Apophyllite with Stilbite and Fluorite

Mineral Association

KCa4(Si8O20)(F,OH)·8H2O (Apophyllite), NaCa4(Si27Al9)O72·28H2O (Stilbite), CaF2 (Fluorite)

Also known as: Zeolite-group minerals with Fluorite

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Description

This is a mineral association rather than a single rock or mineral. It consists of Apophyllite, Stilbite, and potentially Fluorite, often found together in vugs or cavities within volcanic rocks. Apophyllite typically forms sharp, pyramidal or tabular crystals, often colorless, white, or pale green. Stilbite forms distinctive sheaf-like or radiating aggregates, commonly salmon-pink, orange, or white. Fluorite, if present, can occur as cubic or octahedral crystals, often transparent to translucent, and can be various colors including purple, green, blue, or colorless. The combination creates visually striking specimens highly prized by collectors.

How to Identify

Color: Apophyllite: Colorless, white, pale green, yellow, pink. Stilbite: Salmon-pink, orange, white, brown. Fluorite: Purple, green, blue, yellow, colorless, pink, black.
Luster: Apophyllite: Vitreous to pearly on cleavage surfaces. Stilbite: Vitreous to pearly. Fluorite: Vitreous.
Texture: Apophyllite: Smooth, glassy crystal faces. Stilbite: Often radiating, sheaf-like aggregates, sometimes fibrous. Fluorite: Smooth, glassy crystal faces.
Crystal Form: Apophyllite: Tetragonal, typically as square prisms terminated by pyramids, or tabular crystals. Stilbite: Monoclinic, often forming distinctive 'sheaf' or 'bow-tie' aggregates, also tabular or radiating. Fluorite: Isometric, typically cubic or octahedral crystals, sometimes dodecahedral.
Cleavage: Apophyllite: Perfect basal cleavage {001}. Stilbite: Perfect cleavage {010}. Fluorite: Perfect octahedral cleavage {111}.
Geological Environment: Amygdaloidal cavities and veins in basaltic and other volcanic rocks, often associated with other zeolites, calcite, and quartz. Also found in some granitic pegmatites and metamorphic rocks.

Key Facts

Hardness: Apophyllite: 4.5-5. Stilbite: 3.5-4. Fluorite: 4.
Specific Gravity: Apophyllite: 2.3-2.4. Stilbite: 2.1-2.2. Fluorite: 3.18.
Crystal System: Apophyllite: Tetragonal. Stilbite: Monoclinic. Fluorite: Isometric.
Color: Apophyllite: Colorless, white, pale green, yellow, pink. Stilbite: Salmon-pink, orange, white, brown. Fluorite: Purple, green, blue, yellow, colorless, pink, black.
Luster: Vitreous to pearly.
Transparency: Transparent to translucent.
Fracture: Apophyllite: Uneven. Stilbite: Uneven. Fluorite: Subconchoidal to uneven.
Cleavage: Apophyllite: Perfect basal {001}. Stilbite: Perfect {010}. Fluorite: Perfect octahedral {111}.
Composition: Apophyllite: Hydrated potassium calcium silicate fluoride/hydroxide. Stilbite: Hydrated sodium calcium aluminum silicate. Fluorite: Calcium fluoride.

Quick Check

Color: Apophyllite: Colorless to pale green/pink. Stilbite: Salmon-pink to orange. Fluorite: Variable, often purple/green.
Luster: Vitreous to pearly.
Streak: White for all three minerals.

Physical Characteristics

Crystal Habit: Apophyllite: Prismatic, pyramidal, tabular. Stilbite: Sheaf-like, radiating, tabular. Fluorite: Cubic, octahedral, dodecahedral.
Cleavage Type: Apophyllite: Perfect basal. Stilbite: Perfect pinacoidal. Fluorite: Perfect octahedral.
Fracture Type: Uneven to subconchoidal.
Tenacity: Apophyllite: Brittle. Stilbite: Brittle. Fluorite: Brittle.
Luster Type: Vitreous to pearly.

Formation

These minerals typically form in the late-stage hydrothermal alteration of basaltic rocks, particularly in amygdaloidal cavities and veins. Apophyllite and stilbite are common secondary minerals in volcanic rocks, often associated with other zeolites. Fluorite, if present, indicates the presence of fluorine-rich fluids during the crystallization process, often at lower temperatures.

Usage

Primarily collected as mineral specimens due to their aesthetic appeal. Apophyllite and stilbite have minor industrial uses as molecular sieves or catalysts, but not typically in this combined form. Fluorite is used in metallurgy as a flux, in ceramics, and in optical lenses.

Age Distribution

Typically Cenozoic to Mesozoic, but can form in older rocks where hydrothermal activity has occurred.

Where to Find

Deccan Traps, India

World-renowned for exceptional specimens of apophyllite, stilbite, and other zeolites, often found in large, well-formed crystals within basaltic vugs.

Poonah (Pune), India

A classic locality within the Deccan Traps, famous for its high-quality apophyllite and stilbite specimens.

Jalgaon, Maharashtra, India

Another prominent locality in India, producing excellent combinations of these minerals.

Nova Scotia, Canada

Basalt flows along the Bay of Fundy yield good specimens of apophyllite and stilbite, though often smaller than Indian material.

Iceland

Volcanic areas in Iceland are known for various zeolite minerals, including apophyllite and stilbite.

Mexico

Some localities produce apophyllite and stilbite, sometimes with fluorite, in volcanic settings.

Finding Tips

Look in Volcanic Rocks

Focus your search on areas with basaltic lava flows, especially those with amygdaloidal structures (cavities filled with secondary minerals).

Check for Vugs and Veins

These minerals typically form in open spaces. Look for vugs (small cavities) and veins within the host rock.

Associated Minerals

Apophyllite and stilbite are often found with other zeolites (e.g., heulandite, chabazite), calcite, and quartz. The presence of these can indicate a good collecting environment.

Careful Extraction

These minerals can be fragile, especially the delicate stilbite aggregates. Use appropriate tools and techniques to avoid damaging specimens.

Safety Precautions

When collecting in quarries or road cuts, be aware of falling rocks and unstable ground. Wear appropriate safety gear.

Similar Rocks

Heulandite with Stilbite

(Ca,Na)2-3Al3(Al,Si)2Si13O36·12H2O (Heulandite), NaCa4(Si27Al9)O72·28H2O (Stilbite)

Also known as: Zeolite-group minerals

Chabazite with Apophyllite

(Ca,Na2,K2,Mg)Al2Si4O12·6H2O (Chabazite), KCa4(Si8O20)(F,OH)·8H2O (Apophyllite)

Also known as: Zeolite-group minerals

Prehnite with Apophyllite

Ca2Al(AlSi3O10)(OH)2 (Prehnite), KCa4(Si8O20)(F,OH)·8H2O (Apophyllite)

Also known as: Hydrous calcium aluminum silicate with Apophyllite

Scientific Classification

Mineral Class: Apophyllite: Phyllosilicate. Stilbite: Tectosilicate (Zeolite Group). Fluorite: Halide.
Group: Apophyllite Group, Zeolite Group, Halide Group
Crystal System: Apophyllite: Tetragonal. Stilbite: Monoclinic. Fluorite: Isometric.
Chemical Formula: KCa4(Si8O20)(F,OH)·8H2O (Apophyllite), NaCa4(Si27Al9)O72·28H2O (Stilbite), CaF2 (Fluorite)
Composition: Apophyllite: Potassium, Calcium, Silicon, Oxygen, Fluorine/Hydroxyl, Water. Stilbite: Sodium, Calcium, Aluminum, Silicon, Oxygen, Water. Fluorite: Calcium, Fluorine.

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