How to identify Apophyllite with Stilbite and Epidote

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

   Apophyllite: Colorless, white, pale green, sometimes pink. Stilbite: Salmon-pink, orange, white, yellowish-brown. Epidote: Pistachio green, yellowish-green, dark green, brownish-green.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Apophyllite: Vitreous to pearly on cleavage surfaces. Stilbite: Vitreous to pearly. Epidote: Vitreous.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Apophyllite: Smooth, often with distinct crystal faces. Stilbite: Often fibrous or radiating aggregates, sometimes platy. Epidote: Prismatic crystals, sometimes granular.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   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.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Apophyllite: Perfect basal cleavage {001}. Stilbite: Perfect cleavage {010}. Epidote: Perfect cleavage {001}.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   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.

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.

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