How to identify Gabbro

To correctly identify Gabbro (Gabbro), 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 green to black, often with a speckled appearance due to the presence of lighter plagioclase and darker mafic minerals.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Vitreous to dull, depending on the mineral components and weathering.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Phaneritic (coarse-grained), meaning individual mineral crystals are visible to the naked eye, typically 1 mm to 5 mm in size, but can be larger in pegmatitic varieties.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Anhedral to subhedral interlocking crystals of plagioclase and pyroxene.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Minerals within gabbro (plagioclase, pyroxene) exhibit distinct cleavage planes. Plagioclase has two good cleavages at nearly 90 degrees, and pyroxene has two good cleavages at approximately 90 degrees.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Forms in the lower oceanic crust, within large layered mafic intrusions, and as sills and dikes in continental settings. It is a fundamental component of ophiolite sequences, representing ancient oceanic crust.

Key Facts

Hardness

6-7 on Mohs scale (due to constituent minerals like plagioclase and pyroxene)

Specific Gravity

2.9-3.2 g/cm3

Crystal System

Not applicable for a rock; constituent minerals are typically monoclinic (pyroxene, hornblende) or triclinic (plagioclase).

Color

Dark green to black, often speckled.

Luster

Vitreous to dull.

Transparency

Opaque (as a rock); individual mineral grains can be translucent to opaque.

Fracture

Conchoidal to uneven (for individual minerals); rock typically exhibits irregular fracture.

Cleavage

Present in constituent minerals (e.g., plagioclase: 2 directions at ~900; pyroxene: 2 directions at ~900).

Composition

Primarily calcic plagioclase feldspar (35-65%) and pyroxene (20-50%), with minor olivine, hornblende, and iron-titanium oxides. Quartz is typically absent or present in very minor amounts (<5%).

Physical characteristics

• Crystal Habit: Anhedral to subhedral interlocking granular texture (phaneritic).
• Cleavage Type: Good to perfect in constituent minerals (e.g., plagioclase, pyroxene).
• Fracture Type: Irregular to subconchoidal.
• Tenacity: Brittle.
• Luster Type: Vitreous to dull.

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