Apophyllite
Apophyllite is the common name for a small group of hydrated potassium calcium silicates, most often KCa4Si8O20(F,OH)·8H2O, crystallizing in the tetragonal system. It typically grows as glassy, pseudo-cubic or pyramidal crystals lining cavities in basaltic lava. The name comes from the Greek for 'to leaf away,' describing how the mineral flakes when heated, losing its water of crystallization.
Apophyllite properties at a glance
| Type | Minerals |
|---|---|
| Chemical formula | KCa4Si8O20(F,OH)·8H2O |
| Mohs hardness | 4.5–5 |
| Crystal system | Tetragonal |
| Luster | Vitreous, pearly on cleavage |
| Streak | White |
| Cleavage | Perfect basal, one direction |
| Fracture | Uneven |
| Specific gravity | 2.3–2.4 |
| Transparency | Transparent to translucent |
| Common colors | Colorless, White, Green, Pink, Pale yellow |
| Rarity | common |
Geology & background
Apophyllite is a classic secondary mineral of basalt vesicles, crystallizing late in the cooling history of lava flows from silica- and calcium-bearing hydrothermal fluids, typically alongside zeolites such as stilbite and heulandite. The single most productive source in the world is the Deccan Traps basalt of Maharashtra, India, particularly around Pune and Nashik, which has supplied the bulk of museum and collector specimens for over a century. Smaller but attractive occurrences are known from Iceland, Brazil's Rio Grande do Sul, and the traprock quarries of New Jersey. Collectors value apophyllite for its glassy, often colorless to pale green crystals, frequently perched on beds of stilbite or other zeolites in striking cavity assemblages. It is soft and cleaves easily, so specimens need careful handling and padded storage. Pale green varieties owe their color to trace vanadium, while rarer pink material contains manganese.
How to identify Apophyllite
Apophyllite forms square pyramidal or pseudo-cubic crystals, often colorless, white, or pale green, with a vitreous luster on crystal faces that turns pearly on its perfect basal cleavage. Hardness is about 4.5 to 5, streak is white, and a knife blade will scratch it. It is commonly confused with quartz or calcite, but its single perfect cleavage, lower hardness than quartz, and lack of a strong effervescent reaction to acid separate it from both.
- Compare the specimen's color, luster, crystal habit, grain size, streak, and locality before confirming.
- Use multiple photos when possible, including a fresh surface and a close view of texture.
Where Apophyllite is found
Reported from multiple localities worldwide; verify locality against specimen provenance when available.
How much is Apophyllite worth?
| Entry-level specimen | $2 |
|---|---|
| Typical collector specimen | $20 |
| Premium specimen | $150+ |
Value depends on size, crystal quality, color, locality, rarity, damage, and provenance.
Common uses
- Collector specimens
- Education
- Field identification
- Industrial or lapidary use when suitable
Interesting facts
- Its name means 'to leaf away' in Greek because the mineral exfoliates into thin flakes when heated, losing its structural water.
- The Deccan Traps of India, a vast basalt province formed by massive volcanic eruptions, remains the world's premier source of fine apophyllite crystals.
- Apophyllite crystals often grow directly on stilbite, creating some of the most photographed mineral combinations in zeolite collecting.
Care & safety
Handle as a collector specimen until hardness, cleavage, and fracture are verified.
Avoid inhaling dust. Do not cut, grind, or polish unknown specimens without eye protection and a proper respirator.
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