Hornblende

Mineral Name: Hornblende

Chemical Composition: Ca2(Mg, Fe, Al)5(Al, Si)8O22(OH)2

Color: Black to a dark green

Streak: Gray to white

Hardness: 5-6

Cleavage/Fracture: The two cleavage planes intersect at about 56 to 124 degrees (1).

Crystal Form: The crystal form of Hornblende is a prism type that looks like a diamond when observed in cross section, or the portion of the mineral that does not have a cleavage surface.

Luster: Vitreous, dull or submetallic

Special Features: The diamond cross section is a unique property of hornblende and the angles of the cleavage. Other unique features of hornblende include the dark to almost black color.

Varieties: The independent elements within hornblende are in such small abundance that they often are just blanketed with the term hornblende to describe them (1). Certain hornblendes contain potassium, titanium, and fluoride.

Mineral Group: Hornblende is contained within the silicates group which contains various amount of silicon and oxygen, this is also the largest group of minerals (3). Amphiboles are a group of minerals that include elements of iron, magnesium, silicon, oxygen, and hydroxyl. A defining feature of amphiboles are their cleavage. Amphiboles intersect at 56 to 124 degrees differentiating themselves from pyroxenes and other groups (5). Hornblende is also part of the Inosilicates group. Insliciates are a silicon atom surrounded by 4 oxygen atoms and can be chained infinitely (4).

Environment: Hornblende is a very common rock-forming mineral. It can be found in both metamorphic and igneous environments. Because of this, hornblende can either be formed under the surface of the Earth due to addition of pressure and heat or the cooling of magma. It can also be formed at the surface of the Earth due in cooling lava. Hornblende forms around 500 to 650 degrees Celsius on the Bowen's reaction series with other minerals such as plagioclase and biotite (6).

Associated Rock Types: Because hornblende is such a common rock-forming mineral, it can be found in many rocks such as granite, andesite, diorite, amphibolite, schist, gneiss and granulite.

Occurrence in North America: Forty out of fifty states in the U.S. contain the mineral Hornblende. (All except Alabama, Delaware, Hawaii, Iowa, Kansas, Louisiana, Mississippi, Nebraska, North Dakota, and Ohio) Larger discoveries found in New Jersey and New York regions in St. Lawrence Co. Also found in Canada (1).

Economic Uses: When hornblende is found, it is commonly referred to as black granite. This shiny black material is cut and polished for household decoration like tile and countertops. Larger quantities are used for decoration on buildings (2).

Industrial Uses: By itself, hornblende doesn’t have many uses. Hornblende is most abundant mineral in a rock known as amphibolite, which is used for many things. It has been used for highway construction as a material for roads, railroad ballast (the rocks surrounding the railroad tracks), building facing, and is sometimes carved into statues (2).

Scholarly Uses: Hornblende has been used to estimate the depth of crystallization of plutonic rocks. Hornblende and other minerals change their composition with the changing environment, such as changes in pressure and/or temperature. These changes allows geologists to see a form resembling a bull’s eye shape when viewed under amplification. This important process, called zoning, shows differing mineral rims that uncover the history of possible magma changing events over time (7).

First Notable Identification: Hornblende was originally named in 1789 by Abraham Gottlieb Werner (8). The name was taken from an old German term for dark colored minerals “with no ore value and from "blende", meaning to deceive” (8).

How We Identified It: Distinguishing two directions of cleavage and its dark black color.

Don’t Confuse It With: Biotite, actinolite, augite (7). Biotite has one direction of cleavage. Actinolite is much softer. Augite has two planes of cleavage, but they come together at 90 degrees.