How to identify Prehnite

To correctly identify Prehnite (Ca2Al2Si3O10(OH)2), check each of these features in order. Many rocks and minerals look alike, so cross-reference multiple properties before deciding.

Step-by-step identification

1. 1

   Color

   Overall color and any zoning or banding

   Typically pale green to yellowish-green, but can also be white, gray, or colorless. The green color is often attributed to trace amounts of iron.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Vitreous (glassy) to pearly, especially on cleavage surfaces.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Often forms botryoidal (grape-like), reniform (kidney-shaped), or stalactitic aggregates. Individual crystals are rare and typically tabular or prismatic.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Orthorhombic system, but well-formed crystals are uncommon. Usually found as radiating aggregates of tabular or prismatic crystals, forming botryoidal or stalactitic masses. Can also be massive or granular.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Good cleavage on {001}, often visible as parallel lines or steps on broken surfaces.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Found in vesicles and amygdules of mafic igneous rocks (basalt, diabase, gabbro), in veins and fractures in various rock types, and as a product of low-grade metamorphism (prehnite-pumpellyite facies) or hydrothermal alteration. Often associated with zeolites, calcite, epidote, datolite, and copper minerals.

Key Facts

Hardness

6-6.5 on the Mohs scale

Specific Gravity

2.80-2.95

Crystal System

Orthorhombic

Color

Pale green, yellowish-green, white, gray, colorless

Luster

Vitreous to pearly

Transparency

Transparent to translucent

Fracture

Uneven to subconchoidal

Cleavage

Good on {001}

Composition

Hydrous calcium aluminum silicate

Physical characteristics

• Crystal Habit: Typically botryoidal, reniform, stalactitic, or massive aggregates of radiating tabular or prismatic crystals. Individual crystals are rare and usually small.
• Cleavage Type: Good cleavage on {001}, often producing platy fragments or visible steps.
• Fracture Type: Uneven to subconchoidal, meaning it breaks with irregular, rough surfaces or slightly curved, shell-like fractures.
• Tenacity: Brittle, meaning it breaks or powders easily when struck.
• Luster Type: Vitreous (glassy) to pearly, especially on cleavage surfaces. Can appear somewhat greasy on some massive forms.

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