igneous

Feldspar Rich Granite with Limonite Staining

K-Feldspar Granite (KAlSi3O8)

Hardness: 6.0-6.5 (Mohs scale), Color: Creamy white to pale gray with yellowish-orange iron staining, Luster: Vitreous to pearly, Crystal structure: Monoclinic or triclinic, Cleavage: Two directions at nearly 90 degrees.

Hardness: 6

Identified January 28, 2026More igneous →

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Physical properties

Formation & geological history

Formed through the slow cooling of silica-rich magma deep within the Earth's crust (plutonic). The specimen shows signs of chemical weathering where iron-bearing minerals have oxidized into limonite.

Uses & applications

Used extensively as dimensional stone in construction, countertops, and floor tiles. Finely ground feldspar is vital in the ceramics and glass manufacturing industries as a flux.

Geological facts

Granite makes up a significant portion of the Earth's continental crust. The yellow-orange 'rust' seen on this specimen is likely limonite, an iron oxide that forms when groundwater reacts with iron-rich minerals like biotite or pyrite.

Field identification & locations

Identify in the field by its interlocking crystalline texture and hard surface that cannot be scratched by a steel knife. Commonly found in mountainous regions or shield areas like the Appalachians or the Canadian Shield.

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Blue Apatite

Fluorapatite (Ca5(PO4)3F)

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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)

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Sodalite

Sodalite - Na8(Al6Si6O24)Cl2

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Blue Apatite

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

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Magnetite

Magnetite (Fe3O4)

mineral