The more easily one mineral scratches another, the greater their difference in hardness. If two
minerals have same hardness, they will scratch each other but it will take some effort. A softer
mineral will not scratch a harder mineral, regardless of the amount of force you put into the
attempt. Take care that you are getting accurate test results. Sometimes it may appear that the
softer mineral has scratched the harder mineral because there is a ‘line’ left behind on the harder
mineral. Close inspection shows that the ‘line’ can be wiped away and there is no scratch under
it, similar to chalk on a chalkboard. In these cases, the line was left by the softer mineral
because the harder one was actually scratching it.
To perform a hardness test, first see if you can scratch the mineral with your fingernail. If you
can, the mineral is the same hardness or softer than your fingernail depending on how much
force is required. If you cannot, see if the mineral can scratch a copper penny, then a glass plate,
and so on up the scale. This will give you a narrow
range
of the hardness. For example, if it
scratches a penny but not a glass plate, the mineral has a
hardness range
of between 3 and 5. If
it scratches glass, the mineral is harder than 5.5. Remember that the easier it is to scratch the
glass, the greater the difference between the hardness of the mineral and the glass, but you
must record ONLY the results of your tests (e.g., you can record ≥5.5, >5.5 or >>5.5 based on
how easy it is to scratch the glass but NOT 8 or 9).
Cleavage and Fracture
Cleavage and fracture refer to the way minerals break. Cleavage is the tendency of some
minerals to break along plane of weakness in the mineral’s crystalline structure. If breaking a
mineral leaves behind relatively flat surfaces that give off flashes of reflected light when the hand
sample is rotated, the mineral has cleavage. These cleavage surfaces are the planes of
weakness. Some minerals, such as mica, have one set cleavage planes that are all parallel. Such
minerals will repeatedly break into smaller and smaller pieces along that one cleavage plane.
Others minerals have two, three, or even four cleavage planes. The quality of the cleavage for
each plane varies. Some minerals have excellent cleavage. For instance, you can peel sheet after
sheet from a mica crystal to see layer after layer of very reflective (shiny) planes. Others have
poor cleavage and only produce relatively flat surfaces with a dull shine. Minerals with no
cleavage are said to fracture.
Figure 3.2 on the next page shows the common cleavage patterns that cause minerals to break
along the planes of weakness, forming certain preferred shapes. Use this figure to help you
evaluate the cleavage of mineral samples. There are three observations you must make:
1. The number of different (non-parallel) cleavage planes.
2. The angle at which different planes intersect.
3. The quality of cleavage: excellent, perfect, good, fair, poor
Fracture occurs when a mineral breaks but not along cleavage planes. Many minerals fracture
because they have no planes of weak bonds in their atomic structure. In these cases, fractures
still can form characteristic shapes or patterns. For example,
conchoidal
fracture creates smooth
and curved surfaces, similar to a clam shell. Conchoidal fracture is commonly seen in the mineral
quartz. Some minerals break into splintery or fibrous fragments. Others fracture into irregular
shapes.