igneous
Glacial Granite with Epidote
Granite featuring Epidote (Ca2(Al2,Fe3+)(SiO4)(Si2O7)O(OH))
Hardness: 6-7 (Mohs scale); Color: Reddish-brown matrix with pistachio-green mineral inclusions; Luster: Vitreous to dull; Crystal Structure: Monoclinic (epidote) in phaneritic matrix; Cleavage: Perfect in one direction (epidote).
- Hardness
- 6-7 (Mohs scale)
- Color
- Reddish-brown matrix with pistachio-green mineral inclusions
- Luster
- Vitreous to dull
Identified More igneous →
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Reddish-brown matrix with pistachio-green mineral inclusions; Luster: Vitreous to dull; Crystal Structure: Monoclinic (epidote) in phaneritic matrix; Cleavage: Perfect in one direction (epidote).
Formation & geological history
Formed from slow-cooling magma at depth (intrusive igneous). While the coordinates are in the sedimentary Upper Mississippi Valley Lead-Zinc District (Ordovician bedrock), this specimen is likely a glacial erratic transported from the Canadian Shield during the Pleistocene epoch.
Uses & applications
Used primarily as decorative stone, construction aggregate (unprocessed), or as a geological educational specimen.
Geological facts
The green mineral is epidote, likely formed via hydrothermal alteration of the granite's feldspars or mafic minerals. Finding such igneous rocks in eastern Iowa/western Illinois is common due to past glacial advances.
Field identification & locations
Identify in the field by the 'pistachio green' color of the epidote against the harder, redder granite matrix. Often found in till or stream beds in the Driftless Area and surrounding glaciated regions.
