Greenstone

Description

Greenstone is a field term for any compact, dark green, altered mafic igneous rock that has undergone metamorphism under greenschist facies conditions. It is not a specific mineral but rather a rock type characterized by its mineral assemblage and color. The green color is imparted by the abundance of chlorite, actinolite, and epidote. It typically exhibits a fine-grained to medium-grained texture and can range from massive to foliated (schistose) depending on the degree of deformation during metamorphism. Pillow structures, amygdales, and other primary volcanic textures may be preserved if metamorphism was not intense.

How to Identify

Color

Typically dark green to greenish-black, often mottled or speckled with lighter green or white minerals.

Luster

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

Texture

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.

Crystal Form

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

Cleavage

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

Geological Environment

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.

Quick Check

• Color: Dark green to greenish-black
• Luster: Dull to sub-vitreous
• Streak: Greenish-white to light green

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.

Formation

Greenstone forms from the low-grade regional metamorphism of mafic igneous rocks, primarily basalt, gabbro, or other mafic volcanic/intrusive rocks. This metamorphism occurs under conditions of relatively low temperature (300-500 °C) and moderate pressure (2-10 kbar), characteristic of the greenschist facies. The original minerals (e.g., pyroxene, plagioclase) are replaced by a characteristic assemblage of hydrous, green-colored minerals such as chlorite, actinolite, epidote, and albite. Quartz and calcite may also be present.

Usage

Historically, greenstone has been used for tools, weapons (e.g., adzes, axes), and ornamental carvings by various indigenous cultures due to its toughness and workability. In modern times, it is sometimes used as a decorative stone, aggregate, or for specific industrial applications where its hardness and resistance to weathering are beneficial. Greenstone belts are also significant hosts for various ore deposits, including gold, nickel, copper, and zinc.

Age Distribution

Predominantly Archean to Proterozoic, but can form in any age where basaltic rocks undergo greenschist facies metamorphism.

Where to Find

Finding Tips

Similar Rocks

Scientific Classification

Mineral Class

Not a mineral, but a rock.

Group

Metamorphic rock (Greenschist facies)

Crystal System

Not applicable for a rock.

Chemical Formula

Variable, reflecting the mineral assemblage (e.g., (Mg,Fe)3(Si,Al)4O10(OH)2·(Mg,Fe)3(OH)6 for chlorite; Ca2(Mg,Fe)5Si8O22(OH)2 for actinolite).

Composition

Silicate minerals, primarily hydrous ferromagnesian silicates (chlorite, actinolite) and calcium-aluminum silicates (epidote, albite).

Explore Greenstone