular, or pyramidal.
- Cleavage
- Okenite: Perfect on {100}. Apophyllite: Perfect basal cleavage on {001}.
- Geological Environment
- Both minerals are typically found in the vesicles and amygdules of basaltic lava flows, often associated with other zeolites and secondary minerals formed by hydrothermal alteration.
Key Facts
- Hardness: Okenite: 5 (Mohs). Apophyllite: 4.5-5 (Mohs).
- Specific Gravity: Okenite: 2.3-2.4. Apophyllite: 2.3-2.5.
- Crystal System: Okenite: Triclinic. Apophyllite: Tetragonal (most common, e.g., Fluorapophyllite-K) or Orthorhombic (e.g., Hydroxyapophyllite-K).
- Color: Okenite: White. Apophyllite: Colorless, white, pale green, pink, yellow, brown.
- Luster: Okenite: Silky to pearly. Apophyllite: Vitreous, pearly on cleavage.
- Transparency: Okenite: Translucent to opaque. Apophyllite: Transparent to translucent.
- Fracture: Okenite: Uneven. Apophyllite: Uneven to conchoidal.
- Cleavage: Okenite: Perfect on {100}. Apophyllite: Perfect basal cleavage on {001}.
- Composition: Okenite: Hydrated calcium silicate. Apophyllite: Hydrated potassium calcium fluorosilicate (or hydroxysilicate).
Quick Check
- Color: White (Okenite), Colorless to pale green/white (Apophyllite)
- Luster: Silky to pearly (Okenite), Vitreous to pearly (Apophyllite)
- Streak: White (both)
Physical Characteristics
- Crystal Habit: Okenite: Spherical or hemispherical aggregates of radiating acicular crystals, often resembling cotton balls. Apophyllite: Tabular, prismatic, or pyramidal crystals, often with well-developed faces.
- Cleavage Type: Okenite: Perfect in one direction. Apophyllite: Perfect basal cleavage.
- Fracture Type: Okenite: Uneven. Apophyllite: Uneven to conchoidal.
- Tenacity: Okenite: Brittle, very delicate. Apophyllite: Brittle.
- Luster Type: Okenite: Silky to pearly. Apophyllite: Vitreous, pearly on {001} cleavage.
Formation
Okenite and Apophyllite are both secondary minerals that form in the vesicles (gas bubbles) and fractures of basaltic and other mafic volcanic rocks. They precipitate from low-temperature hydrothermal solutions rich in calcium, silicon, and water, often in the presence of fluorine for apophyllite. Okenite typically forms later than apophyllite, often encrusting or growing on pre-existing apophyllite crystals.
Usage
Primarily a collector's mineral due to its aesthetic appeal. Okenite's delicate, fibrous nature and apophyllite's well-formed crystals make for attractive specimens. No significant industrial or commercial uses.
Age Distribution
Typically Cenozoic, associated with basaltic volcanism, but can occur in older formations where hydrothermal alteration has taken place.
Where to Find
Deccan Traps, India
The most famous and prolific locality for high-quality Okenite on Apophyllite specimens, particularly from the Maharashtra region (e.g., Nashik, Pune, Mumbai). These specimens are renowned for their aesthetic appeal and size.
Faroe Islands
Known for basaltic occurrences where both minerals can be found, though specimens are generally less abundant and smaller than those from India.
Greenland
Occurrences in basaltic rocks, sometimes yielding good specimens.
Iceland
Volcanic regions can host these minerals, often in association with other zeolites.
Brazil
Some localities in basaltic terrains have produced specimens, though less commonly than India.
Finding Tips
Geological Context
Focus on areas with basaltic lava flows, especially those that have undergone low-temperature hydrothermal alteration. Look for vugs, geodes, and fractures within the basalt.
Associated Minerals
Okenite and Apophyllite often occur with other zeolites (e.g., Stilbite, Heulandite, Scolecite, Laumontite), Calcite, and Quartz. The presence of these minerals can indicate a favorable environment.
Delicate Nature
Okenite is extremely delicate due to its fibrous structure. Handle specimens with extreme care to prevent damage. Apophyllite, while harder, can still be brittle.
Collecting Tools
Standard rockhounding tools (hammer, chisel, pry bar) for extracting from basalt. For delicate specimens, soft brushes and careful extraction techniques are essential.
Similar Rocks
Stilbite on Apophyllite
Stilbite-Ca ((Na,Ca0.5)9[Al9Si27O72]·nH2O) on Apophyllite group minerals (KCa4(Si4O10)2F·8H2O)
Also known as: Stilbite-Apophyllite assemblage
Scolecite on Apophyllite
Scolecite (CaAl2Si3O10·3H2O) on Apophyllite group minerals (KCa4(Si4O10)2F·8H2O)
Also known as: Scolecite-Apophyllite assemblage
Heulandite on Apophyllite
Heulandite-Ca ((Ca,Na)2-3Al3(Al,Si)2Si13O36·12H2O) on Apophyllite group minerals (KCa4(Si4O10)2F·8H2O)
Also known as: Heulandite-Apophyllite assemblage
Scientific Classification
- Mineral Class
- Okenite: Phyllosilicate. Apophyllite: Phyllosilicate (specifically, a tectosilicate with a layered structure).
- Group
- Okenite: Zeolite group (though structurally distinct from typical framework silicates). Apophyllite: Apophyllite Group.
- Crystal System
- Okenite: Triclinic. Apophyllite: Tetragonal or Orthorhombic.
- Chemical Formula
- Okenite: Ca10Si18O46·18H2O. Apophyllite: KCa4(Si4O10)2F·8H2O (Fluorapophyllite-K, common member).
- Composition
- Okenite: Calcium, Silicon, Oxygen, Hydrogen (as water). Apophyllite: Potassium, Calcium, Silicon, Oxygen, Fluorine (or Hydroxyl), Hydrogen (as water).
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