What is 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.
FAQs on Hornblende
1. What is hornblende and in which types of rocks is it commonly found?
Hornblende is a complex inosilicate mineral, representing the most common member of the amphibole group. It is not a single mineral but a series of minerals with variable chemical compositions. It is a key rock-forming mineral found primarily in igneous and metamorphic rocks such as granite, diorite, gabbro, schist, and gneiss. It is a major constituent of the metamorphic rock called amphibolite.
2. What are the key physical properties used to identify hornblende?
Hornblende can be identified by its distinct physical properties. Key characteristics include:
- Color: Typically dark green, dark brown, or black.
- Lustre: Vitreous (glassy) to dull.
- Hardness: 5 to 6 on the Mohs hardness scale.
- Cleavage: Possesses two distinct cleavage planes that intersect at approximately 56 and 124 degrees. This is a crucial identifying feature.
- Crystal System: Monoclinic, often forming prismatic or elongated crystals.
- Streak: Pale green to greyish-white.
3. What are the main uses of hornblende in industry and geology?
Hornblende itself has limited direct uses. Its main value comes from being a major component of the rock amphibolite, which is crushed and used extensively as a construction aggregate, for road paving, and as railroad ballast. High-quality amphibolite, polished and sold as 'black granite', is used for countertops and building tiles. In geology, hornblende's presence helps geologists estimate the depth of crystallization of plutonic rocks, providing clues about their formation history.
4. Is hornblende considered a valuable mineral?
Generally, hornblende is not considered a valuable mineral on its own. It is often found as a gangue mineral, which is the commercially worthless material surrounding a valuable ore. However, its presence can contribute to the value of other materials. For instance, the desirable black streaks in some commercial 'black granite' are due to the presence of hornblende.
5. How can one distinguish hornblende from a similar-looking mineral like augite?
The most reliable way to distinguish hornblende (an amphibole) from augite (a pyroxene) is by examining their cleavage angles. Hornblende crystals break along two planes that meet at approximately 56 and 124 degrees, giving them a diamond-shaped cross-section. In contrast, augite has two cleavage planes that meet at nearly a right angle (around 87 and 93 degrees), resulting in a more square or rectangular cross-section. This difference in cleavage is a definitive diagnostic feature for students and geologists.
6. What is the specific relationship between hornblende and the amphibole group?
The relationship is one of a specific member to a larger group. The amphibole group is a large family of silicate minerals with a similar crystal structure. Hornblende is not a single, chemically distinct mineral but rather the most common, well-known series within this broader amphibole group. Think of it like this: 'Amphibole' is the general category (like 'dog'), while 'Hornblende' is a very common and well-known type within that category (like 'Labrador Retriever').
7. Why is the presence of hornblende in an igneous rock so significant for geologists?
The presence of hornblende provides crucial information about the conditions under which an igneous rock formed. Hornblende is stable only in magmas that contain water and solidify under specific temperature and pressure conditions, typically found at greater depths in the Earth's crust. Its presence, therefore, acts as a geobarometer, indicating that the rock crystallized deep underground rather than at the surface. This helps geologists reconstruct the geological history of a region.
8. Why is it difficult to write one simple chemical formula for hornblende?
It is difficult to write a single formula because hornblende is a complex solid solution with a highly variable composition. Its basic structure allows for extensive elemental substitution. A generalized formula is often written as (Ca,Na)₂(Mg,Fe,Al)₅(Al,Si)₈O₂₂(OH)₂. This shows that elements like calcium, sodium, magnesium, iron, and aluminium can substitute for one another in the crystal lattice, leading to a wide range of specific compositions, all of which are still classified as hornblende.
