Mica, Gypsum, and Hardness
Date: Winter 2011-2012
I have a question regarding the hardness of mica and gypsum. Gypsum is a 2 on Moh's hardness scale and mica is somewhere between a 2 and 3. However, mica breaks and scratches very easily by a penny, paper clip, and fingernail and gypsum does not. What is a good way to explain to my students how gypsum is still "softer" on the scale? Obviously, mica breaks apart very easily.
Mineral hardness and mineral cleavage are separate issues.
Mineral hardness is a measure of a mineral's ability to resist being scratched.
Mineral cleavage is the ability for a mineral to split along weakly-bonded atomic planes.
The micas have one good mineral cleavage. That is, mica splits nicely in along one plane. Halite (table salt), on the other hand, splits equally well in three orthogonal directions (that is, along three planes, each of which is at 90 degrees to the others). Shake out some table salt on an overhead projector and have the students walk up to the screen. They will see that all salt has cubic mineral cleavage. I don't know how much snow you get in North Carolina, but if you get some salt that is used to melt ice you will note that it too has cubic cleavage. When you break up the large salt crystals you will observe that they break into smaller cubes. When you break the mica, it will break into smaller sheets.
Each mineral has a characteristic hardness and a defined number of mineral cleavage planes. Consider diamond. Diamond is a very hard mineral but it has numerous cleavage planes (by the way, lonsdaleite is 58% harder than diamond). The ability to cleave diamond allows for the light to refract off of the numerous surfaces and thus give rise to the sparkle effect.
Quartz is softer than diamond, but it has no cleavage planes. Hit it with a hammer (and use safety glasses and keep the students far away) and you will see that it breaks into shards -- no planes of weakness in quartz. The atoms have equal bond strengths in all directions. Although quartz grows into beautiful hexagonal crystals (known as a mineral's habit), it has no mineral cleavage.
Now you have to consider habit, cleavage, and hardness...
I know it is hard for a younger students relate hardness to scratchability. But almost any reasonably sized lump of rock or mineral is hard, in the everyday sense, if it hits you in the head! Mineralogostists had to come up with some other way of measuring properties of rocks and minerals. It is too bad that we use hardness when we really mean scratchability".
Mineral hardness is defined by what can scratch what. Mica breaks into sheets easily because its atoms are arranged in sheets. The sheets are held together my a very force known as the Van Der Waals force. The fact that mica breaks into sheets has nothing to do with the scratchability of the surfaces of the individual sheets.
The hardness of a mineral, using the Moh’s hardness scale is a rough semi-quantitative measure of the resistance of a mineral to scratching using various materials. It does not take into account minerals that have a different hardness depending upon the symmetry of the sample. In addition to mica, other minerals, for example graphite is very soft in the direction where the planes of carbon glide across one another but is “hard” in the direction that pushes planes against one another. It is a qualitative test. The best approach is just not to over-sell the test. It is a “field” test, not a quantitative analytical procedure.
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Update: June 2012