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Hornblende

A green to black amphibolic mineral, CaNa(Mg,Fe)₄(Al,Fe,Ti)₃Si₆O₂₂(OH,F)₂, formed in the late stages of cooling in igneous rock. Hornblende is the most common example of the group of silicates called amphibole. The sample above left is about 12x8 cm and is from Ontario, Canada.

Hornblende has two cleavage directions that intersect at roughly 60° and 120°. Fracture uneven. It is usually dark green to black in color and is similar to augite. It is commonly found in continental rocks which are igneous in origin. It often makes up the dark part of rocks which are composed of light and dark crystals. Prismatic crystals, often with hexagonal cross section and twinning. Also in massive, fibrous, bladed, columnar, and granular aggregates.

Hornblende is a common rock-forming mineral in medium- and high-grade metamorphic rocks, particularly those of mafic and ultramafic composition. In mafic rocks, it first appears in the upper part of the low grade by a chemical reaction involving the disappearance of actinolite, a nonaluminous calcic amphibole. This change is extremely noticeable in thin sections, very pale-green actinolite giving way to blue-green hornblende. With prograde metamorphism, the composition of the hornblende gradually changes in a highly complex manner that is a function of temperature, pressure, oxygen fugacity (a measure of the activity of oxygen), and the chemical composition of the rock. This causes a gradual color change from blue-green through various shades of green to olive green and brown. At the middle of the high grade, hornblende becomes unstable and breaks down to form pyroxene (plus other minerals). The prominence of hornblende in medium-grade metabasic rocks has led to these rocks being called amphibolites.

The name comes from a old German miners word, "horn", which may be related to the color of horn, and the German word "blenden", which means "to deceive". It looked like many of the other mineral ores which could be smelted to produce metals, but hornblende failed to be smelted. Often black amphiboles are identified as hornblende without x-ray or microprobe verification. Hornblende is a very common mineral found in many geologic environments. Hornblende is common as small needle-like grains with plagioclase and biotite.

Material Notes:

Two cleavage directions. Weathering end product(s): Brown clay, limonite. An important rock-forming mineral that occurs in igneous (granite, granodiorite, gabbro) and metamorphic (granite-gneiss, horneblende-dschist, greenstone) rocks. Weathers slowly; Gives strength and toughness to rocks.

Hornblende generally forms as stubby, prismatic crystals. It can also occur in massive, compact, granular, columnar, or fibrous habits. Its color ranges from green to greenish-brown to black. A common rock-forming mineral, hornblende can be found in granite, basalt, diorite, schist, and gneiss.

Although hornblende is a common mineral that is found in various rock types, it has almost no commercial value. It is sometimes mistaken for black tourmaline, but tourmaline cleaves poorly. Hornblende and actinolite are both amphiboles. Actinolite is found in metamorphosed mafic to intermediate rocks.

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