Hornblende is a inosilicate amphibole minerals, which are two type hornblende minerals. They are ferrohornblende and magnesiohornblende. 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 two end-member hornblendes—iron-rich ferrohornblende and magnesiumrich magnesiohornblende—are both calcium-rich and monoclinic in crystal structure. Other elements, such as chromium, titanium, and nickel, can also appear in the crystal structures of the group. The concentrations of these elements are an indicator of the metamorphic grade of the mineral. Specimens are green, dark green, or brownish green to black in color. Crystals are usually bladed and unterminated, and they often show a pseudohexagonal cross section. Well-formed crystals are short to long prisms. They also occurs as cleavable masses and radiating groups. The mineral forms in metamorphic rocks, especially gneisses, hornblende schists, amphibolites, and magnesium- and iron-rich igneous rocks.
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.
Ferro-hornblende Association: Hedenbergite (granite); biotite, epidote, albite, quartz (amphibolite)
Magnesio-hornblende: Quartz, orthoclase, plagioclase, biotite, magnetite, apatite (granite).
Polymorphism & Series: Forms a series with magnesiohornblende (Magnesio-hornblende). Forms a series with ferrohornblende (Ferro-hornblende)
Mineral Group: Amphibole supergroup
Contents
Chemical Properties
| Chemical Classification | Silicate mineral |
| General Formula | (Ca,Na)2–3(Mg,Fe,Al)5(Al,Si)8O22(OH,F)2. |
| Ferro-hornblende | {Ca2}{Fe2+4Al}(AlSi7O22)(OH)2 |
| Magnesio-hornblende | {Ca2}{Mg4Al}(AlSi7O22)(OH)2 |
| Common Impurities | Ti,Mn,Na,K |
Hornblende Physical Properties
| Color | Usually black, dark green, dark brown |
| Streak | White, colorless – (brittle, often leaves cleavage debris behind instead of a streak) |
| Luster | Vitreous |
| Cleavage | Two directions intersecting at 124 and 56 degrees |
| Diaphaneity | Translucent to nearly opaque |
| Mohs Hardness | 5 to 6 |
| Crystal System | Monoclinic |
Ferro-hornblende Optical Properties
| Color / Pleochroism | Pleochroic in various shades of green and brown. In PPL a thin section of Hornblende ranges from yellow -green to dark brown. Green varieties usually have X= light yellow green, Y=green or grey-green and Z=dark green. Brownish varieties have X=greenish-yelow/brown, Y=yellowish to reddish brown and Z=grey to dark brown. |
| 2V: | Measured: 12° to 76°, Calculated: 30° to 62° |
| RI values: | nα = 1.687 – 1.694 nβ = 1.700 – 1.707 nγ = 1.701 – 1.712 |
| Optic Sign | Biaxial (-) |
| Birefringence | δ = 0.014 – 0.018 |
| Relief | High |
| Dispersion: | r > v or r < v |
| Extinction | Symmetrical to cleavages |
| Distinguishing Features | Cleavages at 56 and 124 degrees which form a distinctive diamond shape in cross section. Hornblende is easly confused with biotite. Distiguishing factors are the lack of birds eye extinction and the two distinct cleavages. Simple twinning is relatively common. Crystal habit and cleavage distinguish hornblende from dark-colored pyroxenes. |
Magnesio-hornblende Optical Properties
| Color / Pleochroism | Pleochroic in various shades of green and brown. In PPL a thin section of Hornblende ranges from yellow -green to dark brown. Green varieties usually have X= light yellow green, Y=green or grey-green and Z=dark green. Brownish varieties have X=greenish-yelow/brown, Y=yellowish to reddish brown and Z=grey to dark brown. |
| 2V: | Measured: 66° to 85°, Calculated: 58° to 88° |
| RI values: | nα = 1.616 – 1.680 nβ = 1.626 – 1.695 nγ = 1.636 – 1.700 |
| Optic Sign | Biaxial (-) |
| Birefringence | δ = 0.020 |
| Relief | Moderate |
| Dispersion: | r > v or r < v |
| Extinction | Symmetrical to cleavages |
| Distinguishing Features | Cleavages at 56 and 124 degrees which form a distinctive diamond shape in cross section. Hornblende is easly confused with biotite. Distiguishing factors are the lack of birds eye extinction and the two distinct cleavages. Simple twinning is relatively common. Crystal habit and cleavage distinguish hornblende from dark-colored pyroxenes. |
Occurrence of Hornblende
It is a common component of many magmatic and metamorphic rocks such as granite, syenite, diorite, gabbro, basalt, andesite, gneiss and schist.
It is the main mineral of amphibolites. Very dark brown to black hornblendes containing titanium are normally called basaltic hornblendes, as they are usually a component of basalt and related rocks. Hornblende easily replaces chlorite and epidote.
A rare kind of hornblende contains less than 5% of iron oxide, is gray to white in color and Edenite is named Edenite from Orange County, New York.
Magnesio-hornblende Occurrence: Common in amphibolites, schists, and pegmatitic alkalic gabbro. Also from welded tu®s, granodiorites, granites, and tonalites.
Ferro-hornblende Occurrence: From granites, granodiorites, and metabasalts; common in amphibolites and schists. As reaction rims on ferroan hedenbergite.
Uses Area
It is the most abundant mineral in a rock known as amphibolite, which has numerous uses.
- Crushed and used as road construction and railway ballast.
- It was cut to be used as a dimension stone.
- 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.
- It was used to estimate the depth of crystallization of plutonic rocks. Those with low aluminum content are associated with crystallization at shallow depth, while those with high aluminum content are associated with greater crystallization depths. This information is also useful for understanding the crystallization of magma and for mineral research.
Distribution
Magnesio-hornblende
Very widespread. A few conrmed localities include:
- at Vesuvius and Monte Somma, Campania, Italy.
- In the granitic batholiths of the Scottish Highlands; Swiss and Italian Alps; Harz Mountains, Germany; Finland and Sweden.
- In the Southern California and Sierra Nevada batholiths, California, USA.
- Widespread in Japan.
Ferro-hornblende
Very widespread, but many locality references lack qualifying chemical analyses. A few historic localities for well-crystallized material include:
- at Monte Somma and Vesuvius, Campania, Italy.
- From Pargas, Finland.
- At KragerÄo, Arendal, and around the Langesundsfjord, Norway.
- From Blina and Schima, Czech Republic.
- In the USA, from Franklin and Sterling Hill, Ogdensburg, Sussex Co., New Jersey; from Edwards, Pierrepont, and Gouverneur, St. Lawrence Co., New York.
- From Bancroft, Pakenham, and Eganville, Ontario, Canada.
- From Broken Hill, New South Wales, Australia.
References
- Bonewitz, R. (2012). Rocks and minerals. 2nd ed. London: DK Publishing.
- Handbookofmineralogy.org. (2019). Handbook of Mineralogy. [online] Available at: http://www.handbookofmineralogy.org [Accessed 4 Mar. 2019].
- Mindat.org. (2019). Orpiment: Mineral information, data and localities.. [online] Available at: https://www.mindat.org/ [Accessed. 2019].
- Smith.edu. (2019). Geosciences | Smith College. [online] Available at: https://www.smith.edu/academics/geosciences [Accessed 15 Mar. 2019].
