Prehnite

Description

Prehnite is a calcium aluminum silicate mineral characterized by its typically pale green to yellowish-green color, though it can also be white, gray, or colorless. It commonly forms in botryoidal (grape-like) or stalactitic aggregates, often with a radiating fibrous internal structure. It is a member of the inosilicate mineral class, though its structure is more complex, often described as a phyllosilicate-like structure due to its layered arrangement of tetrahedra. Prehnite is known for its vitreous to pearly luster and its relatively good hardness, making it suitable for lapidary work. It is often associated with zeolites, calcite, epidote, and copper minerals.

How to Identify

Color

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.

Luster

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

Texture

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

Crystal Form

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.

Cleavage

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

Geological Environment

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

Quick Check

• Color: Pale green to yellowish-green, white, gray, colorless
• Luster: Vitreous to pearly
• Streak: White

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.

Formation

Prehnite is a hydrous calcium aluminum silicate mineral that typically forms in the low-grade metamorphic facies (prehnite-pumpellyite facies) and as a secondary mineral in the vesicles and fractures of mafic igneous rocks (basalts, gabbros, diabases), and less commonly in granitic rocks. It precipitates from hydrothermal fluids circulating through these rocks, often replacing other minerals like plagioclase feldspar, pyroxene, and amphibole. It can also form in veins and cavities in metamorphic rocks, such as schists and gneisses, and in serpentinites.

Usage

Prehnite is primarily used as a collector's mineral due to its attractive botryoidal and stalactitic habits and appealing colors. Gem-quality prehnite is cut into cabochons, beads, and occasionally faceted stones for jewelry. It has limited industrial use, though its presence can be an indicator of specific metamorphic conditions in geological studies.

Age Distribution

Prehnite forms in a variety of geological settings, often associated with low-grade metamorphic and hydrothermal alteration processes. It is not typically dated directly but is found in rocks of various ages, from Precambrian to Cenozoic, wherever the specific formation conditions are met.

Where to Find

Finding Tips

Similar Rocks

Scientific Classification

Mineral Class

Silicates

Group

Inosilicates (though sometimes classified as a phyllosilicate-like structure due to its layered arrangement of tetrahedra)

Crystal System

Orthorhombic

Chemical Formula

Ca2Al2Si3O10(OH)2

Composition

Calcium aluminum silicate with hydroxyl groups. The ideal formula is Ca2Al2Si3O10(OH)2, but minor substitutions can occur, such as Fe3+ for Al3+.

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