Olivine in Basalt
Olivine (Forsterite-Fayalite series) in Mafic Basalt · Mineral Specimen / Volcanic Rock (Basaltic)
Crystal System
Orthorhombic for Olivine grains; Matrix is a microcrystalline igneous mass with an aphanitic/vesicular habit.
Mineral Group
Silicates (Nesosilicate subgroup for Olivine; Tectosilicate and Inosilicate for matrix)
Chemical Formula
(Mg,Fe)2SiO4; matrix primarily composed of Plagioclase Feldspar and Pyroxene
Formation Process
Igneous (Volcanic). Formed by the rapid cooling of magnesium-rich and iron-rich lava at or near the Earth's surface.
About This Crystal
The specimen features an opaque, dark charcoal to black volcanic matrix with a dull to sub-metallic luster. Embedded within are small, translucent to transparent granules of pale olive to golden-yellow olivine. These grains are typically rounded or subhedral due to rapid cooling.
Physical Characteristics
Color: Black matrix with green/yellow phenocrysts; Streak: White to pale gray; Fracture: Conchoidal (olivine) to uneven (matrix); Specific Gravity: 2.8-3.4; Fluorescence: Typically inert.
Optical Properties
Biaxial (+) or (-); Refractive Index of Olivine: 1.635–1.817; Birefringence: 0.033–0.052; No significant optical phenomena in this raw form.
Hardness & Durability
Origin Region
Notable sources include Hawaii, Iceland, Canary Islands, and Arizona (Peridot Mesa). Formed in volcanic basalt flows.
Hardness & Durability
Olivine Hardness: 6.5–7.0 on Mohs scale; Basalt Matrix Hardness: 5.0–6.0. Generally durable but the matrix can be brittle and susceptible to mechanical weathering.
Care & Maintenance
Avoid acidic cleaners which may etch the olivine. Clean with warm soapy water and a soft brush. Keep away from extreme thermal shock.
Rarity & Value
Common as a rock-forming mineral; low value for raw basaltic specimens. Value increases significantly if olivine crystals are large and clear enough to be cut into 'Peridot' gemstones.
Special Characteristics
Features vesicular texture (tiny holes from gas bubbles) and distinct chemical zoning between the magnesium-rich olivine and iron-rich host rock.
Lore & History
Historical significance as an indicator of volcanic activity. Peridot (gem-quality olivine) was valued by Ancient Egyptians as the 'gem of the sun.' In folk lore, it is believed to provide protection against nightmares and bring prosperity.