Hornblende is a complex inosilicate series of minerals, belonging to the amphibole group, which are characterized by their double chain silicate structures. Here are some key details about hornblende:
Composition and Structure
- Chemical Formula: Hornblende's general formula is (Ca,Na)2-3(Mg,Fe,Al)5(Si,Al)8O22(OH,F)2. This formula indicates a wide range of possible substitutions for various elements within the crystal structure.
- Crystal System: It crystallizes in the monoclinic system, although some varieties can also be triclinic.
- Cleavage: Two perfect cleavages intersect at about 56° and 124°.
- Color: Hornblende can range from dark green to black, brown, or even a dark blue-green.
- Luster: Its luster is vitreous to pearly on cleavage surfaces.
Geological Occurrence
History and Etymology
- The name "hornblende" derives from the German words "horn" (meaning horn) and "blenden" (meaning to blind or deceive), alluding to its typical dark color and the fact that it can be confused with other dark minerals like biotite or pyroxene.
- The mineral was first described in scientific literature in the 18th century, with its name reflecting its deceptive appearance.
Uses and Significance
- While not directly used as a gem or industrial material, hornblende is crucial in petrography and geology for rock classification and understanding geological processes.
- It is often used as a geobarometer and geothermometer due to its stability at various pressure and temperature conditions.
Sources for Further Reading
- Mindat.org - Detailed information on hornblende's mineralogy.
- Geology.com - Overview of hornblende's properties and occurrences.
- Galleries.com - Provides photos and basic information about hornblende.
- Britannica - Encyclopedic entry on hornblende.
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