Crystal Stilebite

Chemical Formula

Stilbite-Ca: NaCa₄Al₉Si₂₇O₇₂ · 28(H₂O)
Stilbite-Na: Na₉Al₉Si₂₇O₇₂ · 28(H₂O)
 

Composition

Hydrous calcium sodium aluminum silicate, occasionally with some potassium. The ratio of calcium to sodium is variable.

Variable Formula

(Na,Ca,K)_4-9Al₉Si₂₇O₇₂ · n(H₂O)

Color

White, beige, peach, pink, orange, light yellow, brown. Less commonly gray, red, or green.

Streak

Colorless

Hardness

3.5 - 4

Crystal System

Monoclinic

Crystal Forms 
and Aggregates

Single crystals are tabular and flattened, and usually with pointed termination. However, single individual crystals are very uncommon; crystals will usually be in platy aggregates, or more often in distinct curved aggregates of fan-shaped or wheat sheaf bundles, and in unique "bow tie" formations. Also radiating, globular, in bladed groups, as rounded balls of radiating crystals, and fibrous. Crystals are often doubly terminated, and aggregates may be twinned in interesting bisecting habits.
 

Transparency

Transparent to translucent

Specific Gravity

2.1 - 2.2

Luster

Vitreous, pearly, dull

Cleavage

1,1

Fracture

Uneven

Tenacity

Brittle

Other ID Marks

Occasionally fluorescent yellow in  UV light.
 

In Group

Silicates; Tectosilicates; Zeolite Group

Striking Features

Crystal aggregate formations and association with other zeolites.

Environment

Most often in volcanic basalt and diabase. Also in altered metamorphic gneiss, and occasionally in granite pegmatites.
 

Rock Type

Igneous, Metamorphic

Popularity (1-4)

2

Prevalence (1-3)

1

Demand (1-3)

2