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Granite vs Diorite: Mineral Proportions and Colour Index

Granite and diorite are coarse-grained intrusive igneous rocks, and both can look speckled. Granite is felsic and normally contains abundant quartz and alkali feldspar with fewer dark minerals. Diorite is intermediate, dominated by plagioclase with more amphibole or pyroxene, and often has a balanced salt-and-pepper appearance. Estimate mineral proportions on a fresh surface. Abundant glassy quartz plus potassium feldspar and relatively few dark minerals supports granite. Plagioclase with abundant hornblende or pyroxene, little quartz, and a stronger salt-and-pepper contrast supports diorite. Use a fresh surface and confirm that crystals are visibly interlocking; both rocks are intrusive and commonly coarse grained. Estimate the abundance of glassy gray quartz across a representative area. Clearly abundant quartz supports granite. Separate pink or cream alkali feldspar from white-gray plagioclase where possible. Alkali feldspar abundance supports granite. Estimate dark amphibole, pyroxene, and biotite. A stronger, more even salt-and-pepper mixture supports diorite. Use a modal count or thin section when the sample lies near granodiorite, tonalite, quartz diorite, or monzonite boundaries. Colour alone is unreliable. Dark granite and light diorite occur, and weathering can change every mineral's appearance. Granodiorite, tonalite, quartz diorite, and related rocks bridge the simple granite-diorite comparison; a hand sample may justify only a broader field name. Do not assign an exact intrusive-rock name from one unusually coarse crystal or a small weathered chip.

  • Granite vs Basalt — Compare granite and basalt using grain size, felsic and mafic minerals, colour index, cooling setting, field context, and porphyritic exceptions.
  • Granite vs Rhyolite — Separate granite from rhyolite using visible crystal size, shared felsic mineralogy, cooling history, porphyritic texture, flow features, and field context.
  • Gneiss vs Granite — Tell gneiss from granite using mineral banding, foliation, grain alignment, interlocking texture, cross-cutting contacts, and metamorphic context.
  • Basalt vs Andesite — Compare basalt and andesite using colour index, olivine, pyroxene, plagioclase, silica range, phenocrysts, and classification limits.
  • All geologic comparison guides — Browse the complete field comparison library.

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