How to identify Granite with Pyrite

To correctly identify Granite with Pyrite (Granite (igneous rock) with Pyrite (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

   Granite: typically light-colored (white, grey, pink, red) due to high quartz and feldspar content. Pyrite: brassy yellow, metallic.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Granite: vitreous (quartz), pearly to vitreous (feldspar), vitreous to submetallic (mica). Pyrite: metallic.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Granite: Phaneritic (coarse-grained), equigranular to porphyritic. Pyrite: Crystalline, often euhedral cubes, pyritohedrons, or massive aggregates.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Granite: Anhedral to subhedral interlocking crystals of quartz, feldspar, and mica. Pyrite: Commonly forms distinct cubes, octahedra, or pyritohedra, or irregular masses.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Granite: Quartz has no cleavage. Feldspar exhibits two distinct cleavages at or near 90 degrees. Mica has perfect basal cleavage. Pyrite: Indistinct cleavage, typically conchoidal to uneven fracture.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Intrusive igneous bodies (plutons, batholiths) formed in continental crust settings, often associated with orogenic belts. Pyrite can be primary magmatic or secondary hydrothermal in origin within these granitic bodies.

Key Facts

Hardness

Granite: 6-7 (Mohs, due to quartz); Pyrite: 6-6.5 (Mohs)

Specific Gravity

Granite: 2.6-2.7 g/cm³; Pyrite: 4.95-5.10 g/cm³

Crystal System

Granite: Not applicable (rock); Pyrite: Isometric

Color

Granite: White, grey, pink, red; Pyrite: Brassy yellow

Luster

Granite: Vitreous to pearly; Pyrite: Metallic

Transparency

Granite: Opaque to translucent (quartz); Pyrite: Opaque

Fracture

Granite: Uneven to conchoidal (quartz), uneven (feldspar); Pyrite: Conchoidal to uneven

Cleavage

Granite: Absent (quartz), good in two directions (feldspar), perfect basal (mica); Pyrite: Indistinct

Composition

Granite: Predominantly quartz (20-60%), alkali feldspar (35-90% of total feldspar), plagioclase feldspar (10-65% of total feldspar), micas, amphiboles. Pyrite: Iron disulfide (FeS2).

Physical characteristics

• Crystal Habit: Granite: Granular, interlocking anhedral to subhedral crystals. Pyrite: Euhedral cubes, octahedra, pyritohedra, or massive, granular aggregates.
• Cleavage Type: Granite: Quartz (none), Feldspar (two directions at ~90°), Mica (perfect basal). Pyrite: Indistinct, poor.
• Fracture Type: Granite: Uneven to conchoidal. Pyrite: Conchoidal to uneven.
• Tenacity: Granite: Brittle. Pyrite: Brittle.
• Luster Type: Granite: Vitreous, pearly, submetallic. Pyrite: Metallic.

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