Rocks: Properties of Rocks and Minerals

Differences between minerals and rocks

A mineral is a naturally occurring solid with a definite chemical composition and a specific crystalline structure.

A rock is an aggregate of one or more minerals.

A rock may also include organic remains and mineraloids.

Some rocks are predominantly composed of just one mineral.

Rocks and minerals are classified by such properties:

Crystal structure and habit.  Shape and size give rise to descriptive terms applied to the typical appearance.

Hardness is usually measured according to the Mohs scale. This scale is relative and goes from 1 to 10.  Minerals with a given Mohs hardness can scratch the surface of any mineral that has a lower hardness than itself.

Luster indicates the way a mineral’s surface interacts with light and can range from dull to glassy.

Color indicates the appearance of the mineral in reflected light or transmitted light for translucent minerals

Streak refers to the color of the powder a mineral leaves after rubbing it on an unglazed porcelain streak plate. Note that this is not always the same color as the original mineral.

Cleavage describes the way a mineral may split apart along various planes. In thin sections, cleavage is visible as thin parallel lines across a mineral.

Fracture is any local separation or discontinuity plane in a geologic formation, such as a joint or a fault that divides the rock into two or more pieces.  Fractures are commonly caused by stress exceeding the rock strength. Fractures can provide permeability for fluid movement, such as water or hydrocarbons. Highly fractured rocks can make good aquifers or hydrocarbon reservoirs, since they may possess both significant permeability and fracture porosity.

Specific gravity relates the mineral mass to the mass of an equal volume of water, namely the density of the material.

Fluorescence responds to ultraviolet light 

Phosphorescence is a specific type of photoluminescence related to fluorescence.  Phosphorescence is a process in which energy absorbed by a substance is released relatively slowly in the form of light.

Opacity or opaque refers to something difficult to see through or perceive.

Porosity is a measure of the void spaces in a material, and is a fraction of the volume of voids over the total volume.

Magnetism is used to describe how materials respond on the microscopic level to an applied magnetic field.

Radioactivity

Tenacity responds to mechanical induced changes of shape or form.

Piezoelectricity is the ability of some materials (notably crystals and certain ceramics, including bone) to generate an electric field or electric potential

Reactivity refers to the rate at which a chemical substance tends to undergo a chemical reaction in time. Reactivity is primarily due to the sub-atomic properties of the compound.

Chemical property is any of a material’s properties that become evident during a chemical reaction.  Chemical properties can be contrasted with physical properties, which can be discerned without changing the substance’s structure.

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Category: Earth Science