Andesite
Andesite (Igneous Volcanic Rock) · Mineral Specimen / Industrial Rock
Crystal System
Varies by component minerals (typically Monoclinic/Triclinic phenocrysts in an Aphanitic groundmass)
Mineral Group
Silicates (specifically Plagioclase Feldspar and Pyroxene/Amphibole groups)
Chemical Formula
Intermediate composition: 52–63% SiO2 with varying Al2O3, FeO, MgO, CaO, Na2O, and K2O
Formation Process
Igneous: Forms via the rapid cooling of intermediate-composition lava at or near the Earth's surface
About This Crystal
Fine-grained (aphanitic) texture, typically grey to salt-and-pepper in appearance. Features small visible crystals (phenocrysts) of plagioclase or hornblende set in a dense, matte matrix.
Physical Characteristics
Grey to medium-dark grey color; white streak; no distinct cleavage in the rock mass; sub-conchoidal to uneven fracture; specific gravity 2.4–2.8; non-fluorescent.
Optical Properties
Generally opaque in hand specimen; phenocrysts may show low birefringence and moderate refractive index under microscopic thin section.
Hardness & Durability
Origin Region
Andes Mountains (South America), Cascade Range (USA), Japan, and Iceland within volcanic arc environments
Hardness & Durability
Mohs hardness 6 to 7; high toughness and durability due to interlocking crystalline structure; excellent stability.
Care & Maintenance
Resilient to most cleaners; wash with warm soapy water; avoid concentrated acids which may etch the feldspar components.
Rarity & Value
Common; low monetary value as a specimen; primarily valued as industrial aggregate or architectural stone.
Special Characteristics
Porphyritic texture (larger crystals in fine groundmass); may contain vesicles (gas bubbles) or flow banding from volcanic movement.
Lore & History
Historically used in the construction of ancient Incan structures; traditionally associated by some practitioners with grounding and the element of Earth, though it is primarily a geological specimen.