Difference Between Biotite and Hornblende

The key difference between biotite and hornblende is that biotite easily forms perfect shiny black cleavage and can be peeled into flakes, whereas hornblende does not flake.

Biotite and hornblende appear similar, and both these minerals are associated with silicate functional groups. However, there are differences between biotite and hornblende, as discussed below.

CONTENTS

1. Overview and Key Difference
2. What is Biotite 
3. What is Hornblende
4. Similarities – Biotite and Hornblende
5. Biotite vs Hornblende in Tabular Form
6. Summary – Biotite vs Hornblende

What is Biotite?

Biotite is a type of phyllosilicate mineral in the mica group, and it has the chemical formula K(Mg,Fe)₃AlSi₃O₁₀(F,OH)₂. Usually, it exists as a solid solution series and occurs between the iron-endmember annite and magnesium-endmember phlogopite. The members of the biotite mineral group include sheet silicates. Chemical elements such as iron, magnesium, aluminum, silicon, oxygen, and hydrogen can form silicate sheets. The atoms of these chemical elements are weakly bound together by potassium ions.

This mineral has a monoclinic crystal system, and it is in the prismatic (2/m) crystal class. The space group of this mineral is C2/m. When considering the appearance of biotite mineral, it appears in dark brown, greenish-brown, blackish brown, yellow of in white color. It has a crystal habitat that is massive to platy. Biotite mineral has a micaceous fracture and occurs in brittle to flexible, elastic tenacity. The hardness of this mineral can range from 2.5 to 3.0 on the Mohs scale. Moreover, biotite has vitreous to pearly luster, and the streak color is white.

Typically, biotite has highly perfect basal cleavage, and it consists of flexible sheets or lamellae that easily flakes off, which is a similar property to most of the mica minerals. Moreover, biotite has tabular to prismatic crystals having obvious pinacoid termination. There are four prism faces, and two pinacoid faces forming pseudohexagonal crystals.

Furthermore, this mineral can dissolve in both acidic and alkaline aqueous solutions. It has the highest dissolution in these solutions at low pH values. But this dissolution is highly anisotropic with crystal edge surfaces that are reacting 45 to 132 times faster than the basal surfaces.

There are some uses of biotite which include constraining ages of rocks by either potassium-argon dating or argon-argon dating. This is because the argon can escape readily from the biotite crystal structure at very high temperatures. Moreover, this mineral is useful in assessing the temperature histories of metamorphic rocks.

What is Hornblende?

Hornblende is a complex inosilicate series of minerals. The general chemical formula for this mineral is Ca₂(Mg,Fe,Al)₅(Al,Si)₈O₂₂(OH)₂. This mineral has a monoclinic crystal system and is in the C2/m space group. Although the above chemical formula is given to this mineral, the composition of metals in this mineral can vary depending on the occurrence and magnitude. For example, manganese and titanium and often present in this mineral.

Hornblende appears in black to dark green or brown color. It has a hexagonal/granular crystal habitat where the fracture of this mineral is uneven. The hardness of hornblende is in the range of 5.0 to 6.0 on the Mohs scale. It has a vitreous to dull luster and pale gray to colorless mineral streak color.

However, this mineral has a few uses compared to other mineral forms. Primarily, this mineral is useful as a mineral specimen. Hornblende occurs in amphibolite rocks abundantly. These types of rocks are useful in highway construction where they are used in the crushed form.

What are the Similarities Between Biotite and Hornblende

1. Biotite and hornblende are silicate minerals.
2. They contain metal atoms.
3. Both have a closely similar appearance.

What is the Difference Between Biotite and Hornblende?

Biotite is a type of phyllosilicate mineral in the mica group, and it has the chemical formula K(Mg,Fe)₃AlSi₃O₁₀(F,OH)_2, while Hornblende is a complex inosilicate series of minerals. The key difference between biotite and hornblende is that biotite easily forms perfect shiny black cleavage and can be peeled into flakes, whereas hornblende does not flake.

The following infographic tabulates the differences between biotite and hornblende in tabular form.

Summary – Biotite vs Hornblende

Biotite and hornblende appear similar and are associated with silicate functional groups. The key difference between biotite and hornblende is that biotite easily forms perfect shiny black cleavage and can be peeled into flakes, whereas hornblende does not flake.

Reference:

1. “Hornblende.” Wikipedia, Wikimedia Foundation, 30 Dec. 2020.

Image Courtesy:

1. “Biotite aggregate – Ochtendung, Eifel, Germany” By Fred Kruijen – (CC BY-SA 3.0 nl) via Commons Wikimedia

2. “Apatite-(CaOH)-Hornblende-38354” By Rob Lavinsky, iRocks.com – (CC-BY-SA-3.0) via Commons Wikimedia