Gabbro
Image: <a href="//commons.wikimedia.org/wiki/User:Zimbres" title="User:Zimbres">Zimbres</a> - Attribute it to Eurico Zimbres (CC BY-SA 3.0)
Mineral NameGabbro
Chemical FormulaComplex silicate (primarily Ca-rich plagioclase and pyroxene)
Mohs Hardness6
Streak ColorGrayish-white
CleavageNot applicable (rock composed of multiple minerals, constituent minerals like plagioclase and pyroxene exhibit good cleavage)
FractureUneven to conchoidal
Specific Gravity2.9
Colorsdark gray, black, greenish-black
TransparencyOpaque
Type LocalityDehesa, San Diego County, California
Mineral GroupIgneous Rock (Intrusive, Mafic)
Usesdimension stone, construction aggregate, decorative stone, countertops
Associated Mineralsplagioclase feldspar, pyroxene (augite), olivine, hornblende
Description
This is a polished slab of dark gray to black orbicular gabbro, characterized by numerous concentric, circular structures (orbicules) embedded in a finer-grained matrix. The orbicules display lighter rims and centers, creating a distinctive target-like pattern.
Geological Context
Gabbro is a coarse-grained, mafic intrusive igneous rock that forms from the slow cooling of magnesium- and iron-rich magma deep within the Earth's crust. Orbicular gabbro is a rare variety where minerals crystallize in spherical layers around a central nucleus, indicating unusual crystallization conditions.
