Crystal Apophyllite

Chemical Formula

Apophyllite-KF: KCa₄Si₈O₂₀(F,OH) · 8(H₂O)
Apophyllite-KOH: KCa₄Si₈O₂₀(OH,F) · 8(H₂O)
Apophyllite-NaF: NaCa₄Si₈O₂₀F · 8(H₂O)

Composition

Hydrous calcium potassium fluoro-hydroxyl-silicate. The ratio fluorine to hyroxyl varies among two group members, and the potassium is replaced with sodium in another member.

Variable Formula

Apophyllite Group: (K,Na)Ca₄Si₈O₂₀(F,OH) · 8(H₂O)

Color

Colorless, white, gray, green, brown. Rarely pink, purple, red, or orange.

Streak

White

Hardness

4.5 - 5

Crystal System

Tetragonal

Crystal Forms 
and Aggregates

Crystals are most often in cubic-shaped crystals, with distinctive triangular corners. Commonly in perfect, pseudocubic or rectangular crystals, as well as sharply pointed prismatic pyramidal crystals that are usually doubly terminated. Also tabular, blocky, in cubic groups, in platy clusters, druzy, and instalactitic formations of crystals. An unusual habit is in blocky radiating sprays and rounded groupings resembling disco balls. Crystals are almost always striated.

(Note: Apophyllite-(KF) and Apophyllite-(KOH) both crystallize in the tetragonal crystal system, whereas Apophyllite-(NaF) crystallizes in the orthorhombic system.)
 

Transparency

Transparent to translucent

Specific Gravity

2.3 - 2.4

Luster

Vitreous to greasy. Pearly on cleavage surfaces.

Cleavage

1,1

Fracture

Uneven

Tenacity

Brittle

Other ID Marks

Occasionally fluorescent yellow or white.
 

Complex Tests

Exfoliates and becomes fluorescent when heated.

In Group

Silicates; Phyllosilicates

Striking Features

Crystal habits and mode of occurrence.

Environment

Mostly in volcanic rock, within basalt and in seams in diabase. Also in low temperature metamorphicveins in gneiss and in cavities in pegmatite dikes.
 

Rock Type

Igneous

Popularity (1-4)

2

Prevalence (1-3)

1

Demand (1-3)

2