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Hornblende


A common rock-forming mineral found in igneous and metamorphic rocks



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.

(Ca,Na)2-3(Mg,Fe,Al)5(Si,Al)8O22(OH,F)2

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 forumla given above.


Physical Properties of Hornblende

Chemical Classification silicate
Color usually black, dark green, dark brown
Streak white, colorless - (brittle, often leaves cleavage debris behind instead of a streak)
Luster vitreous
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
Chemical Composition (Ca,Na)23(Mg,Fe,Al)5(Al,Si)8O22(OH,F)2
Crystal System monoclinic
Uses very little industrial use


Hornblende Minerals



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 can not be distinguished from one another without laboratory analysis. A small list of the hornblende minerals is given below with their chemical compositions.

Mineral Chemical Composition
Edenite Ca2NaMg5(AlSi7)O22(OH)2
Ferro-actinolite Ca2(Fe,Mg5)(Si8O22(OH)2
Ferro-edenite Ca2NaFe5(AlSi7)O22(OH)2
Ferro-pargasite Ca2NaFe4Al(Al2Si6)O22(OH)2
Ferro-tschermakite Ca2Fe3Al2(Al2Si6)O22(OH)2
Glaucophane Na2Mg3Al2Si8O22(OH)2
Kaersutite Ca2Na(Fe,Mg)4Ti(Al2Si6O22(OH)2
Pargasite Ca2NaMg4Al(Al2Si6)O22(OH)2
Tremolite Ca2(Mg,Fe5)(Si8O22(OH)2
Tschermakite Ca2Mg3Al2(Al2Si6)O22(OH)2


Hornblende as a Rock-Forming Mineral



Hornblende is an important constituent in acid 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 is 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.


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. [1]



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Hornblende
Hornblende with a typical black granular to fibrous appearance from Faraday Township, Ontario, Canada. This specimen is approximately 3 inches (7.6 centimeters) across.




biotite hornblende granite
Hornblende is an important constituent in many igneous rocks. This piece of biotite hornblende granite is an example. Image by NASA.


hornblende andesite
Hornblende is an important constituent in many igneous rocks. In extrusive rocks hornblende sometimes crystallizes below the ground, in the magma, before eruption. That can produce large phenocrysts of hornblende in a fine-grained rock. This piece of hornblende andesite is an example. Image by NASA.


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