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The Apophyllite Group consists of hydrated potassium calcium sodium fluorosilicates. The most common members are Fluorapophyllite-(K), Fluorapophyllite-(Na), and Hydroxyapophyllite-(K). These minerals are characterized by their distinctive crystal habits, often forming pyramidal, tabular, or prismatic crystals, frequently with a pearly luster on cleavage surfaces. They are known for their excellent cleavage and often occur in association with zeolites.
How to Identify
- Color
- Colorless, white, green, yellow, pink, violet. Green is particularly characteristic of some varieties.
- Luster
- Vitreous (glassy) on crystal faces, pearly on cleavage surfaces.
- Texture
- Smooth, often with distinct crystal faces.
- Crystal Form
- Commonly forms square-pyramidal, tabular, or prismatic crystals. Often found in drusy coatings or radiating aggregates.
- Cleavage
- Perfect basal cleavage {001}, producing pearly surfaces.
- Geological Environment
- Typically found in the amygdaloidal cavities of basalt and other volcanic rocks, often associated with zeolites, calcite, and prehnite. Also in some granitic pegmatites and hydrothermal veins.
Key Facts
- Hardness: 4.5-5 on the Mohs scale
- Specific Gravity: 2.3-2.5
- Crystal System: Tetragonal (for Fluorapophyllite-(K), Fluorapophyllite-(Na)) or Orthorhombic (for Hydroxyapophyllite-(K))
- Color: Colorless, white, green, yellow, pink, violet
- Luster: Vitreous on prism faces, pearly on basal cleavage
- Transparency: Transparent to translucent
- Fracture: Uneven
- Cleavage: Perfect basal cleavage {001}
- Composition: Hydrated potassium calcium sodium fluorosilicate
Quick Check
- Color: Colorless, white, green, yellow, pink, violet
- Luster: Vitreous to pearly
- Streak: White
Physical Characteristics
- Crystal Habit: Commonly square-pyramidal, tabular, or prismatic crystals; also massive, lamellar, or radiating aggregates. Often forms pseudo-cubic crystals.
- Cleavage Type: Perfect on {001} (basal)
- Fracture Type: Uneven to conchoidal
- Tenacity: Brittle
- Luster Type: Vitreous to pearly
Formation
Apophyllite minerals typically form as secondary minerals in the vesicles (amygdules) and cavities of basaltic and other mafic volcanic rocks. They can also be found in granitic pegmatites, hydrothermal veins, and metamorphic rocks.
Usage
Primarily a collector's mineral due to its aesthetic crystal forms and colors. It has no significant industrial uses. Some metaphysical and spiritual communities attribute healing and meditative properties to it.
Age Distribution
Not applicable (mineral formation, not rock age)
Where to Find
Deccan Traps, India
World-renowned for producing some of the finest and largest apophyllite crystals, often associated with stilbite, heulandite, and cavansite, in basaltic geodes and cavities.
Poonah (Pune), India
Historically significant locality within the Deccan Traps, famous for exceptional green and colorless apophyllite specimens.
Jalgaon, Maharashtra, India
Another prominent locality in India, known for high-quality apophyllite specimens.
Brazil
Various localities produce apophyllite, often in association with other zeolites.
Mexico
Known for some occurrences, particularly in volcanic terrains.
United States
Occurrences in various states, including Oregon, Washington, and New Jersey, often in basaltic environments.
Iceland
Found in basaltic lavas.
Canada
Localities in Nova Scotia and Quebec.
Finding Tips
Look in Volcanic Rocks
Focus on areas with basaltic lava flows, especially those with amygdaloidal structures (cavities filled with secondary minerals).
Associated Minerals
Apophyllite often occurs with zeolites (stilbite, heulandite, scolecite), calcite, prehnite, and chalcedony. Finding these minerals can indicate the presence of apophyllite.
Check for Cavities
Apophyllite crystals typically line or fill vugs and geodes within the host rock. Look for exposed cavities or weathered-out areas.
Examine Crystal Habit
Familiarize yourself with the characteristic pyramidal, tabular, or prismatic crystal forms and the pearly luster on cleavage surfaces.
Similar Rocks
Stilbite
Stilbite Group
Also known as: Stilbite-Ca, Stilbite-Na
Heulandite
Heulandite Group
Also known as: Heulandite-Ca, Heulandite-Na, Heulandite-K, Heulandite-Sr
Scolecite
Scolecite
Also known as: None
Scientific Classification
- Mineral Class
- Silicates
- Group
- Apophyllite Group
- Crystal System
- Tetragonal (Fluorapophyllite-(K), Fluorapophyllite-(Na)); Orthorhombic (Hydroxyapophyllite-(K))
- Chemical Formula
- General formula: KCa4(Si4O10)2F·8H2O (Fluorapophyllite-(K)); NaCa4(Si4O10)2F·8H2O (Fluorapophyllite-(Na)); KCa4(Si4O10)2(OH)·8H2O (Hydroxyapophyllite-(K))
- Composition
- Hydrated potassium calcium sodium fluorosilicate. The specific composition varies slightly between the end-members of the group, primarily in the alkali metal (K or Na) and the halogen/hydroxyl (F or OH) content.
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