We just learned about how to identify minerals on the basis of luster and color. There are two more
important characteristics we will also need to discuss. One of the best ways to identify minerals
is by examining their cleavage, or lack thereof. A mineral is said to have cleavage if it breaks
along a planar surface in one or more directions. The way that a mineral breaks is fundamentally
controlled by its crystalline structure, and more specifically its relative strength along different
crystallographic directions. For example, graphite is composed of stacks of two-dimensional sheets of
carbon atoms bonded together through van der Waals forces. The carbon atoms themselves are strongly
bonded within a sheet, but the bonds between the sheets are extremely weak. Because of this, when
graphite is broken, it breaks into planar sheets along the direction of the weak van der Waals
bonds, but breaks irregularly along the direction of the stronger, more isotropic covalent bonds.
A mineral that has one direction of cleavage, like graphite, is said to have basal cleavage.
A class of minerals called phyllosilicates, or sheet silicates, are easily identified
based on their perfect basal cleavage, causing them to break apart like the crust of baklava.
Some minerals totally lack cleavage and break along random, uneven fractures. The mineral
quartz, or silicon dioxide, has a lattice composed of equal-strength silicon-oxygen bonds in all
directions, and therefore lacks cleavage. However, quartz does have a characteristic fracture pattern
called conchoidal fracture. A conchoidal fracture appears as a concave “scoop” with
a series of parallel arc-like steps along its surface. Think of a broken piece of thick glass,
like the bottom of a glass soda bottle. When a mineral has more than one plane of cleavage,
it is useful to estimate the angle between them, which can also be diagnostic. The minerals quartz,
calcite, and halite often appear very similar in color and luster. The easiest way to tell them
apart, is to look for cleavage. Quartz has no cleavage, whereas calcite has 3 directions of
cleavage. In addition, while both halite and calcite have three directions of cleavage,
the cleavage planes of calcite are 60- and 120-degrees apart vs. 90 degrees for halite. If
you were to smash halite with a hammer, it would break into tiny cubes. Do the same to calcite
and it would break into tiny rhombohedrons. Two of the most common minerals in Earth’s crust,
quartz and plagioclase, can also look similar in color and luster, especially to the untrained eye.
But plagioclase has two directions of cleavage at 90 degrees in contrast with quartz’s lack thereof.
Even scratching a rock with your fingernail can be a quick way to classify and identify minerals,
telling you something about the strength of its chemical bonds. A typical human fingernail has a
hardness, or resistance to scratching, similar to the mineral calcite. Austrian mineralogist,
Friedrich Mohs, determined this literal rule of thumb while developing the Mohs Hardness Scale.
The Mohs scale ranks minerals by their ability to scratch each other, and is used to measure
the relative hardness of a mineral sample. The scale goes from 1, the softest mineral,
talc, to 10, the hardest mineral, diamond. Geologists also use common objects like coins or a
metal fingernail file to approximate the hardness of minerals. Pennies have a hardness of about 3.5
on the Mohs scale, similar to fluorite, and metal files have a hardness of 6.5 on the Mohs scale,
similar to quartz. For example, say you have an unknown mineral that can be scratched by a steel
file, but not by a penny. This would indicate that your mineral has a hardness between 3.5 and 6.5.
So that wraps up a discussion surrounding some of the physical characteristics of minerals.
Now that we’ve related the atomic structure of minerals to their macroscopic properties,
we can move on to the different classes of minerals
and discover how crystal chemistry and crystal structure relate to one another.