Stilbite with Apophyllite and Chalcopyrite

Description

This is a mineral assemblage featuring three distinct minerals: Stilbite-Ca, Apophyllite-(KF), and Chalcopyrite. Stilbite-Ca is a zeolite mineral, often forming sheaf-like or radiating crystal aggregates. Apophyllite-(KF) is a phyllosilicate, typically forming blocky, pyramidal, or tabular crystals. Chalcopyrite is a sulfide mineral, appearing as metallic, brassy yellow crystals. The combination creates visually striking specimens, with the delicate, often translucent zeolites and apophyllite contrasting with the opaque, metallic chalcopyrite. The paragenesis of these minerals provides insights into the complex hydrothermal processes that formed them.

How to Identify

Color

Stilbite-Ca: White, cream, salmon-pink, orange-brown. Apophyllite-(KF): Colorless, white, pale green, pink. Chalcopyrite: Brassy yellow, often tarnishing to iridescent blues, purples, and reds.

Luster

Stilbite-Ca: Vitreous to pearly on cleavage surfaces. Apophyllite-(KF): Vitreous, sometimes pearly on basal cleavage. Chalcopyrite: Metallic.

Texture

Stilbite-Ca: Often forms radiating, sheaf-like aggregates or tabular crystals. Apophyllite-(KF): Blocky, pyramidal, or tabular crystals, often with a distinct basal cleavage. Chalcopyrite: Massive, disseminated grains, or distinct tetrahedral/sphenoidal crystals.

Crystal Form

Stilbite-Ca: Monoclinic, typically forming tabular crystals often twinned to create 'bow-tie' or sheaf-like aggregates. Apophyllite-(KF): Tetragonal, forming blocky, pseudo-cubic, or pyramidal crystals. Chalcopyrite: Tetragonal, often forming sphenoidal or pseudo-tetrahedral crystals.

Cleavage

Stilbite-Ca: Perfect on {010}. Apophyllite-(KF): Perfect on {001} (basal). Chalcopyrite: Poor on {011}.

Geological Environment

Amygdules and veins in basaltic volcanic rocks, cavities and fractures in granitic and metamorphic rocks, hydrothermal ore deposits. Often found in association with other zeolites, prehnite, calcite, and other sulfide minerals.

Key Facts

• Hardness: Stilbite-Ca: 3.5-4. Apophyllite-(KF): 4.5-5. Chalcopyrite: 3.5-4.
• Specific Gravity: Stilbite-Ca: 2.1-2.2. Apophyllite-(KF): 2.3-2.4. Chalcopyrite: 4.1-4.3.
• Crystal System: Stilbite-Ca: Monoclinic. Apophyllite-(KF): Tetragonal. Chalcopyrite: Tetragonal.
• Color: Stilbite-Ca: White, cream, salmon-pink. Apophyllite-(KF): Colorless, white, pale green. Chalcopyrite: Brassy yellow.
• Luster: Stilbite-Ca: Vitreous to pearly. Apophyllite-(KF): Vitreous. Chalcopyrite: Metallic.
• Transparency: Stilbite-Ca: Transparent to translucent. Apophyllite-(KF): Transparent to translucent. Chalcopyrite: Opaque.
• Fracture: Stilbite-Ca: Uneven. Apophyllite-(KF): Uneven to conchoidal. Chalcopyrite: Conchoidal to uneven.
• Cleavage: Stilbite-Ca: Perfect on {010}. Apophyllite-(KF): Perfect on {001}. Chalcopyrite: Poor on {011}.
• Composition: Stilbite-Ca: Hydrated sodium calcium aluminum silicate. Apophyllite-(KF): Hydrated potassium calcium silicate fluoride hydroxide. Chalcopyrite: Copper iron sulfide.

Quick Check

• Color: Stilbite: White to salmon-pink. Apophyllite: Colorless to pale green. Chalcopyrite: Brassy yellow.
• Luster: Stilbite: Vitreous to pearly. Apophyllite: Vitreous. Chalcopyrite: Metallic.
• Streak: Stilbite: White. Apophyllite: White. Chalcopyrite: Greenish-black.

Physical Characteristics

• Crystal Habit: Stilbite-Ca: Sheaf-like, radiating aggregates, tabular crystals. Apophyllite-(KF): Blocky, pyramidal, tabular crystals. Chalcopyrite: Sphenoidal, pseudo-tetrahedral, massive.
• Cleavage Type: Stilbite-Ca: Perfect. Apophyllite-(KF): Perfect. Chalcopyrite: Poor.
• Fracture Type: Stilbite-Ca: Uneven. Apophyllite-(KF): Uneven to conchoidal. Chalcopyrite: Conchoidal to uneven.
• Tenacity: Stilbite-Ca: Brittle. Apophyllite-(KF): Brittle. Chalcopyrite: Brittle.
• Luster Type: Stilbite-Ca: Vitreous to pearly. Apophyllite-(KF): Vitreous. Chalcopyrite: Metallic.

Formation

This assemblage forms in hydrothermal environments, often within amygdules and veins in basaltic volcanic rocks, or in cavities and fractures in metamorphic and granitic rocks. Stilbite and Apophyllite are typically late-stage minerals, crystallizing from silica-rich, low-temperature hydrothermal fluids. Chalcopyrite, a sulfide mineral, usually forms earlier or concurrently from higher-temperature, sulfur-rich hydrothermal solutions, often associated with metallic ore deposition. The presence of all three together indicates a complex hydrothermal history with varying fluid compositions and temperatures.

Usage

Primarily a collector's specimen due to its aesthetic appeal and the interesting paragenesis of the minerals. Chalcopyrite is an important ore of copper, but in this assemblage, it is usually not in sufficient quantities for economic extraction. Stilbite and Apophyllite have minor industrial uses, such as in water purification (zeolites) or as a source of potassium (apophyllite), but again, not typically from these aesthetic specimens.

Age Distribution

Variable, typically associated with late-stage hydrothermal activity in volcanic or metamorphic rocks, ranging from Cenozoic to Precambrian depending on the host rock formation.

Where to Find

Finding Tips

Similar Rocks

Scientific Classification

Mineral Class

Stilbite-Ca: Zeolite Group (Tectosilicate). Apophyllite-(KF): Phyllosilicate. Chalcopyrite: Sulfide.

Group

Stilbite-Ca: Zeolite. Apophyllite-(KF): Apophyllite. Chalcopyrite: Sulfide.

Crystal System

Stilbite-Ca: Monoclinic. Apophyllite-(KF): Tetragonal. Chalcopyrite: Tetragonal.

Chemical Formula

Stilbite-Ca: NaCa2Al5Si13O36·14H2O. Apophyllite-(KF): KCa4(Si8O20)(F,OH)·8H2O. Chalcopyrite: CuFeS2.

Composition

Stilbite-Ca: Hydrated sodium calcium aluminum silicate. Apophyllite-(KF): Hydrated potassium calcium silicate fluoride hydroxide. Chalcopyrite: Copper iron sulfide.

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