Degrees to each other you can see the traces of

degrees to each other. You can see the traces of halite cleavage in this photo as sort of vague
cracks running through the crystal. Cleavage is one excellent way to distinguish halite from
calcite.
So different minerals have different cleavages. Quartz has no cleavage at all, it breaks like glass
and we call this conchoidal fracture. Micas have one cleavage, sometimes called planar cleavage,
or basal cleavage.
Feldspars have two cleavages, at about ninety degrees to each other .hornblende also has to
cleaves but the angles are about 56-124 degrees between the cleavages. Recall that we saw this
microscope view of hornblende just a few slides ago. Here are two more examples of minerals
with two cleavages, augite on the left, cleavage angle is about 9o degrees, and plagioclase on the
right also has cleavage angles of about 90 degrees. And we saw before the halite and calcite each
have three cleavages, with the halite cleavages being perpendicular, and the calcite cleavages not
perpendicular. Fluorite is one the few common minerals that has four directions of cleavage so
florite cleaves into eight sided shapes called octahedra. Florite crystals are cubes, as shown on
the left here, but when florite cleaves we get octahedra such as those seen on the right. The
drawing here shows the relationships between the cubic crystals and the octahedral cleavage
fragments.

Minerals have various degrees of hardness. Hardness is a mineral’s ability to withstand
scratching and its ability to scratch other minerals. I think we all know that the hardest mineral is
diamond. Mineralogist use a ten point scale to describe mineral hardness, it is called the Moh’s
hardness scale, named after the gentleman that created it. Each of the ten levels on the scale is
associated with a particular diagnostic mineral, as seen on this chart. From bottom to top the
minerals are talc, gypsum, calcite, fluorite, apatite, orthoclase, quartz, topaz, corundum, and
diamond. For comparison on the right hand side of this chart, some common objects are shown
with their relative hardness indicated by arrows. The softest minerals gypsum and talc, can be
scratched with your fingernail, this makes identifying those two minerals relatively easy. Here
are photos of the minerals on Mohs hardness scale from the hardest to the softest. Numbers ten
nine eight seven six are shown here, Diamond, corundum, topaz, quartz, and feldspar, and five
four three two and one are shown here, apatite, fluorite, calcite, gypsum, and talc. When working
with mineral samples in a mineralogy laboratory we frequently have samples of all ten of these
minerals so if a new unknown mineral comes in we can determine its hardness by trying to
scratch the known minerals with the unknown mineral or vice versa. Sometimes however that is
not practical, and we use proxies to get a rough idea of a minerals hardness. Here we see four
common things we can use. Our fingernails have a hardness about two and a half, a copper coin
has a hardness of about three, a glass plate has a hardness of five, steel nail has a hardness of five