Hornblende vs Amphibole: What Are They and What’s the Difference

Hornblende and Amphibole are minerals that are a part of a group of inorganic compounds with structures that are characterized by silicate tetrahedrons that are arranged in chainlike formations.

This article will explain just what hornblende and amphibole minerals are, how they are similar, and how they are different.

Hornblende vs Amphibole (EXPLAINED)

What Is Hornblende?

Hornblende is a complex inosilicate series of minerals found in various forms of igneous and metamorphic rocks.

While it is not recognized as a mineral on its own, hornblende rather refers to a general or field term that references a dark amphibole.

Hornblende is a member of the amphibole group of more complex silicates, where the tetrahedra form a chain twice the width of the pyroxene (a large class of rock-forming silicate minerals containing calcium, magnesium, iron, and typically occurring as prismatic crystals) chains.

Hornblende can be divided into orthorhombic and monoclinic symmetry.

Hornblende is frequently thought of as the most common clinoamphibole and is a very common mineral found in intrusive and extrusive igneous rocks.

It can be identified by its dark color and ranges from black to dark/opaque green to brown, with pale grey, gray-white, white, or sometimes colorless streaks.

They have two directions of cleavage at intersections of 124 and 56 degrees.

They are members of the monoclinic crystal system (a group of crystalline solids with three axes of unequal length, two are perpendicular), with uneven fractures, a vitreous to dull luster, and are considered brittle.

Hornblende contains the minerals edenite, Ferro-actinolite, Ferro-edenite, Ferro-pargasite, Ferro-tschermakite, glaucophane, kaersutite, pargasite, tremolite, tschermakite, and many others.

Since it is a rock-forming mineral, it is made up of igneous rocks like rhyolite, andesite, syenite, diorite, granite, and metamorphic rocks like schist and gneiss.

Its chemical composition is generally thought of as (Ca,Na)2-3(Mg,Fe,AI)5(Si,Ai)8O22(OH,F)2.

It maintains a hardness between 5-6 on Mohs scale.

While it varies wildly upon each rocks specific composition, has a gravity between 2.9 and 3.5. hornblende is a inosilicate amphibole minerals containing two type hornblende minerals, ferrohornblende and magnesiohornblende.

They are isomorphous combinations of three different molecular structures; calcium-iron-magnesium, aluminium-iron-magnesium, and iron-magnesium silicates.

The name hornblende is applied to a group of amphibole minerals distinguished only by extensive chemical findings.

The double ended hornblendes ferrohornblende and magnesiumrich magnesiohornblend are found to be calcium-rich, with elements such as chromium, titanium, and nickel, which also frequently appear in the crystal structures of the hornblende group classification.

The concentrations of chromium, titanium, and nickel are also solid indicators of the metamorphic grade of the mineral in question.

Crystals are normally found bladed and unterminated or without serious fracture, and often display pseudohexagonal cross sections.

Well-formed crystals look short and stubby with long and slim prisms.

Hornblende minerals can also come as cleavable masses and are found as radiating groups.

Hornblende as a mineral form, like gneisses, hornblende schists, amphibolites, magnesium, and iron, covered igneous rocks.

While hornblende itself has very few practical uses outside of its examination and identification as a mineral specimen, it is the most abundant mineral of the amphibole minerals.

What Is Amphibole?

Amphibole rocks are a group of inosilicate minerals that form long, prismatic, acicular (slender and fragile), fibrous, or needle-like crystals, are composed of double chain SiO4 tetrahedra (triangular pyramid composed of four triangle faces), linked at the vertices, and contain iron and or magnesium in their structures.

Because hornblende is thought of as a type of Amphibole (a distinction which is obviously very confusable in the average person), they carry the same physical properties described above.

Amphibole minerals are found in both metamorphic (mostly in mafic or those containing dark ferromagnesian minerals and siliceous dolomites) and igneous rock (most common in intermediate and felsic because of the higher silica and dissolved water content of the more evolved magmas) formations and maintain the same physical properties of hornblende.

The Amphibole International Mineralogical Association symbol is AMP and can be black, brown, blue, colorless, white, yellow, and green.

Amphiboles crystalize in monoclinic and orthorhombic (three unequal axes).

Amphiboles are two chained silicates and have a general formula consisting of A0-1B2C5T8O22(OH, F)2.

The two chains are (Si, Al) O4 tetrahedrons or T site, which are combined to form another chain with (Si, Al)4O11 and extend far throughout the long or c-axis of the mineral. (Si, Al) O4 tetrahedrons switch with varying differences in double chains which produces rings of six tetrahedrons which ultimately make up and define the chain.

The center of the ring has a large hydroxyl area, containing OH or F.

The structure contains 2 hydroxyl sites for every 8 tetrahedral.

Double chains are broken up by strips of octahedral sites or what are referred to as M sites.

There are a total of four distinctive types of octahedral sites in the structure with varying lengths and sizes, such as M1, M2, M3, and M4. M1, M2, and M3 are small and are found in the middle of two double chains of tetrahedrons with (Si, Al) O4 vertices pointing towards them.

The first order can produce a series of tetrahedral chains with octahedral sites in between or T-O-T, which are found to display 5 octahedral sites for each 8 tetrahedrons.

The T-O-Ts are also called I-beams and appear on top of a large cation site or A, which is in the middle of the double chain, and two M4 sites, which appear larger than the previous octahedral sites while also occupying positions respective to the previous chains.

This newly created structure is monoclinic but there are a few amphiboles which lack cation A sites and contain both Fe and Mg in the M-sites, which results in orthorhombic structures.

T-O-T chains have strong bonds and produce two planes parallel to the long axis.

Double chains of amphiboles are larger than single chains of pyroxene.

Thus, the planes in the intersect at about 125° and 55° determine the intersecting prism cleavage of the amphibole.

The most common types of amphiboles are glaucophane, and the most common or mineral with the highest amphibole content is andesite, which contains around 20% Amphibole.

Since Amphibole is harder than limestone and heavier than granite, takes a high polish, maintains a dark coloring, and its texture, Amphibole is used as a dimension stone in a lot of road and railroad bed construction.

Since it is so commonly found at the source, the cost of transporting this mineral is very low, and it is sometimes used as veneer for the faces of buildings, and even as a gemstone, since it can be rounded and polished finely for use in jewelry and fine countertops.

Another wildly popular amphibole is Glaucophane, which is grey to lavender blue with pale grey to blueish-gray streaks, translucent, with a vitreous luster, and its main amphobole from other amphoboles is its distinct blue color.

According to the Wisconsin Geological and Natural History Survey division of extension at the University of Wisconsin, Hornblende as a field name has recently been discredited.

It was used as a field name for dark Amphibole but has since been retracted due to advancements in x-ray or microprobe verification practices.