Physical and Chemical Properties of Minerals

The observable properties of minerals, hardness, cleavage, luster, color, streak, and density, follow directly from their bonding and crystal structure and are the basis of identification.

Definition

The measurable and observable mechanical, optical, and chemical characteristics of minerals that arise from their composition and structure and serve to identify and distinguish them.

Scope

This topic covers the diagnostic properties used to recognize minerals: hardness on the Mohs scale, cleavage and fracture, luster and diaphaneity, color and streak, specific gravity, tenacity, magnetism, fluorescence, and reactions to acid. It explains how each property reflects the underlying bond strength, structural anisotropy, and chemistry of the mineral.

Core questions

• How does the Mohs scale rank relative hardness, and what controls absolute hardness?
• Why do some minerals cleave along planes while others fracture irregularly?
• How do color and streak differ in diagnostic reliability?
• What structural features explain a mineral's specific gravity?

Key theories

Mohs hardness scale

A relative ordinal scale from talc to diamond ranks minerals by scratch resistance, which reflects the strength and density of chemical bonds in the structure.

Cleavage as structural expression

Cleavage occurs along planes of weakest bonding within the crystal lattice, so its number, quality, and angles are direct, reproducible indicators of internal structure.

Clinical relevance

Property-based identification remains the fastest and most accessible method in the field and classroom, supports gemstone evaluation, and provides the link between observable behavior and the bonding and structure determined by laboratory methods.

History

Friedrich Mohs introduced his comparative hardness scale in 1812, providing the first standardized diagnostic property; subsequent work tied cleavage, density, and optical traits to the crystal structures revealed by X-ray analysis, unifying descriptive and structural mineralogy.

Key figures

• Friedrich Mohs
• Cornelis Klein
• William D. Nesse

Seminal works

• klein2007
• nesse2016

Frequently asked questions

Why is color an unreliable identifier?

Many minerals show variable color from trace impurities or structural defects, so color alone can be misleading; streak, the color of the powdered mineral, is far more consistent.

What is the difference between cleavage and fracture?

Cleavage is breakage along smooth planes determined by structure, whereas fracture is irregular breakage (such as conchoidal fracture in quartz) where no plane of structural weakness exists.