A common rock-forming mineral found in igneous and metamorphic rocks
Article by: Hobart M. King, Ph.D., RPG
What is Hornblende?
Hornblende is a field and classroom name used for a group of dark-colored amphibole minerals found in many types of igneous and metamorphic rocks. These minerals vary in chemical composition but are all double-chain inosilicates with very similar physical properties. A generalized composition for the hornblende group is shown below.
Note that calcium, sodium, magnesium, iron, aluminum, silicon, fluorine and hydroxyl can all vary in abundance. This creates a huge number of compositional variants. Chromium, titanium, nickel, manganese, and potassium can also be part of the complex composition and further indicates the generalization of the formula given above.
As noted above, hornblende is a name used for a number of dark-colored amphibole minerals that are compositional variants with similar physical properties. These minerals cannot be distinguished from one another without laboratory analysis. A small list of the hornblende minerals is given below with their chemical compositions.
Hornblende as a Rock-Forming Mineral
Hornblende is an important constituent in acidic and intermediate igneous rocks such as granite, diorite, syenite, andesite, and rhyolite. It is also found in metamorphic rocks such as gneiss and schist. A few rocks consist almost entirely of hornblende. Amphibolite is the name given to metamorphic rocks that are mainly composed of amphibole minerals. Lamprophyre is an igneous rock that is mainly composed of amphibole and biotite with a feldspar ground mass.
Identification of Hornblende
Hornblende minerals as a group are relatively easy to identify. The diagnostic properties are their dark color (usually black) and two directions of excellent cleavage that intersect at 124 and 56 degrees. The angle between the cleavage planes and hornblende's elongate habit can be used to distinguish it from augite and other pyroxene minerals that have a short blocky habit and cleavage angles intersecting at about 90 degrees. The presence of cleavage can be used to distinguish it from black tourmaline that often occurs in the same rocks.
Identifying the individual members of the hornblende group is difficult to impossible unless a person has the skills and equipment to do optical mineralogy, x-ray diffraction, or elemental analysis. The introductory student or the beginning mineral collector can be satisfied to assign the name of "hornblende" to a specimen.
Physical Properties of Hornblende
|Color||Usually black, dark green, dark brown|
|Streak||White, colorless - (brittle, often leaves cleavage debris behind instead of a streak)|
|Diaphaneity||Translucent to nearly opaque|
|Cleavage||Two directions intersecting at 124 and 56 degrees|
|Mohs Hardness||5 to 6|
|Specific Gravity||2.9 to 3.5 (varies depending upon composition)|
|Diagnostic Properties||Cleavage, color, elongate habit|
|Uses||Very little industrial use|
Uses of Hornblende
The mineral hornblende has very few uses. Its primary use might be as a mineral specimen. However, hornblende is the most abundant mineral in a rock known as amphibolite which has a large number of uses. It is crushed and used for highway construction and as railroad ballast. It is cut for use as dimension stone. The highest quality pieces are cut, polished, and sold under the name "black granite" for use as building facing, floor tiles, countertops, and other architectural uses.
Hornblende has been used to estimate the depth of crystallization of plutonic rocks. Those with low aluminum content are associated with shallow depths of crystallization, while those with higher aluminum content are associated with greater depths of crystallization. This information is useful in understanding the crystallization of magma and also useful for mineral exploration. 
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