How to identify Okenite on Apophyllite

To correctly identify Okenite on Apophyllite (Okenite (Ca10Si18O46·18H2O) on Apophyllite group minerals (KCa4(Si4O10)2F·8H2O)), 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

   Okenite is typically white, sometimes with a very faint yellowish or bluish tint. Apophyllite is commonly colorless, white, or pale green, but can also be pink, yellow, or brown.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Okenite has a silky to pearly luster. Apophyllite has a vitreous (glassy) luster, sometimes pearly on cleavage surfaces.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Okenite forms delicate, radiating fibrous aggregates, often described as 'cotton balls' or 'snowballs'. Apophyllite forms distinct, blocky, tabular, or pyramidal crystals with smooth faces.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Okenite: Spherical or hemispherical aggregates of acicular (needle-like) crystals. Apophyllite: Tetragonal or orthorhombic crystals, often pseudo-cubic, tabular, or pyramidal.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Okenite: Perfect on {100}. Apophyllite: Perfect basal cleavage on {001}.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Both minerals are typically found in the vesicles and amygdules of basaltic lava flows, often associated with other zeolites and secondary minerals formed by hydrothermal alteration.

Key Facts

Hardness

Okenite: 5 (Mohs). Apophyllite: 4.5-5 (Mohs).

Specific Gravity

Okenite: 2.3-2.4. Apophyllite: 2.3-2.5.

Crystal System

Okenite: Triclinic. Apophyllite: Tetragonal (most common, e.g., Fluorapophyllite-K) or Orthorhombic (e.g., Hydroxyapophyllite-K).

Color

Okenite: White. Apophyllite: Colorless, white, pale green, pink, yellow, brown.

Luster

Okenite: Silky to pearly. Apophyllite: Vitreous, pearly on cleavage.

Transparency

Okenite: Translucent to opaque. Apophyllite: Transparent to translucent.

Fracture

Okenite: Uneven. Apophyllite: Uneven to conchoidal.

Cleavage

Okenite: Perfect on {100}. Apophyllite: Perfect basal cleavage on {001}.

Composition

Okenite: Hydrated calcium silicate. Apophyllite: Hydrated potassium calcium fluorosilicate (or hydroxysilicate).

Physical characteristics

• Crystal Habit: Okenite: Spherical or hemispherical aggregates of radiating acicular crystals, often resembling cotton balls. Apophyllite: Tabular, prismatic, or pyramidal crystals, often with well-developed faces.
• Cleavage Type: Okenite: Perfect in one direction. Apophyllite: Perfect basal cleavage.
• Fracture Type: Okenite: Uneven. Apophyllite: Uneven to conchoidal.
• Tenacity: Okenite: Brittle, very delicate. Apophyllite: Brittle.
• Luster Type: Okenite: Silky to pearly. Apophyllite: Vitreous, pearly on {001} cleavage.

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