igneous
Pegmatitic granite with tourmaline and iron staining
Granitic pegmatite; primarily quartz (SiO₂), feldspar, mica, and black tourmaline (schorl, NaFe₃Al₆(BO₃)₃Si₆O₁₈(OH)₄)
AI-generated identification · may be incorrectCoarse, interlocking crystals with cream feldspar, translucent quartz, reddish iron oxides, and black tourmaline; generally Mohs 6–7, vitreous to pearly luster, massive texture, and feldspar cleavage. Exact identification requires fresh surfaces or testing.
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Physical properties
Coarse, interlocking crystals with cream feldspar, translucent quartz, reddish iron oxides, and black tourmaline; generally Mohs 6–7, vitreous to pearly luster, massive texture, and feldspar cleavage. Exact identification requires fresh surfaces or testing.
Formation & geological history
Formed from the final, volatile-rich melt of a cooling granite intrusion, allowing exceptionally large crystals to grow; typically Precambrian to Phanerozoic, depending on the host intrusion.
Uses & applications
Used mainly as a collector specimen; pegmatites may supply feldspar, quartz, mica, lithium, tantalum, beryl, and gem tourmaline, while sound granite is used as dimension stone.
Geological facts
The black grains are likely schorl tourmaline, commonly associated with granitic pegmatites; red-brown coloration is usually hematite or limonite weathering.
Field identification & locations
Common in granitic terrains worldwide, including Brazil, Madagascar, the United States, and Afghanistan. Look for very coarse crystals, glassy quartz, blocky feldspar, and tourmaline prisms; a streak, hardness, and close inspection can confirm minerals.
