Crystal Prehnite

Chemical Formula

Ca₂Al₂Si₃O₁₂(OH)

Composition

Basic calcium aluminum silicate, sometimes with some iron
 

Variable Formula

(Ca,Fe)₂Al₂Si₃O₁₂(OH)

Color

Light green, dark green, mint-green, yellowish-green, brownish-green, and bluish-green. Rarely brown, yellow, orange, pink, gray, white, or colorless.

Streak

White

Hardness

6 - 6.5

Crystal System

Orthorhombic

Crystal Forms 
and Aggregates

Most often as rounded, bubbly globular masses. These rounded masses are usually composed of small saddle-shaped crystals or radiating balls. Commonly botryoidal, reniform, encrusting, stalactitic,acicular, as rounded balls, and as finger-shaped formations. Individual crystals can sometimes be seen in rounded aggregates as curved wedge shaped crystals or rounded groups of blocky rectangles. Larger individual crystals, which are elongated dipyramidal and often with flattened terminations, as well flattened tabular, are very rare and found in only a few unique localities. An interesting habit is the "Roman Helmet", which is a hemispherical ball with a protruding rounded crystal arc developed in the center. Prehnite frequently forms an epimorph over other dissolved minerals, forming elongatedrounded prisms and fingers as well as rounded, triangular-shaped crystals.
 

Transparency

Transparent to translucent

Specific Gravity

2.8 - 3.0

Luster

Vitreous, waxy, pearly

Cleavage

1,1;3,1

Fracture

Uneven

Tenacity

Brittle

In Group

Silicates; Inosilicates

Striking Features

Color and crystal forms

Environment

Most often in igneous basalt and diabase traprock environments, usually associated with zeolites. Also in alpine cavities and metamorphosed gneiss.
 

Rock Type

Igneous, Metamorphic

Popularity (1-4)

2

Prevalence (1-3)

2

Demand (1-3)

1