How to identify Calcite with Marcasite

To correctly identify Calcite with Marcasite (Calcite (CaCO3) and Marcasite (FeS2)), 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, yellow, brown, or gray. Marcasite: Pale bronze-yellow to off-white, often tarnishing to a darker yellow or brown.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Calcite: Vitreous to pearly. Marcasite: Metallic.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Calcite: Crystalline, often with distinct crystal faces. Marcasite: Crystalline, often forming radiating, coxcomb, or spear-shaped aggregates, sometimes botryoidal.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Calcite: Rhombohedral, scalenohedral, prismatic, tabular. Marcasite: Orthorhombic, typically forming tabular, pyramidal, or radiating crystals; often twinned.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Calcite: Perfect rhombohedral cleavage in three directions (1011). Marcasite: Indistinct cleavage, typically parting on {101} or {010}.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Sedimentary rocks (shales, limestones, coal seams), low-temperature hydrothermal veins, concretions, and geodes.

Key Facts

Hardness

Calcite: 3 (Mohs); Marcasite: 6-6.5 (Mohs)

Specific Gravity

Calcite: 2.71 g/cm³; Marcasite: 4.85-4.92 g/cm³

Crystal System

Calcite: Trigonal; Marcasite: Orthorhombic

Color

Calcite: Variable (white, colorless, yellow, brown); Marcasite: Pale bronze-yellow, off-white

Luster

Calcite: Vitreous to pearly; Marcasite: Metallic

Transparency

Calcite: Transparent to opaque; Marcasite: Opaque

Fracture

Calcite: Conchoidal to uneven; Marcasite: Uneven to conchoidal

Cleavage

Calcite: Perfect rhombohedral {1011}; Marcasite: Indistinct {101}, {010}

Composition

Calcite: Calcium Carbonate (CaCO3); Marcasite: Iron Disulfide (FeS2)

Physical characteristics

• Crystal Habit: Calcite: Rhombohedral, scalenohedral, prismatic, tabular, massive, granular; Marcasite: Tabular, pyramidal, radiating, coxcomb, spear-shaped, botryoidal, massive.
• Cleavage Type: Calcite: Perfect rhombohedral; Marcasite: Indistinct.
• Fracture Type: Calcite: Conchoidal to uneven; Marcasite: Uneven to conchoidal.
• Tenacity: Calcite: Brittle; Marcasite: Brittle.
• Luster Type: Calcite: Vitreous to pearly; Marcasite: Metallic.

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