Mineral - Opal
Opal
Chemical Properties
Mineral Class
Silicates
Toxicity
none
Formula
SiO₂·nH₂O
Unit Cell
Physical Properties
Hardness
5.5 - 6.5
Streak
white
Density
Normal (2-3,5)
Cleavage
Fracture
conchoidal,
splintery,
uneven
Tenacity
Brittle
Optical Properties
Color
browngrayredorangeyellowgreenbluecolorless
Birefringence
Luster
Greasy
Transparency
translucent,
transparent,
opaque
Pleochroism
Geomineralogical Properties
Crystal System
amorphous
Point Group
Space Group
Habit
massive
Formation
(1) Through the crystallization of amorphous silica from supersaturated, hydrothermal, or meteoric waters as a result of cooling or evaporation, usually within cavities in volcanic rocks or sediments.
(2) Through biogenic accumulation and diagenesis of skeletal substances from siliceous organisms (e.g., diatoms).
(3) As pseudomorphic replacement of organic or mineral structures in weathering zones.
Paragenesis
Twinning
Rarity
Common
Synonyms
Opal