WHAT IS HARDNESS?

"What is hardness?"
"Hardness is a measure of how difficult or easy it is for a substance to be penetrated or scratched! For example, steel (like a steel nail) can scratch your fingernail, so steel is harder than fingernail!!"
"That's what I thought! My sentiments exactly!"
"But the question is: how can you describe the amount of hardness!"
"Some question! Just like you said: something is harder than this or softer than that! Like steel is harder than fingernail! Like my French test is harder than my geology test - harder for me to penetrate its meaning (heh! heh!)!! Makes me scratch my head (heh! heh!)!!"
"Wise guy! That won't do, because I don't know how hard your French test is! Or your skull! What we need is some way of setting up some standards to which the hardness of things can be compared!"
"Sounds good to me! What do you suggest?!"
"One very simple thing to do is to choose a couple of very common substances and compare everything to them. For example, a simple classification can be devised using a fingernail and a steel nail!"
"You're right! Most people can get hold of those!!"
"Using the fingernail and the steel nail, a simple scale consisting of three classes of hardness may be established: 1) materials that can be scratched with a fingernail are 'soft'; 2) materials that cannot be scratched with a fingernail but can be scratched with a steel nail are 'intermediate'; and 3) materials that cannot be scratched with a steel nail are 'hard'!"
"Useful, I suppose, but kind of crude!!!"
"Well, another way was suggested in 1812 by a German mineralogist named Friedrich Mohs. He chose ten common minerals (minerals that anyone interested in minerals is likely to have in their collections) and used them to set up a 'Hardness Scale'! He assigned each of the ten minerals a hardness number from 1 to 10, with 1 being the softest and 10 being the hardest! All other minerals (and things that aren't minerals, too) can be tested against the Mohs Scale minerals and given a value. For example, the mineral wulfenite has a hardness (H) = 3."
"A good guy, this Mohs!"
What are the advantages and disadvantages of the 'simple' hardness scale as compared to the Mohs hardness scale? (See Hardness Exercises A, B and C for more insight.)
"There are some complications... Many minerals have a range of hardness! Scheelite, for example, has H = 4.5 to 5."
"How come the hardness can vary?"
"Because the chemical composition of some minerals can vary slightly. Also, many minerals are harder if you scratch them in one direction than if you scratch them in another direction. That's because the atoms are arranged differently and 'bonded' (glued together) differently in different directions within the mineral. But with two exceptions (kyanite and calcite), the difference in hardness cannot be detected without delicate instruments. We shan't concern ourselves with these particulars!"
"So what are the Mohs minerals and what are their hardnesses on the Mohs Scale?!"
"That's for you to find out, Lou!"


2010, David Leveson/Revised by G.Rocha and Michelle O'Dea