Page 96 - Laboratory Manual in Physical Geology

shown in FIGURE 3.9 or pieces of the minerals in Mohs Scale.

Hardness test
Commercial hardness kits contain a set of all of the minerals
in FIGURE 3.9 or a set of metal scribes of known hardnesses.

When using such kits to make hardness comparisons, remem-
ber that the harder mineral/object is the one that scratches,
and the softer mineral/object is the one that is scratched.
Cleavage and Fracture. Cleavage is the tendency of
Glass some minerals to break ( cleave ) along flat, parallel surfaces
plate ( cleavage planes ) like the flat surfaces on broken pieces
of galena ( FIGURE 3.2B ). Cleavage planes are surfaces of

weak chemical bonding (attraction) between repeating,
parallel layers of atoms in a crystal. Each different set of
parallel cleavage planes is referred to as a cleavage direction .
Cleavage can be described as excellent, good, or poor
( FIGURE 3.10 ). An excellent cleavage direction reflects

light in one direction from a set of obvious, large, flat,
parallel surfaces. A good cleavage direction reflects light
in one direction from a set of many small, obvious, flat,
FIGURE 3.8 Hardness test. You can test a mineral’s hardness parallel surfaces. A poor cleavage direction reflects light
(resistance to scratching) using a glass plate, which has a hardness of from a set of small, flat, parallel surfaces that are difficult

5.5 on Mohs Scale of Hardness ( FIGURE 3.9 ). Be sure the edges of the
glass have been dulled. If not, then wrap the edges in masking tape to detect. Some of the light is reflected in one direction
or duct tape. Hold the glass plate firmly against a flat table top, then from the small cleavage surfaces, but most of the light
forcefully try to scratch the glass with the mineral sample. A mineral that is scattered randomly by fracture surfaces separating the
scratches the glass is a hard mineral (i.e., harder than 5.5). A mineral that cleavage surfaces.
does not scratch the glass is a soft mineral (i.e., less than or equal to 5.5).

Mohs Scale
of Hardness of Some Common Objects
Hardness* (Harder objects scratch softer objects)

10 Diamond

9 Corundum
HARD 8 Topaz

7 Quartz
6.5 Streak plate
6 Orthoclase Feldspar
5.5 Glass, Masonry nail, Knife blade
5 Apatite
4.5 Wire (iron) nail
4 Fluorite
3.5 Brass (wood screw, washer)
SOFT 3 Calcite 2.9 Copper coin (penny)

2 Gypsum 2.5 Fingernail

1 Talc
* A scale for measuring relative mineral hardness (resistance to scratching).

FIGURE 3.9 Mohs Scale of Hardness (resistance to scratching). Hard minerals have a Mohs hardness number greater than 5.5, so they
scratch glass and cannot be scratched with a knife blade or masonry (steel) nail. Soft minerals have a Mohs hardness number of 5.5 or less, so
they do not scratch glass and are easily scratched by a knife blade or masonry (steel) nail. A mineral’s hardness number can be determined by
comparing it to the hardness of other common objects or minerals of Mohs Scale of Hardness.

Mineral Properties, Identification, and Uses ■ 81