Komatiite is a rare and distinctive ultramafic volcanic rock, characterized by its unusually high magnesium content and unique physical properties, often indicative of its formation from extremely hot magmas in the early Earth.
Physical Properties of Komatiite
Komatiites exhibit a range of physical properties that differentiate them from most other volcanic rocks, primarily due to their unique chemical composition (very high MgO, low SiO₂, K₂O, and Na₂O) and rapid cooling at high temperatures.
Key Characteristics
One of the most defining physical characteristics of komatiite is its spinifex texture, which is a distinctive arrangement of large, often blade-like, skeletal, or dendritic olivine and pyroxene crystals. This texture forms during rapid cooling of highly superheated komatiitic lavas.
Here's a breakdown of its primary physical attributes:
| Property | Description |
|---|---|
| Color | Fresh komatiite typically ranges from very dark green to black. Due to alteration and weathering processes, it often develops brown, reddish-brown, or yellowish-brown hues on its surface, as iron-bearing minerals oxidize. |
| Texture | Predominantly characterized by spinifex texture, appearing as long, radiating, or randomly oriented bladed crystals (often olivine or pyroxene) set in a finer-grained or glassy matrix. Other textures include massive (fine-grained to porphyritic) or cumulate (layered accumulations of early-formed crystals). |
| Hardness | The hardness of komatiite varies significantly depending on its mineralogy and degree of alteration. Fresh olivine, a primary mineral, is relatively hard (around 6.5-7 on the Mohs scale). However, komatiites are often extensively altered to softer serpentine, talc, or chlorite, which would reduce the overall rock hardness. |
| Density/Specific Gravity | Komatiites have a relatively high density and specific gravity due to their abundant heavy magnesium-iron silicates. Fresh samples typically have a specific gravity in the range of 3.0 to 3.3 g/cm³. |
| Luster | The luster of komatiite is generally dull to vitreous on fresh surfaces, reflecting the presence of silicate minerals. Altered surfaces may appear more earthy or dull. |
| Structure | Can occur as massive flows, pillow lavas (indicating subaqueous eruption), or as intrusive sills and dikes. Lava channels and thermal erosion features are also common. |
Associated Minerals and Alteration Products
Komatiites are highly susceptible to alteration due to their unstable mineralogy (especially olivine) when exposed to lower temperatures and aqueous conditions. This alteration often transforms the primary minerals into secondary minerals like:
- Serpentine: A common alteration product, giving the rock a greasy feel.
- Talc: Another soft, secondary mineral often found in altered komatiites.
- Chlorite: A green, flaky mineral.
- Carbonates: Such as magnesite.
Due to their significant iron content, komatiites frequently undergo oxidation upon weathering, leading to the formation of various iron oxides. One common iron oxide is hematite, which exhibits distinct physical properties:
| Property | Description |
|---|---|
| Chemical Composition | Fe₂O₃ - Ferric Iron oxide |
| Hardness | Generally 5-6 (hard) in most varieties, though earthy forms can be very soft (<2). |
| Specific Gravity | 5.3 (feels notably heavy, which is typical for metallic minerals). |
| Luster | Ranges widely from splendent metallic to a dull earthy appearance. |
| Streak | Distinctive red-brown to rusty red. |
The presence of these alteration products, including iron oxides like hematite, significantly impacts the overall physical appearance and properties of weathered komatiite outcrops.
