How to identify Quartz with Pyrite and Calcite

To correctly identify Quartz with Pyrite and Calcite (SiO2 (Quartz), FeS2 (Pyrite), CaCO3 (Calcite)), 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

   Quartz: Colorless, white, milky, or smoky. Pyrite: Brassy yellow, metallic. Calcite: White, colorless, gray, or various pastel shades.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Quartz: Vitreous (glassy). Pyrite: Metallic. Calcite: Vitreous to sub-vitreous, sometimes pearly on cleavage surfaces.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Crystalline, often with distinct crystal forms of each mineral. Quartz can be massive or euhedral. Pyrite typically forms distinct cubes or other isometric forms. Calcite can be massive, granular, or well-crystallized.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Quartz: Hexagonal prisms with pyramidal terminations. Pyrite: Isometric, commonly cubes, octahedra, or pyritohedra, often striated. Calcite: Hexagonal-rhombohedral, commonly rhombohedra, scalenohedra, or tabular forms.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Quartz: None (conchoidal fracture). Pyrite: Poor or indistinct cubic cleavage. Calcite: Perfect rhombohedral cleavage in three directions, producing rhomb-shaped fragments.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Hydrothermal veins, often associated with fault zones, shear zones, and intrusive igneous bodies. Found in various ore deposits (gold, silver, base metals) and metamorphic terrains.

Key Facts

Hardness

Quartz: 7 (Mohs); Pyrite: 6-6.5 (Mohs); Calcite: 3 (Mohs).

Specific Gravity

Quartz: 2.65 g/cm³; Pyrite: 4.95-5.10 g/cm³; Calcite: 2.71 g/cm³.

Crystal System

Quartz: Trigonal; Pyrite: Isometric; Calcite: Trigonal.

Color

Quartz: Colorless, white, milky; Pyrite: Brassy yellow; Calcite: White, colorless.

Luster

Quartz: Vitreous; Pyrite: Metallic; Calcite: Vitreous to sub-vitreous.

Transparency

Quartz: Transparent to translucent; Pyrite: Opaque; Calcite: Transparent to translucent.

Fracture

Quartz: Conchoidal; Pyrite: Uneven to conchoidal; Calcite: Conchoidal to uneven.

Cleavage

Quartz: None; Pyrite: Poor/indistinct cubic; Calcite: Perfect rhombohedral (3 directions at 74° and 106°).

Composition

Quartz: Silicon dioxide (SiO2); Pyrite: Iron disulfide (FeS2); Calcite: Calcium carbonate (CaCO3).

Physical characteristics

• Crystal Habit: Quartz: Prismatic, massive, granular; Pyrite: Cubic, octahedral, pyritohedral, massive; Calcite: Rhombohedral, scalenohedral, tabular, massive, granular.
• Cleavage Type: Quartz: None; Pyrite: Poor/indistinct; Calcite: Perfect rhombohedral.
• Fracture Type: Quartz: Conchoidal; Pyrite: Uneven to conchoidal; Calcite: Conchoidal to uneven.
• Tenacity: Quartz: Brittle; Pyrite: Brittle; Calcite: Brittle.
• Luster Type: Quartz: Vitreous; Pyrite: Metallic; Calcite: Vitreous to sub-vitreous.

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