BH250-90
Title
Subject
Description
Minor Minerals: titanite, zircon, apatite, iron-titanium oxide, sericite (alteration)
Mineralogy comments: apatite visible inside an amphibole
Diorite is a common coarse-grained intrusive igneous rock that lies compositionally between granite and diorite. It forms when magma of intermediate composition slowly cools and crystallizes beneath the Earth’s surface, typically within continental crustal settings.
BH250-90 is an excellent example of a dioritic rock in both appearance (salt and pepper) and chemistry. Granodiorite’s mineral assemblage is dominated by:
Quartz: Typically 20–30% of the rock, providing a glassy component.
Plagioclase feldspar: Usually the dominant feldspar, commonly andesine to oligoclase in composition.
Alkali feldspar: Present in lesser amounts than in granite, often orthoclase or microcline.
Biotite mica: A common accessory mafic mineral that gives granodiorite its characteristic dark spots.
Hornblende: Often present, sometimes accompanied by other amphiboles.
Accessory minerals: Magnetite, zircon, apatite, sphene (titanite), and occasionally garnet.
The texture is typically phaneritic (coarse-grained), with well-formed crystals visible to the naked eye. The color ranges from light gray to grayish pink, punctuated by dark biotite and hornblende spots.