Steven Dutch, Professor Emeritus, Natural and Applied Sciences, Universityof Wisconsin - Green Bay
"Musheral" is a tongue-in cheek name I coined for minerals that are more water than anything else. They tend to occur in frozen brines, protected wet weathering environments like mines, and fumaroles. Some occur in hypersaline Antarctic lakes, and may occur in the frozen subsurface of Mars. They tend to be very soluble and unstable in dry settings. Also the frozen brine minerals tend to undergo incongruent melting and break down to water or brine plus some less hydrated mineral.
I started off with the data from webmineral.com, and listed all the minerals containing hydrogen and oxygen (which is most minerals), then I selected only the minerals with both components and copied the data to a spreadsheet. Sorting out the water is relatively easy, since most formulas list water separated from the rest of the formula by a dot, so I was able to convert the formula to two columns using the dot as a separator. Erase the H2O and parentheses and voila, a column listing numbers of water molecules in the formula. After numerous trials I came up with a method of parsing the chemical formulas reasonably accurately. As with all data converted from text to spreadsheets, numerous manual corrections were needed. Per cent water by weight is much easier to get than number of atoms, but is relatively useless since heavy elements in the formula can make water look much less important than it is. For example, arseuranospathite has more water molecules than other atoms but is only 31% water by weight because of the massive arsenic and uranium atoms.
I didn't count methane hydrates or ice since they're in a class by themselves, nor did I count minerals with "nH2O" in the formula since the water would not really be integral to the structure of the mineral. Hydroxyl (OH) was not counted in the water tally. The list includes minerals with at least as many water molecules as other atoms in the formula. The column H2O/atoms gives the ratio, and Delta gives the excess of H2O over other atoms. Also included are some "honorable mentions" where Delta = -1, that is, there is only one fewer water molecule than other atoms.
|Name and Link to webmineral.com||Formula and link to structure page||Anhydrous||Atoms||H2O||H2O/atoms||Delta|
None of these minerals are clathrates, where atoms or molecules are mechanically trapped inside a cage of hydrogen-bonded water molecules. In almost all cases, the polar nature of the water molecule is used to bridge between anions and cations. The bonding is weak, since the effective charge on the oxygen in a water molecule is -0.7 and the charge on each hydrogen is +0.35. So these aren't especially durable minerals. On the other hand, let's not sell hydrogen bonding short. Just ask anyone on the Titanic.
For example in carnallite (KMgCl3*6(H2O)), one of the better known minerals on the list, magnesium atoms are octahedrally coordinated to the oxygens in water molecules, with the hydrogens all pointing outward. Potassium atoms are octahedrally coordinated with chlorine. The positive hydrogen atoms on the Mg octahedra link to the negative Cl ions on the K-Cl octahedra. So the Mg atoms are completely surrounded by water, but the water is electrostatically attracted to the Mg atom and not to other water molecules. The polar nature of water creates an interesting result. The K-Cl octahedra have a cation in the center and negative charge on the outside. The Mg-H2O octahedra have a cation in the center, but the outward-pointing hydrogens give the octahedron a positive charge on the outside.
Chalcanthite (CuSO4*5(H2O)) is another well known mineral that makes the "honorable mention" list and is known as a mineral that disintegrates in collections. The fully hydrated heptahydrate, called boothite, is described as an "Ephemeral mineral and virtually impossible to preserve in an artificial environment." Hydration to boothite followed by dehydration causes chalcanthite to crumble unless special measures are taken. The copper atoms in chalcanthite are octahedrally coordinated with a ring of water molecules around each copper atom and two oxygen atoms at either end. These are shared with sulfate tetrahedra, forming kinked chains of copper and sulfate units. Independent water molecules link the sulfate oxygens in adjacent chains.
In contrast to many minerals with complex anions, where oxygen atoms surround a central cation like carbon, sulfur or silicon, most of these minerals have water molecules clustered around a central cation. In complex anions, the charges on the oxygens are greater than the charge on the cation, resulting in an overall negative charge. Since water molecules are neutral, the effective charge on a cation surrounded by a cluster of water molecules is equal to the charge on the cation. Effectively, these are complex cations.
We can recognize several patterns in these minerals.
It's common for references to dismiss the water as not structurally bound to the mineral. While that may be true for some layer silicates and zeolites where water is mechanically trapped in void spaces in the lattice, in all these minerals the water is integral to the mineral structure. The water forms coordination polyhedra around the cations, which are in turn linked to other units. In general, removing the water does not merely expose an anhydrous core structure, but results in extensive rearrangement of the entire lattice.
What's the largest possible H2O/atoms ratio? Although there are lots of chemicals with a dozen or more water molecules in their formulas, they all tend to be fairly complex so the H2O/atoms ratio is low. One might envision a hypothetical compound XY where the X cation is big enough to be surrounded by water molecules in 12-fold coordination, with the Y anions bonding the clusters. This material would have the formula XY*12(H2O) for an H2O/atoms ratio of 6.0. In practice, the only compound, natural or synthetic, that I have found with a ratio greater than 2 is lithium chloride pentahydrate (LiClÂ·5H2O) with a ratio of 2.5. Compounds with a ratio of 2 are fairly common.
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Created 20 March 2011, Last