Stilbite on Chalcedony with Mordenite

Description

This is a mineral assemblage featuring Stilbite-Ca, Chalcedony, and Mordenite, often found together in volcanic environments. Stilbite-Ca typically forms radiating, sheaf-like or tabular crystals, often white, pink, or orange. Chalcedony, a microcrystalline variety of quartz, forms botryoidal, mammillary, or stalactitic masses, providing a base or matrix, usually white, gray, or blue. Mordenite, another zeolite, often appears as delicate, fibrous, or acicular crystals, typically white to pale pink, sometimes forming cotton-like aggregates. The combination creates aesthetically pleasing specimens, with the contrasting forms and colors of the three minerals.

How to Identify

Color

Stilbite-Ca: White, pink, orange, salmon-red, brown. Chalcedony: White, gray, blue, brown, often translucent. Mordenite: White, pale pink, yellowish, colorless.

Luster

Stilbite-Ca: Vitreous to pearly on cleavage surfaces. Chalcedony: Waxy to dull. Mordenite: Silky to vitreous.

Texture

Stilbite-Ca: Sheaf-like, tabular, radiating aggregates. Chalcedony: Botryoidal, mammillary, stalactitic, microcrystalline. Mordenite: Fibrous, acicular, cotton-like aggregates.

Crystal Form

Stilbite-Ca: Monoclinic, typically tabular crystals often twinned into 'bow-tie' or 'sheaf' aggregates. Chalcedony: Cryptocrystalline quartz, no macroscopic crystal form, but forms botryoidal or mammillary masses. Mordenite: Orthorhombic, typically as fine acicular or fibrous crystals, often radiating or in felted masses.

Cleavage

Stilbite-Ca: Perfect on {010}. Chalcedony: None (conchoidal fracture). Mordenite: Good on {100} and {010}.

Geological Environment

Typically found in the amygdaloidal cavities and veins of basaltic and other mafic volcanic rocks, as well as in some granitic pegmatites and hydrothermal alteration zones. It forms under low-temperature, low-pressure hydrothermal conditions.

Key Facts

• Hardness: Stilbite-Ca: 3.5-4. Chalcedony: 6.5-7. Mordenite: 3-5.
• Specific Gravity: Stilbite-Ca: 2.1-2.2. Chalcedony: 2.55-2.65. Mordenite: 2.1-2.12.
• Crystal System: Stilbite-Ca: Monoclinic. Chalcedony: Trigonal (cryptocrystalline). Mordenite: Orthorhombic.
• Color: Variable, as described above, depending on the dominant mineral and impurities.
• Luster: Variable, as described above.
• Transparency: Stilbite-Ca: Transparent to translucent. Chalcedony: Translucent to opaque. Mordenite: Transparent to translucent.
• Fracture: Stilbite-Ca: Uneven. Chalcedony: Conchoidal. Mordenite: Uneven to splintery.
• Cleavage: Stilbite-Ca: Perfect on {010}. Chalcedony: None. Mordenite: Good on {100} and {010}.
• Composition: Stilbite-Ca: Hydrated calcium sodium aluminum silicate. Chalcedony: Silicon dioxide (SiO2). Mordenite: Hydrated sodium calcium potassium aluminum silicate.

Quick Check

• Color: Stilbite: White, pink, orange. Chalcedony: White, gray, blue. Mordenite: White, pale pink.
• Luster: Stilbite: Vitreous to pearly. Chalcedony: Waxy to dull. Mordenite: Silky to vitreous.
• Streak: White for all three components.

Physical Characteristics

• Crystal Habit: Stilbite-Ca: Sheaf-like, tabular, radiating. Chalcedony: Botryoidal, mammillary, stalactitic. Mordenite: Fibrous, acicular, cotton-like.
• Cleavage Type: Stilbite-Ca: Perfect. Chalcedony: None. Mordenite: Good.
• Fracture Type: Stilbite-Ca: Uneven. Chalcedony: Conchoidal. Mordenite: Uneven to splintery.
• Tenacity: Stilbite-Ca: Brittle. Chalcedony: Brittle. Mordenite: Brittle.
• Luster Type: Stilbite-Ca: Vitreous to pearly. Chalcedony: Waxy to dull. Mordenite: Silky to vitreous.

Formation

This assemblage forms in the vugs and amygdales of volcanic rocks (basalts, andesites) or in hydrothermal veins. Chalcedony often forms first as a lining, followed by mordenite, and then stilbite-Ca, or they can precipitate concurrently or in different sequences depending on the fluctuating hydrothermal fluid chemistry and temperature. The presence of all three indicates a complex paragenetic sequence within a low-temperature, silica-rich, and alkali-rich hydrothermal environment.

Usage

Primarily a collector's specimen due to its aesthetic appeal. Zeolites (stilbite, mordenite) have industrial uses as molecular sieves, catalysts, and ion-exchange materials, but this specific assemblage is not typically mined for these purposes due to its mixed composition and occurrence as aesthetic specimens.

Age Distribution

Typically Cenozoic, but can occur in older rocks where hydrothermal alteration has taken place.

Where to Find

Finding Tips

Similar Rocks

Scientific Classification

Mineral Class

Silicates (Tectosilicates - Zeolite Group for Stilbite and Mordenite; Quartz Group for Chalcedony)

Group

Zeolite Group (Stilbite, Mordenite), Quartz Group (Chalcedony)

Crystal System

Stilbite-Ca: Monoclinic. Chalcedony: Trigonal (cryptocrystalline). Mordenite: Orthorhombic.

Chemical Formula

Stilbite-Ca: (Ca,Na,K)9Al9Si27O72·28H2O. Chalcedony: SiO2. Mordenite: (Na2,Ca,K2)Al2Si10O24·7H2O.

Composition

Stilbite-Ca: Hydrated calcium sodium potassium aluminum silicate. Chalcedony: Silicon dioxide. Mordenite: Hydrated sodium calcium potassium aluminum silicate.

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