Why doesn't it cleave nicely along the (100) or (010) or any other planes? What determines its cleavage plane? I know it has something to do with the fact that it crystallizes in face centered cubic system but I don't understand why things happen the way they do.
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$\begingroup$ Fun fact: I keep getting up-votes but no answers. $\endgroup$– TamásJun 1, 2014 at 18:24
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$\begingroup$ I think the fcc that you mention is the key. I can imagine it in my head, but hard to explain. Try drawing an fcc crystal on paper and plotting the 110 plane - it will cut diagonally across the side of the face-centered cell. By extrapolation, I would also expect cleavage on 101 and 011 planes. $\endgroup$– winwaedJun 2, 2014 at 13:40
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$\begingroup$ According to mindat.org it has perfect cleavage on {001}. Do the curly brackets include those planes mentioned by you? $\endgroup$– TamásJun 2, 2014 at 15:36
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$\begingroup$ Yes. So where did you get the (110) from? $\endgroup$– winwaedJun 4, 2014 at 19:49
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$\begingroup$ My mineralogy professor who has a PhD in geology... Don't ask $\endgroup$– TamásJun 5, 2014 at 8:28
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1 Answer
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I think the commenters have identified the issue. Halite does not have perfect cleavage along the {110} plane. As recorded in Mindat, Halite has perfect cleavage alone {100}, {010}, and {001}. This makes sense, as Halite is a cubic mineral (fcc), these are the planes you would expect it to cleave on.
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1$\begingroup$ This does control the cleavage direction but there are other cubic minerals where the ion sizes and charge mean the cleavage is stronger in different directions. A good example is fluorite, calcium fluoride, which often forms cubic-shaped crystals but has octahedral cleavage. $\endgroup$– uhohOct 8, 2022 at 0:13
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1$\begingroup$ i.stack.imgur.com/OqmOm.png from aflow.org/prototype-encyclopedia/ABCD_cF16_216_c_d_b_a.html $\endgroup$– uhohOct 8, 2022 at 0:17
