How to identify Greenstone

To correctly identify Greenstone (Metabasalt or Metavolcanic Rock), 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 greenish-black, often mottled or speckled with lighter green or white minerals.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to sub-vitreous, depending on the dominant minerals and grain size.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Fine-grained to medium-grained, often massive, but can be schistose (foliated) if deformation was significant. Relict textures like pillow structures or amygdales may be visible.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Individual crystals are usually anhedral to subhedral, forming an interlocking mosaic. Macroscopic crystal forms are rare.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Poor to absent in massive varieties; schistose varieties may exhibit a well-developed foliation (schistosity) due to the alignment of platy minerals like chlorite.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Found in ancient cratonic areas, particularly in Archean greenstone belts, which represent metamorphosed oceanic crust and volcanic arcs. Also occurs in orogenic belts where mafic rocks have undergone low-grade regional metamorphism.

Key Facts

Hardness

5-6 on the Mohs scale (due to constituent minerals like actinolite, epidote, albite)

Specific Gravity

2.8-3.1 g/cm³

Crystal System

Not applicable for a rock; constituent minerals are monoclinic (chlorite, actinolite, epidote) or triclinic (albite).

Color

Dark green, greenish-black

Luster

Dull to sub-vitreous

Transparency

Opaque

Fracture

Conchoidal to uneven, tough

Cleavage

Poor to absent (massive varieties); good schistosity (foliated varieties)

Composition

Predominantly chlorite, actinolite, epidote, albite, with minor quartz, calcite, sphene, and magnetite. Derived from mafic igneous protoliths.

Physical characteristics

• Crystal Habit: Not applicable for a rock; constituent minerals are typically anhedral to subhedral, forming a fine-grained aggregate.
• Cleavage Type: Not applicable for a rock; constituent minerals have distinct cleavages (e.g., chlorite: perfect basal; actinolite: two at 56° and 124°).
• Fracture Type: Conchoidal to uneven, splintery in some foliated varieties.
• Tenacity: Tough, resistant to breaking.
• Luster Type: Dull to sub-vitreous, sometimes greasy on fresh surfaces.

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