Hornblende

Lore & Background

Hornblende is part of the calcium-amphibole group and includes solid solution series such as magnesiohornblende–ferrohornblende, tschermakite–ferrotschermakite, edenite–ferroedenite, pargasite–ferropargasite, and magnesiohastingstite–hastingsite. Different chemical types are almost impossible to distinguish even by optical or X-ray methods, requiring detailed chemical analysis using an electron microprobe. There is a solid solution series between hornblende and tremolite–actinolite at elevated temperature, but a miscibility gap exists at lower temperatures, often resulting in exsolution lamellae of grunerite. Hornblende is a common constituent of many igneous and metamorphic rocks such as granite, syenite, diorite, gabbro, basalt, andesite, gneiss, and schist. It crystallizes in preference to pyroxene minerals from cooler magma richer in silica and water. It is the principal mineral of amphibolites, which form during medium- to high-grade metamorphism of mafic to intermediate igneous rock in the presence of pore water. Hornblende alters easily to chlorite, biotite, or other mafic minerals. A rare variety of hornblende contains less than 5% iron oxide, is gray to white, and is named edenite from its locality in Edenville, Orange County, New York. Oxyhornblende is a variety in which most iron has been oxidized to the ferric state, enriched in titanium, found almost exclusively in volcanic rock, and sometimes called basaltic hornblende.

Reader's Guide

Hornblende is significant as a widespread and compositionally complex mineral series that serves as a key indicator in petrology for understanding the conditions of igneous and metamorphic rock formation. Its presence in rocks such as granite, basalt, and amphibolite helps geologists infer the cooling history, water content, and silica saturation of magmas, as well as the metamorphic grade of host rocks. The mineral's high variability in composition, including substitutions of titanium, manganese, chromium, and halogens, makes it a valuable subject for geochemical studies using electron microprobe analysis. Hornblende's tendency to alter to chlorite or biotite provides clues about retrograde metamorphic processes. The term 'hornblende' itself, derived from German words meaning 'horn' and 'deceive,' reflects its historical confusion with metal-bearing ores. Overall, hornblende is a fundamental mineral for interpreting the petrogenesis of many common rock types.

Did You Know?

• Hornblende is not a recognized mineral in its own right but a general term for a dark amphibole.
• A rare variety of hornblende containing less than 5% iron oxide is gray to white and named edenite from Edenville, Orange County, New York.
• Oxyhornblende is a variety enriched in titanium, found almost exclusively in volcanic rock, and sometimes called basaltic hornblende.