How to identify Stilbite on Chalcedony with Mordenite

To correctly identify Stilbite on Chalcedony with Mordenite (Stilbite-Ca, Chalcedony, Mordenite), 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

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

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

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

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

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

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   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.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

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

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   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.

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.

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