The mineral forms in metamorphic rocks, especially gneisses, hornblende schists, amphibolites, and magnesium- and iron-rich igneous rocks. Series: Forms a series with magnesiohornblende (Magnesio-hornblende). Hornblende is a rock-forming mineral that is an important constituent in acidic and intermediate igneous rocks such as granite, diorite, syenite, andesite, and rhyolite. Norway. Image by NASA. Relief: High positive. Ferro-hornblende Occurrence: Inexpensive mineral collections are available in the Geology.com Store. In the USA, from Franklin and Sterling Hill, pyroxenes. From Broken Hill, New South Wales, Australia. varieties have X=greenish-yelow/brown, Y=yellowish to reddish brown and Magnesio-hornblende. Name: The word it is derived from the German horn and blenden, to ‘deceive’ in allusion to its similarity in appearance to metal-bearing ore minerals. These minerals vary in chemical composition but are all double-chain inosilicates with very similar physical properties. It was used to estimate the depth of crystallization of plutonic rocks. Cleavages at 56 and 124 degrees which form a Also found as anhedral to strongly irregular grains, which in some cases may poikilitically enclose associated minerals. That can produce large phenocrysts of hornblende in a fine-grained rock. Hornblende in Hand Sample. It is crushed and used for highway construction and as railroad ballast. distinctive diamond shape in cross section. Measured: 12° to 76°, Calculated: 30° to 62°, nα = 1.687 – 1.694 nβ = 1.700 – 1.707 nγ = 1.701 – Also from welded Ferro-hornblende Association: Hedenbergite (granite); biotite, epidote, albite, quartz (amphibolite). Z=grey to dark brown. Its primary use might be as a mineral specimen. colorless – (brittle, often leaves cleavage debris behind instead of a From granites, granodiorites, and metabasalts; common in amphibolites and These minerals cannot be distinguished from one another without laboratory analysis. They are an isomorphous mixture of three molecules; a calcium-iron-magnesium silicate, an aluminium-iron-magnesium silicate, and an iron-magnesium silicate.The name hornblende is applied to a group of minerals that can be distinguished from each other only by detailed chemical analysis. 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. In extrusive rocks, hornblende sometimes crystallizes below the ground, in the magma, before eruption. schists. It is the most abundant mineral in a rock known as amphibolite, which has numerous uses. It is cut for use as Polymorphism & Distiguishing factors are the lack of birds material include: Cite this article as: Geology Science. Hornblende is a inosilicate amphibole minerals, which are two type hornblende minerals. This information is also useful for understanding the crystallization of magma and for mineral research. replaces chlorite and epidote. A generalized composition for the hornblende group is shown below. St. Lawrence Co., New York. As noted above, hornblende is a name used for a number of dark-colored amphibole minerals that are compositional variants with similar physical properties. Crystal habit and cleavage distinguish hornblende from dark-colored Magnesio-hornblende Occurrence: They are ferrohornblende and magnesiohornblende. black hornblendes containing titanium are normally called basaltic hornblendes, The diagnostic properties are their dark color (usually black) and two directions of excellent cleavage that intersect at 124 and 56 degrees. A few rocks consist almost entirely of hornblende. 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. Measured: 66° to 85°, Calculated: 58° to 88°, nα = 1.616 – 1.680 nβ = 1.626 – 1.695 nγ = 1.636 – Common in amphibolites, schists, and pegmatitic alkalic gabbro. as they are usually a component of basalt and related rocks. (2020). Hornblende has been used to estimate the depth of crystallization of plutonic rocks. A rare kind of hornblende contains less than 5% of iron tu®s, granodiorites, granites, and tonalites. a series with ferrohornblende (Ferro-hornblende). Width of the grain is approximatly 500 microns. In the Southern California and Sierra Nevada Hornblende. Those with low aluminum content are associated with crystallization at shallow depth, while those with high aluminum content are associated with greater crystallization depths. Hornblende easily Crushed and used as road construction and railway ballast. They also occurs as cleavable masses and radiating groups. It is the main mineral of amphibolites. Hornblende andesite: Hornblende is an important constituent in many igneous rocks. However, hornblende is the most abundant mineral in a rock known as amphibolite which has a large number of uses. From Bancroft, Pakenham, and Eganville, Ontario, Width of field of view is 400 microns. These minerals vary in chemical composition but are all double-chain inosilicates with very similar physical properties. Hornblende: Hornblende with a typical black granular to fibrous appearance from Faraday Township, Ontario, Canada. oxide, is gray to white in color and Edenite is named Edenite from Orange They are ferrohornblende and magnesiohornblende. Note that calcium, sodium, magnesium, iron, aluminum, silicon, fluorine and hydroxyl can all vary in abundance. Image by NASA. Beautiful hornblende grain with simple twinning visible. Green varieties usually Hornblende is a inosilicate amphibole minerals, which are two type hornblende minerals. Brownish at Monte Somma and Vesuvius, Campania, Italy. Highlands; Swiss and Italian Alps; Harz Mountains, Germany; Finland and Sweden. at Vesuvius and Monte Somma, Campania, Italy. Very widespread. They are an isomorphous mixture of three molecules; a calcium-iron-magnesium silicate, an aluminium-iron-magnesium silicate, and an iron-magnesium silicate.The name hornblende is applied to a group of minerals that can be distinguished … 1.712. Close up of the same grain showing 56 and 124 degree cleavages. Hornblende minerals as a group are relatively easy to identify. 1.700. Well-formed crystals are short to long prisms. The two end-member hornblendes—iron-rich ferrohornblende and magnesiumrich magnesiohornblende—are both calcium-rich and monoclinic in crystal structure. 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. [1], White, colorless - (brittle, often leaves cleavage debris behind instead of a streak), Two directions intersecting at 124 and 56 degrees, 2.9 to 3.5 (varies depending upon composition). Lamprophyre is an igneous rock that is mainly composed of amphibole and biotite with a feldspar ground mass. Forms Hornblende has a hardness of 5–6, a specific gravity of 2.9–3.4 and is typically an opaque green, greenish-brown, brown or black color. Amphibolite is the name given to metamorphic rocks that are mainly composed of amphibole minerals. A few conrmed localities include: Very widespread, but many locality references lack Magnesio-hornblende: Quartz, orthoclase, plagioclase, biotite, magnetite, apatite (granite). [online] Available at: https://geologyscience.com/minerals/hornblende/ [29th November 2020 ], Economically Important Metal Concentrations in Earth’s Crust, Porphyry Deposits: General characteristics and modeling, https://geologyscience.com/minerals/hornblende/, White, A small list of the hornblende minerals is given below with their chemical compositions. streak), Two directions intersecting at 124 and 56 degrees. The highest quality pieces are cut, polished and sold under the name “black granite” for use as building facades, floor tiles, countertops and other architectural uses. brown. In PPL a thin section of Specimens are green, dark green, or brownish green to black in color. The mineral hornblende has very few uses. Hornblende from K-84 in plane light. Other elements, such as chromium, titanium, and nickel, can also appear in the crystal structures of the group. It is a common component of many magmatic and metamorphic rocks such as granite, syenite, diorite, gabbro, basalt, andesite, gneiss and schist. Habit/Form: Commonly found as slender prismatic to bladed crystals with a diamon-shaped cross section showing the amphibole cleavage at 56° and 124°. eye extinction and the two distinct cleavages. At KragerÄo, Arendal, and around the Langesundsfjord, Hornblende Hornblende in amphibolite Hornblende in hornblende muscovite schist Black hornblende and pinkish-red corundum (ruby) in green zoisite Scanning electron micrograph of hornblende Scanning electron micrograph of a hornblende grain plucked from a garnet-rich sand from the Moray Coast in northeastern Scotland (there’s diatom tests on it!) As reaction rims on ferroan hedenbergite. 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. Chromium, titanium, nickel, manganese, and potassium can also be part of the complex composition and further indicates the generalization of the formula given above. Hornblende is easly confused with biotite.
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