igneous

Granodiorite

Granodiorite (phaneritic intrusive igneous rock)

Hardness: 6-7 Mohs; Color: Grayish-white with dark specks (mottled appearance); Luster: Vitreous to dull; Crystal structure: Phaneritic (coarsegrained), containing plagioclase feldspar, quartz, and phlogopite/biotite; Specific Gravity: 2.6-2.7.

Hardness: 6-7 Mohs
Color: Grayish-white with dark specks (mottled appearance)
Luster: Vitreous to dull

Identified March 2, 2026More igneous →

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Physical properties

Formation & geological history

Formed through the slow cooling of silica-rich magma beneath the Earth's surface (plutonic). It is typically found in large batholiths associated with mountain-building events and subduction zones throughout various geological ages from the Archean to the Cenozoic.

Uses & applications

Extensively used in construction as crushed stone for road base, concrete aggregate, and railroad ballast. When polished, it is used as ornamental 'granite' for countertops, floor tiles, and grave markers.

Geological facts

The famous Rosetta Stone, which was key to deciphering Egyptian hieroglyphs, was long thought to be basalt but was later correctly identified as granodiorite. It is intermediate in composition between granite and diorite.

Field identification & locations

Identify in the field by its interlocking coarse crystals. Unlike true granite, it contains more plagioclase feldspar than orthoclase (potassium) feldspar, giving it a salt-and-pepper look without the pinkish hues often seen in granite. Common in the Sierra Nevada and Andes ranges.

Other igneous specimens

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Blue Apatite

Fluorapatite (Ca5(PO4)3F)

mineral

Dark Igneous Rock (Likely Basalt or Diabase)

Basalt (extrusive igneous rock) or Diabase (intrusive igneous rock - also known as Dolerite), largely composed of plagioclase feldspar, pyroxene (augite), and sometimes olivine and amphibole. Exact mineralogy would require thin section analysis.

Igneous

Magnetite

Iron(II,III) oxide (Fe3O4)

mineral

Sodalite

Sodalite - Na8(Al6Si6O24)Cl2

mineral

Blue Apatite

Apatite (Group), Ca5(PO4)3(F,Cl,OH)

mineral

Magnetite

Magnetite (Fe3O4)

mineral