Malachite and Plancheite (#36)

Lustrous opaque primary malachite crystals to ½" in a cluster with minor plancheite.
Africa Onganja Mine, Onganja, Namibia 3 ½" x 3" x 2"

 

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In Learn More for specimen #35, we saw the chemical formula for malachite: Cu2(CO3)(OH)2. The formula of plancheite (the blue mineral in this specimen) is Cu8Si8O22(OH)2 • (H2O). Right away you can see big differences. Malachite is a carbonate, as evinced by "CO3" in the formula. Plancheite is a silicate, indicated by the "Si8O22" in the formula.

The Mohs hardness (see Learn More for #7) of malachite is 3.5-4. That of plancheite is 6. Why the difference? You may recall from reading about specimen #7 that Mohs hardness is determined by the arrangement of the weakest bonds in a mineral. There are ways to quantitatively estimate the strengths of atomic bonds, and it turns out the the C-O bond is stronger than the Si-O bond. So why should malachite, which is a carbonate, be softer than the silicate, plancheite?

There are only so many outer electrons (the ones involved in chemical bonding) to go around in a crystal structure, and if some are used in forming very strong (i.e., short) bonds, then there must be fewer for other bonds, and those bonds (such as Cu-O in these minerals) must then be weaker (i.e., longer). In malachite, the Cu- O bonds are a bit longer, on average, than those in plancheite. This is a fairly simplistic way of looking at relative hardness in these two minerals, though, because we have ignored the placement of OH (called hydroxyl) in both, as well as water molecules in plancheite. To determine the reason for the substantial difference in hardness would require a more careful examination of the two crystal structures - not something we can pursue here.