How to identify Prehnite

To correctly identify Prehnite (Ca2Al(AlSi3O10)(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 soft and apple-like.

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) or reniform (kidney-shaped) aggregates with a smooth, waxy feel. Individual crystals, when present, are typically tabular or prismatic.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Commonly found as botryoidal, reniform, or stalactitic masses. Individual crystals are rare but can be tabular or prismatic, often with a distinct termination.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Good cleavage in one direction ({001}), often visible as parallel lines or steps on crystal faces or fractured surfaces.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Found in veins, vugs, and amygdules within mafic igneous rocks (basalts, gabbros, diabases) that have undergone low-grade metamorphism or hydrothermal alteration. Also occurs in some granitic rocks and gneisses.

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

Translucent to transparent

Fracture

Uneven to subconchoidal

Cleavage

Good on {001}

Composition

Hydrous calcium aluminum silicate

Physical characteristics

• Crystal Habit: Botryoidal, reniform, stalactitic, granular, massive; rarely as tabular or prismatic crystals.
• Cleavage Type: Good in one direction ({001}), often producing platy fragments or steps.
• Fracture Type: Uneven to subconchoidal, meaning it breaks with irregular, rough surfaces or slightly curved, shell-like surfaces.
• Tenacity: Brittle, meaning it breaks or crumbles easily when struck.
• Luster Type: Vitreous (glassy) on most surfaces, becoming pearly on cleavage surfaces.

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