How to identify Diorite

To correctly identify Diorite (Diorite), 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

   Typically dark gray to black with white speckles, giving a 'salt-and-pepper' appearance. The dark minerals (hornblende, biotite) are black or dark green, while the plagioclase feldspar is white to light gray.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Vitreous to sub-vitreous on fresh surfaces of individual mineral grains.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Phaneritic (coarse-grained), meaning individual mineral crystals are visible to the naked eye. Grains are typically interlocking.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Anhedral to subhedral crystals, forming an interlocking mosaic.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Minerals within diorite (e.g., hornblende, plagioclase) exhibit characteristic cleavage planes. Hornblende has two cleavage planes at approximately 56° and 124°, while plagioclase has two at nearly 90°.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Intrusive igneous bodies (plutons, dikes, sills) in continental arc settings, subduction zones, and orogenic belts. Often found in association with granodiorite and tonalite.

Key Facts

Hardness

6-7 on Mohs scale (due to constituent minerals like feldspar and hornblende)

Specific Gravity

2.8-3.0 g/cm³

Crystal System

Not applicable for a rock; constituent minerals have their own crystal systems (e.g., plagioclase is triclinic, hornblende is monoclinic).

Color

Dark gray to black with white speckles

Luster

Vitreous to sub-vitreous

Transparency

Opaque (as a rock)

Fracture

Irregular to conchoidal (on individual mineral grains)

Cleavage

Present in constituent minerals (e.g., plagioclase, hornblende)

Composition

Predominantly plagioclase feldspar (typically andesine, 35-65% by volume) and ferromagnesian minerals (hornblende, biotite, pyroxene, 30-50% by volume). Minor quartz (<5-10%) and K-feldspar (<5-10%) may be present. Accessory minerals include magnetite, ilmenite, apatite, and zircon.

Physical characteristics

• Crystal Habit: Granular, interlocking anhedral to subhedral crystals.
• Cleavage Type: Good to perfect in constituent minerals (e.g., plagioclase: 2 directions at ~90°; hornblende: 2 directions at ~56° and ~124°).
• Fracture Type: Irregular to sub-conchoidal.
• Tenacity: Brittle.
• Luster Type: Vitreous to sub-vitreous.

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