igneous
Iron-stained quartz-rich breccia (probable)
Hydrothermal quartz–iron oxide breccia; mainly SiO₂ quartz with iron oxides/hydroxides (hematite/goethite)
AI-generated identification · may be incorrectTan to reddish-brown, fine-grained and strongly fractured, with a pale gray-white quartz vein or cement. Likely hardness ~6–7 where quartz dominates, earthy to dull luster, irregular massive structure, and no obvious cleavage; iron-rich areas may be softer and crumbly.
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Physical properties
Tan to reddish-brown, fine-grained and strongly fractured, with a pale gray-white quartz vein or cement. Likely hardness ~6–7 where quartz dominates, earthy to dull luster, irregular massive structure, and no obvious cleavage; iron-rich areas may be softer and crumbly.
Formation & geological history
Likely formed when silica-rich hydrothermal fluids filled and cemented fractures in an older rock, followed by oxidation of iron-bearing minerals. Exact age cannot be determined visually; northeastern Minnesota commonly contains Precambrian volcanic, intrusive, and hydrothermal rocks.
Uses & applications
Primarily a collector specimen or lapidary material; quartz-rich pieces may take a polish, though this specimen’s fractures make it unsuitable for high-quality gemstones or construction aggregate.
Geological facts
The contrasting gray-white material is probably quartz, while orange-brown coloration commonly reflects goethite, limonite, or hematite. A magnet test may detect magnetite if present, but iron staining alone is not magnetic.
Field identification & locations
A useful field identification is the angular breccia texture, hard glassy quartz, and reddish iron staining. Near 46.7°N, 92.0°W, this is geologically plausible in the Precambrian rocks of the Duluth–Lake Superior region; acid testing and hardness testing would improve identification.
