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Prehnite is a distinctive mineral known for its typically pale green to yellowish-green color, though it can also be white, gray, or colorless. It often forms botryoidal (grape-like) or reniform (kidney-shaped) aggregates, sometimes with a radiating fibrous internal structure. Individual crystals are less common but can be tabular or prismatic. It has a vitreous to pearly luster and is translucent to transparent. Prehnite is a calcium aluminum silicate with hydroxyl groups, belonging to the inosilicates, though its structure is often described as intermediate between sheet silicates and chain silicates. It is a common mineral in low-grade metamorphic and hydrothermal environments.
How to Identify
- Color
- Typically pale green to yellowish-green, but can also be white, gray, or colorless. The green color is often soft and apple-green.
- Luster
- Vitreous (glassy) to pearly, especially on cleavage surfaces.
- Texture
- 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.
- Crystal Form
- 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.
- Cleavage
- Good cleavage in one direction ({001}), often difficult to observe in massive or botryoidal forms.
- Geological Environment
- 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.
Quick Check
- Color: Pale green to yellowish-green (most common), white, gray, colorless.
- Luster: Vitreous to pearly.
- Streak: White.
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.
Formation
Prehnite is a hydrous calcium aluminum silicate mineral that typically forms as a secondary mineral in the vesicles and cavities of mafic volcanic rocks (like basalt and diabase), and in altered gabbros, granites, and gneisses. It is characteristic of the prehnite-pumpellyite metamorphic facies, indicating low-grade metamorphic conditions (temperatures typically 200-350 C and pressures 2-7 kbar). It can also form in veins and fractures within these rocks, often associated with zeolites, calcite, epidote, and copper minerals. Its formation involves the alteration of plagioclase feldspar and mafic minerals under hydrous conditions.
Usage
Prehnite is primarily used as a collector's mineral and for lapidary purposes. Its attractive color and translucent to transparent nature make it suitable for cabochons, beads, and carvings. It is also occasionally used in ornamental objects. Due to its relatively low hardness, it is not typically used in high-wear jewelry.
Age Distribution
Prehnite forms in various geological settings and is not restricted to a specific geological age. It is commonly found in rocks that have undergone low-grade metamorphism or hydrothermal alteration, which can occur throughout Earth's history.
Where to Find
Karoo Basin, South Africa
Famous for producing large, fine botryoidal masses of apple-green prehnite, often associated with zeolites and copper minerals, particularly in the basaltic lavas.
Paterson, New Jersey, USA
Known for excellent specimens of prehnite, often forming botryoidal masses in diabase quarries, sometimes with associated copper minerals.
Mali
Produces distinctive, often gem-quality, tabular prehnite crystals, sometimes with a yellowish-green hue, found in granitic pegmatites.
Australia (e.g., Wave Hill, Northern Territory)
Sources of good quality prehnite, often in botryoidal forms, sometimes with inclusions of epidote or other minerals.
India (Deccan Traps)
Found in the basaltic flows, often in association with zeolites, similar to the South African occurrences.
Finding Tips
Target Mafic Volcanic Rocks
Focus your search on areas with basalt, diabase, or gabbro, especially where these rocks show signs of alteration or contain vesicles and cavities. Prehnite often forms in these open spaces.
Look for Hydrothermal Veins
Examine veins and fractures within altered igneous and metamorphic rocks. Prehnite can crystallize from hydrothermal fluids circulating through these structures.
Associated Minerals
Prehnite is frequently found with zeolites (e.g., stilbite, heulandite), calcite, epidote, pumpellyite, and native copper. The presence of these minerals can be a good indicator.
Botryoidal Forms
Keep an eye out for grape-like or kidney-shaped aggregates, which are characteristic habits of prehnite. These can range from small coatings to large masses.
Color and Luster
The distinctive pale green to yellowish-green color and vitreous to pearly luster are key visual identifiers.
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Scientific Classification
- Mineral Class
- Silicates
- Group
- Inosilicates (though sometimes considered an intermediate between sheet and chain silicates)
- Crystal System
- Orthorhombic
- Chemical Formula
- Ca2Al(AlSi3O10)(OH)2
- Composition
- Calcium aluminum silicate hydroxide.
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