igneous
Iron-stained weathered granite or granitic gneiss
Granite/granitic gneiss, dominated by quartz, feldspar, and mica, with iron-oxide weathering
AI-generated identification · may be incorrectCoarse-grained, reddish-brown surface with gray crystalline patches and dark mica; quartz is Mohs 7, feldspar 6–6.5, and mica 2–3. Luster is generally vitreous to dull from weathering; cleavage occurs in feldspar and mica, while quartz lacks cleavage.
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Physical properties
Coarse-grained, reddish-brown surface with gray crystalline patches and dark mica; quartz is Mohs 7, feldspar 6–6.5, and mica 2–3. Luster is generally vitreous to dull from weathering; cleavage occurs in feldspar and mica, while quartz lacks cleavage.
Formation & geological history
Formed by slow cooling of silica-rich magma, likely during Precambrian or Paleozoic continental crust formation; the reddish coating results from oxidation of iron-bearing minerals during surface weathering. The exact rock could be granite or weakly foliated granitic gneiss.
Uses & applications
Commonly used as dimension stone, crushed aggregate, building stone, and decorative landscaping rock; uncommonly collected as a specimen.
Geological facts
Rust-red coloration is typically produced by hematite and goethite films rather than the rock’s primary composition. Exposed surfaces may weather much faster than fresh gray interior material.
Field identification & locations
Look for interlocking quartz and feldspar grains, visible mica, and a hard crystalline texture; a streak test is usually non-diagnostic. Common in continental shields, mountain belts, and granitic terrains worldwide.
