How to identify Apophyllite with Stilbite and Fluorite

To correctly identify Apophyllite with Stilbite and Fluorite (KCa4(Si8O20)(F,OH)·8H2O (Apophyllite), NaCa4(Si27Al9)O72·28H2O (Stilbite), CaF2 (Fluorite)), 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, yellow, pink. Stilbite: Salmon-pink, orange, white, brown. Fluorite: Purple, green, blue, yellow, colorless, pink, black.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

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

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Apophyllite: Smooth, glassy crystal faces. Stilbite: Often radiating, sheaf-like aggregates, sometimes fibrous. Fluorite: Smooth, glassy crystal faces.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   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.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Apophyllite: Perfect basal cleavage {001}. Stilbite: Perfect cleavage {010}. Fluorite: Perfect octahedral cleavage {111}.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   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.

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.

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