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Gemstones, Minerals & Fossils
Gemstones and Minerals
❓Your Mineral Identification Questions answered here
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<blockquote data-quote="user 4386" data-source="post: 443309" data-attributes="member: 4386"><p>Both composition and orientation variations in hardness can give the stated hardness range, For most minerals it is small, but kyanite and to a lesser extent tourmaline are examples that I know where it can be larger than most. The orientation variation is related to the different strength of atomic bonding in different directions, the composition variation is because composition can vary dramatically in the different types of a single mineral. I suspect orientation is the bigger issue of the two. Corundum and quartz have no cleavage and have conchoidal fracture, quartz being hexagonal and corundum trigonal. As you mention, corundum (sapphire, ruby) does have weak parting planes, despite having no real cleavage, and I am not surprised by what you have found (compositional variation is negligible in corundum). The length of oxygen bonds are not all the same. I see that "The toughness of corundum is sensitive to ..crystallographic orientation" - although toughness and hardness are not quite the same thing, this suggests bonding strength might also vary with orientation. One study of Knoop hardness of corundum found that hardness varied with orientation.</p></blockquote><p></p>
[QUOTE="user 4386, post: 443309, member: 4386"] Both composition and orientation variations in hardness can give the stated hardness range, For most minerals it is small, but kyanite and to a lesser extent tourmaline are examples that I know where it can be larger than most. The orientation variation is related to the different strength of atomic bonding in different directions, the composition variation is because composition can vary dramatically in the different types of a single mineral. I suspect orientation is the bigger issue of the two. Corundum and quartz have no cleavage and have conchoidal fracture, quartz being hexagonal and corundum trigonal. As you mention, corundum (sapphire, ruby) does have weak parting planes, despite having no real cleavage, and I am not surprised by what you have found (compositional variation is negligible in corundum). The length of oxygen bonds are not all the same. I see that "The toughness of corundum is sensitive to ..crystallographic orientation" - although toughness and hardness are not quite the same thing, this suggests bonding strength might also vary with orientation. One study of Knoop hardness of corundum found that hardness varied with orientation. [/QUOTE]
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Gemstones, Minerals & Fossils
Gemstones and Minerals
❓Your Mineral Identification Questions answered here
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