Apophyllite with Stilbite and Epidote

Mineral Assemblage

KCa4(Si8O20)(OH,F)·8H2O (Apophyllite), NaCa4(Si27Al9)O72·28H2O (Stilbite), Ca2(Al,Fe3+)3(SiO4)3(OH) (Epidote)

Also known as: Zeolite-Epidote Assemblage

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Description

This is a striking mineral assemblage featuring three distinct minerals: Apophyllite, Stilbite, and Epidote. Apophyllite typically forms sharp, pyramidal or tabular crystals, often colorless, white, or pale green, with a pearly to vitreous luster. Stilbite commonly occurs as sheaf-like aggregates, radiating clusters, or individual tabular crystals, often salmon-pink, orange, or white, with a vitreous to pearly luster. Epidote usually forms prismatic crystals, often dark green to yellowish-green, with a vitreous luster. The combination creates a visually appealing specimen, often with contrasting colors and crystal habits, making it highly sought after by mineral collectors. The minerals typically grow together in vugs or fractures within host rocks, showcasing their individual crystal forms.

How to Identify

Color: Apophyllite: Colorless, white, pale green, sometimes pink. Stilbite: Salmon-pink, orange, white, yellowish-brown. Epidote: Pistachio green, yellowish-green, dark green, brownish-green.
Luster: Apophyllite: Vitreous to pearly on cleavage surfaces. Stilbite: Vitreous to pearly. Epidote: Vitreous.
Texture: Apophyllite: Smooth, often with distinct crystal faces. Stilbite: Often fibrous or radiating aggregates, sometimes platy. Epidote: Prismatic crystals, sometimes granular.
Crystal Form: Apophyllite: Tetragonal, typically pyramidal, tabular, or blocky crystals. Stilbite: Monoclinic, often forming sheaf-like aggregates, bow-tie shapes, or tabular crystals. Epidote: Monoclinic, typically prismatic, often striated parallel to length.
Cleavage: Apophyllite: Perfect basal cleavage {001}. Stilbite: Perfect cleavage {010}. Epidote: Perfect cleavage {001}.
Geological Environment: Amygdules and vugs in basaltic and other mafic volcanic rocks, hydrothermal veins, granitic pegmatites, and metamorphic rocks (for epidote).

Key Facts

Hardness: Apophyllite: 4.5-5. Stilbite: 3.5-4. Epidote: 6-7.
Specific Gravity: Apophyllite: 2.3-2.5. Stilbite: 2.1-2.2. Epidote: 3.3-3.5.
Crystal System: Apophyllite: Tetragonal. Stilbite: Monoclinic. Epidote: Monoclinic.
Color: Variable, as described above for each mineral.
Luster: Vitreous to pearly.
Transparency: Transparent to translucent.
Fracture: Apophyllite: Uneven. Stilbite: Uneven. Epidote: Uneven to conchoidal.
Cleavage: Apophyllite: Perfect basal {001}. Stilbite: Perfect {010}. Epidote: Perfect {001}.
Composition: Apophyllite: Hydrated potassium calcium silicate hydroxide fluoride. Stilbite: Hydrated sodium calcium aluminum silicate. Epidote: Calcium aluminum iron silicate hydroxide.

Quick Check

Color: Apophyllite: White/pale green. Stilbite: Salmon-pink/orange. Epidote: Pistachio green.
Luster: Vitreous to pearly.
Streak: White (Apophyllite, Stilbite), Grayish-white (Epidote).

Physical Characteristics

Crystal Habit: Apophyllite: Pyramidal, tabular, blocky. Stilbite: Sheaf-like, radiating, tabular. Epidote: Prismatic, granular.
Cleavage Type: Apophyllite: Perfect basal. Stilbite: Perfect {010}. Epidote: Perfect {001}.
Fracture Type: Uneven to conchoidal.
Tenacity: Apophyllite: Brittle. Stilbite: Brittle. Epidote: Brittle.
Luster Type: Vitreous to pearly.

Formation

This mineral assemblage forms primarily in the cavities (vugs and amygdules) of basaltic and other mafic volcanic rocks, and less commonly in granitic pegmatites or hydrothermal veins. Apophyllite and stilbite are common secondary minerals formed during the late-stage hydrothermal alteration of these rocks, often at relatively low temperatures (typically 50-200°C). Epidote, on the other hand, is a common mineral in medium-grade metamorphic rocks (greenschist to amphibolite facies) and also forms in hydrothermal alteration zones, often at slightly higher temperatures than zeolites, or in skarn deposits. The co-occurrence of all three suggests a complex hydrothermal history, possibly with varying temperature and fluid compositions over time, or localized microenvironments. The fluids responsible are typically rich in silica, calcium, potassium, sodium, and aluminum.

Usage

Primarily a collector's mineral due to its aesthetic appeal. Apophyllite and stilbite are sometimes used in water purification and as molecular sieves, though not typically in this combined form. Epidote has no significant industrial uses.

Age Distribution

Typically Cenozoic to Mesozoic, but can occur in older metamorphic and igneous rocks.

Where to Find

Deccan Traps, India

World-renowned for exceptional specimens of apophyllite and stilbite, often found together in basaltic amygdules. Epidote can also be found in these regions, though less commonly in direct association with the finest zeolite specimens.

Poonah (Pune), India

A classic locality within the Deccan Traps, famous for large, well-formed crystals of apophyllite and stilbite.

Brazil

Various localities produce good quality apophyllite and stilbite, sometimes with epidote, from volcanic and pegmatitic environments.

Iceland

Known for zeolite occurrences, including apophyllite and stilbite, in basaltic lavas.

United States (e.g., Oregon, Washington)

Basalt flows in the Pacific Northwest can yield apophyllite and stilbite, and epidote is common in metamorphic terrains.

Alpine regions (e.g., Switzerland, Austria)

Epidote is a characteristic mineral of the Alps, often found in metamorphic rocks and hydrothermal veins. Apophyllite and stilbite are less common but can occur in specific hydrothermal settings.

Finding Tips

Look in Volcanic Rocks

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

Check Hydrothermal Veins

Explore areas with evidence of past hydrothermal activity, such as altered rock zones or quartz veins.

Examine Metamorphic Terrains

For epidote, search in medium-grade metamorphic rocks like greenschists and amphibolites.

Use Proper Tools

A rock hammer, chisels, and safety glasses are essential for extracting specimens from hard rock matrices. A hand lens will help identify crystal forms.

Respect Private Property and Regulations

Always obtain permission before collecting on private land and adhere to all local and national regulations regarding mineral collecting.

Similar Rocks

Heulandite with Stilbite

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

Also known as: Zeolite Assemblage

Prehnite with Epidote

Ca2Al(AlSi3O10)(OH)2 (Prehnite), Ca2(Al,Fe3+)3(SiO4)3(OH) (Epidote)

Also known as: Hydrothermal Assemblage

Quartz with Epidote

SiO2 (Quartz), Ca2(Al,Fe3+)3(SiO4)3(OH) (Epidote)

Also known as: Hydrothermal Vein

Scientific Classification

Mineral Class: Apophyllite: Phyllosilicate. Stilbite: Tectosilicate (Zeolite Group). Epidote: Sorosilicate (Epidote Group).
Group: Apophyllite Group, Zeolite Group, Epidote Group
Crystal System: Apophyllite: Tetragonal. Stilbite: Monoclinic. Epidote: Monoclinic.
Chemical Formula: KCa4(Si8O20)(OH,F)·8H2O (Apophyllite), NaCa4(Si27Al9)O72·28H2O (Stilbite), Ca2(Al,Fe3+)3(SiO4)3(OH) (Epidote)
Composition: Complex silicates with varying amounts of potassium, calcium, sodium, aluminum, iron, and water.

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