Hornblende

A Category of silicate mineral, Hornblende is a crucial element of the amphibole group of complex silicates in which the tetrahedra are associated to form an ongoing chain twice the width of the pyroxene chains. The hornblende rock is often found in igneous and metamorphic rocks.

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Discovery - Who Discovered Hornblende?

The name Hornblende was originally given in 1789 by Abraham Gottlieb Werner.

So what does the name mean?

Etymology

The name hornblende is derived from German words horn and blenden which refers to its similarity in appearance to the metal-bearing mineral ores.

Chemical Formula of Hornblende Mineral

Hornblende chemical formula is as given: (Ca,Na)2-3(Mg,Fe,Al)5(Si,Al)8O22(OH,F)2

These constituents can be present in different amounts. Thus it is notable that the hornblende mineral has a varied composition. There are different varieties with similar physical properties which can only be differentiated in the laboratories

Let us look further into the chemical nature of the mineral.

Chemical Composition of Hornblende

It is an isomorphous mixture of three molecules and the hornblende chemical composition is as stated:

• Calcium-Iron-Magnesium Silicate

• Aluminum-Iron-Magnesium Silicate

• Iron-Magnesium Silicate.

These minerals are difficult to distinguish by physical means. The iron, magnesium, and aluminum ions can freely provide the alternative for each other and form what has been distinctive as separate minerals. The minerals are assigned the names Magnesio-hornblende, Ferrohornblende, Aluminio-ferro-hornblende and Aluminum-magnesio-hornblende.

Sodium, potassium are often present. Manganese and titanium can also be present. In crystalline structures, fluoride is more common than hydroxide.

Occurrence - Where Do You Find Hornblende?

This element of silicate mineral is a key constituent of igneous and metamorphic rocks. Recall what are they?

Hornblende has rock-forming properties. It forms both igneous and metamorphic rocks.

Igneous Rocks - These rocks, also known as the magmatic rock, are formed by the cooling and solidification of the lava or the magma. Hornblende is present in acidic and intermediate igneous rocks.

Example - granite, diorite, syenite, gabbro

Metamorphic Rocks - These are rocks formed from some other type of previously present rocks. They are formed due to factors such as extreme pressures, temperature, or a mixture of both.

Hornblende also forms metamorphic rocks. Examples- Gneiss, schist

Edenite is a rare variety of hornblende that has 5% iron oxide and thus appears white to gray.

Hornblende Properties

Let us further look into the physical and chemical properties of hornblende.

Category - Silicate mineral; inosilicates; amphibole group

Physical Properties

Mohs hardness- 5-6 ( Mohs scale 1 is talc, while 10 is for diamond)

Specific gravity - 2.9-3.4

Transparency - Opaque

Color - green. Greenish-brown, black

Luster - vitreous, dull, submetallic

Streak - pale gray, gray-white, colorless

Tenacity- brittle

Chemical Properties

Diagnostic Properties - Cleavage (intersect at 124-56 degrees), color

Hornblende thin section in PPL ( plane-polarized light) image ranges from green to dark brown.

Isotropy/Anisotropy - Anisotropic i.e. it shows different properties when observed in different planes and axes.

Crystal- monoclinic.

Further details about the crystal form of hornblende are mentioned below.

Hornblende Crystal

As mentioned above, it is a monoclinic type of crystal. These are prismatic crystals with a diamond-shaped cross-section. It is rarely found as an individual crystal, instead always as platy or grainy crystals.

Hornblende Thin Section

Hornblende thin section in PPL (plane-polarized light) image ranges from green to dark brown.

Green Varieties have

• X= light yellow-green

• Y=green or grey-green

• Z=dark green.

Brownish Varieties have

• X=greenish-yellow/brown,

• Y=yellowish to reddish-brown

•  Z=grey to dark brown.

 Geographical Locations

Hornblende is a commonly occurring mineral but only specific places harbor good quality crystals. A lot of the Lustrous, well-formed crystals, mostly as microcrystals, come from -

• The Italian volcanoes at Monte Somma, Vesuvius

• The Montenero Quarry, Latera, Lazio Province

Exceptional Crystals in Relatively Large Prismatic Crystals Come From -

• Studsdalen, near Kragero, Telemark, Norway.

• Arendal, Langesund Fjord

• Risor, Aust-Agder.

Dark green Hornblende was found in Malmberget, Gällivare, Sweden; and doubly terminated floater crystals come from Žim (Schima), Teplice, Bohemia, in the Czech Republic.

In the U.S, crystal plates have come from the iron mines in the Jersey Highlands in Bergen, Passaic, and Sussex Counties, New Jersey. Good Hornblende crystals also come from upstate New York regions in St. Lawrence Co. at Edwards, Pierrepont, Gouverneur, and Russel. In Canada, well-formed stubby crystals come from Bancroft, Ontario; and at the Bear Lake Diggings in Gooderham,Haliburton Co., Ontario.

Conclusion

Thus it is an important mineral ore that forms a constituent of various rocks used for day-to-day purposes.