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Apophyllite

Crystal system · Tetragonal

Apophyllite is a silicate mineral prized by collectors for its exceptional color range.

About Apophylliteextended article

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Mohs 4.5–5
Vickers (~) 540 HV
Knoop (~) 620 HK
Geological setting
Volcanic
Element composition by mass

Formula: KCa₄Si₈O₂₀(F,OH)·8H₂O · molar mass: 780.09 g/mol

O 44.09%
Si 28.8%
Ca 20.55%
K 5.01%
F 1.22%
H 0.32%

Computed from atomic weights (IUPAC 2021). Site-occupancy groups (Fe,Mn) split equally.

Mohs Hardness 4.5–5

Apophyllite sits at 4.5–5 on the Mohs scale — can be scratched by a steel knife.

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Colors:
Streak
White
Crystal system
Tetragonal
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SilicatesSilicates (Phyllosilicates)
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TL;DR · 1 min read
Apophyllite (KCa₄Si₈O₂₀(F,OH)·8H₂O) is a tetragonal phyllosilicate famous for its glassy, water-clear pyramidal crystals on chalcedony or stilbite matrix in basalt cavities. The Indian Deccan Traps — particularly the Pune and Jalgaon districts in Maharashtra — produce the world's most prized Apophyllite specimens, with sharp single crystals reaching 10+ cm and complete pyramid terminations.
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Apophyllite (KCa₄Si₈O₂₀(F,OH)·8H₂O) is a tetragonal phyllosilicate famous for its glassy, water-clear pyramidal crystals on chalcedony or stilbite matrix in basalt cavities. The Indian Deccan Traps — particularly the Pune and Jalgaon districts in Maharashtra — produce the world’s most prized Apophyllite specimens, with sharp single crystals reaching 10+ cm and complete pyramid terminations. The pearly luster on basal pinacoid surfaces is diagnostic.

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Identification & care

Apophyllite typically forms prismatic to tabular tetragonal crystals; often steep pyramidal terminations; cubic-looking; drusy. Its color range is broad, including colorless, white, pale green (most popular collector form), pink, yellow, and violet. The luster is vitreous, pearly (on {001}), the streak is white, and specimens range from transparent to translucent. The cleavage is perfect on {001} — basal cleavage gives pearly luster. The fracture is uneven, which aids identification.

Collector context

How it forms

In terms of geology, Apophyllite forms in vesicles and voids in basalt and volcanic rocks; hydrothermal veins; contact metamorphic zones; often with zeolites and calcite. It is commonly found in association with stilbite, heulandite, calcite, prehnite, gyrolite, okenite, natrolite.

Why collectors care

Collectors pursue Apophyllite for the clarity of its crystal form and, in good material, saturated color that reads instantly across a display case. A well-terminated apophyllite on clean matrix photographs well, identifies quickly, and anchors a cabinet piece. Top Chinese specimens over the last two decades have reset the bar for what apophyllite looks like at collector grade.

What affects value

Value in Apophyllite is assessed, in typical order of weight, against: (1) locality provenance; (2) size relative to the species norm; (3) crystal form and termination sharpness; (4) color saturation and zoning; (5) transparency and internal clarity; (6) matrix quality and aesthetic balance; (7) condition (absence of damage, chips, or repair). Cleaning quality and verified locality documentation act as multipliers across the above.

Naming history

The name Apophyllite has a specific etymological and historical context — see Mindat's reference entry for provenance details. We have retained naming data at the record level; published prose is paraphrased from factual fields rather than copied from source.