How to identify Prehnite

To correctly identify Prehnite (Prehnite), 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-green.

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. When broken, it may show a radiating fibrous internal structure. Individual crystals are rare but can be tabular or prismatic.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Orthorhombic system. Crystals are typically tabular or prismatic, but more commonly found as botryoidal, reniform, or stalactitic masses with a radiating internal structure. Well-formed crystals are often small and embedded within these aggregates.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Good cleavage in one direction ({001}), often difficult to observe in massive or botryoidal forms.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Found as a secondary mineral filling vesicles and cavities in mafic igneous rocks (basalt, diabase, gabbro), and in veins and fractures in altered granites, gneisses, and other metamorphic rocks. It is indicative of low-grade metamorphic conditions (prehnite-pumpellyite facies) or hydrothermal alteration.

Key Facts

Hardness

6-6.5 on the Mohs scale.

Specific Gravity

2.80-2.95 g/cm.

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 in one direction ({001}).

Composition

Hydrous calcium aluminum silicate.

Physical characteristics

• Crystal Habit: Typically botryoidal, reniform, or stalactitic aggregates with a radiating fibrous internal structure. Less commonly, tabular or prismatic crystals.
• Cleavage Type: Good on {001}, often not easily observed in massive forms.
• Fracture Type: Uneven to subconchoidal.
• Tenacity: Brittle.
• Luster Type: Vitreous to pearly.

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