How to identify Calcite with Sphalerite

To correctly identify Calcite with Sphalerite (Calcite (CaCO3) with Sphalerite ((Zn,Fe)S)), 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

   Calcite: Typically white, colorless, gray, but can be yellow, pink, blue, green, brown, or black due to impurities. Sphalerite: Commonly dark brown to black (iron-rich), but can be yellow, reddish-brown, green, or colorless (pure ZnS).

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Calcite: Vitreous to sub-vitreous, sometimes pearly on cleavage surfaces. Sphalerite: Resinous to adamantine, sometimes sub-metallic.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Crystalline, often with distinct crystal forms of both minerals visible. Calcite may show characteristic rhombohedral or scalenohedral faces. Sphalerite may show tetrahedral or dodecahedral forms, or granular masses.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Calcite: Rhombohedral, scalenohedral, prismatic, tabular, massive, granular. Sphalerite: Tetrahedral, dodecahedral, massive, botryoidal, granular.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Calcite: Perfect rhombohedral cleavage in three directions at 74° and 106°. Sphalerite: Perfect dodecahedral cleavage in six directions.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Hydrothermal veins, Mississippi Valley-Type (MVT) deposits, skarn deposits, volcanogenic massive sulfide (VMS) deposits, and sedimentary exhalative (SEDEX) deposits. Often found in carbonate host rocks (limestone, dolomite).

Key Facts

Hardness

Calcite: 3 (Mohs). Sphalerite: 3.5-4 (Mohs).

Specific Gravity

Calcite: 2.71. Sphalerite: 3.9-4.1 (increases with Fe content).

Crystal System

Calcite: Trigonal. Sphalerite: Isometric.

Color

Calcite: White, colorless, gray, various. Sphalerite: Dark brown to black, yellow, red, green.

Luster

Calcite: Vitreous. Sphalerite: Resinous to adamantine.

Transparency

Calcite: Transparent to translucent. Sphalerite: Transparent to opaque.

Fracture

Calcite: Conchoidal to uneven. Sphalerite: Conchoidal to uneven.

Cleavage

Calcite: Perfect rhombohedral (3 directions). Sphalerite: Perfect dodecahedral (6 directions).

Composition

Calcite: Calcium Carbonate (CaCO3). Sphalerite: Zinc Iron Sulfide ((Zn,Fe)S).

Physical characteristics

• Crystal Habit: Calcite: Rhombohedral, scalenohedral, prismatic, massive, granular. Sphalerite: Tetrahedral, dodecahedral, massive, botryoidal, granular.
• Cleavage Type: Calcite: Perfect rhombohedral. Sphalerite: Perfect dodecahedral.
• Fracture Type: Conchoidal to uneven for both.
• Tenacity: Calcite: Brittle. Sphalerite: Brittle.
• Luster Type: Calcite: Vitreous to sub-vitreous. Sphalerite: Resinous to adamantine.

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