in sedimentary rocks such as limestone.

A brief examination of a specimen of calcite

shows a clearly non metallic luster.

With any ordinary sample of calcite,

one should be able to find several flat, cleavage

surfaces.

One important thing to notice about these cleavage surfaces

is that when they intersect they never

do so at 90 degree angles.

These intersections give calcite a tendency

to form parallelogram shapes.

This is a very important feature that distinguishes calcite

from another mineral on our list, halite.

When we do a test of the hardness of calcite

we start with a glass which has a hardness a 5.5.

When we attempt to scratch the glass with a crystal of calcite

we see that it does not leave a scratch.

This tells us that the hardness of calcite

is somewhere below 5.5.

When we attempt to scratch the calcite crystal

with a fingernail, which has a hardness is 2.5,

we find that the fingernail does not scratch it.

This would indicate that calcite's hardness

is above 2.5.

So overall we can place the hardness

of calcite somewhere between 5.5 and 2.5.

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