How to identify Apophyllite with Stilbite

To correctly identify Apophyllite with Stilbite (Apophyllite Group (KCa4(Si8O20)(F,OH)·8H2O) and Stilbite-Ca (NaCa2(Si7Al2)O18·7H2O)), 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, green, yellow, pink, brown. Stilbite: White, pink, orange, brown, yellowish-brown.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

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

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Apophyllite: Smooth, glassy crystal faces. Stilbite: Often fibrous or radiating aggregates, sometimes with a silky feel.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Apophyllite: Tetragonal, typically forming pseudo-cubic, pyramidal, or tabular crystals. Stilbite: Monoclinic, commonly forming sheaf-like aggregates, bow-tie shapes, or tabular crystals.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

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

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Vesicles and amygdules in basaltic and other mafic volcanic rocks, hydrothermal veins, and less commonly in granitic pegmatites.

Key Facts

Hardness

Apophyllite: 4.5-5; Stilbite: 3.5-4

Specific Gravity

Apophyllite: 2.3-2.5; Stilbite: 2.1-2.2

Crystal System

Apophyllite: Tetragonal; Stilbite: Monoclinic

Color

Apophyllite: Colorless, white, green, yellow, pink; Stilbite: White, pink, orange, brown

Luster

Vitreous to pearly

Transparency

Transparent to translucent

Fracture

Apophyllite: Uneven; Stilbite: Uneven to subconchoidal

Cleavage

Apophyllite: Perfect basal {001}; Stilbite: Perfect {010}

Composition

Apophyllite: Hydrated potassium calcium silicate fluoride/hydroxide; Stilbite: Hydrated sodium calcium aluminum silicate

Physical characteristics

• Crystal Habit: Apophyllite: Pseudo-cubic, pyramidal, tabular, massive. Stilbite: Sheaf-like, radiating, tabular, fibrous, granular.
• Cleavage Type: Apophyllite: Perfect in one direction (basal). Stilbite: Perfect in one direction.
• Fracture Type: Apophyllite: Uneven. Stilbite: Uneven to subconchoidal.
• Tenacity: Brittle for both.
• Luster Type: Vitreous to pearly.

Explore Apophyllite with Stilbite