What's in a Name - Hydro
Mineral Matters #354
The current crop of minerals (for the Illustrated Minerals of Australia in the next issue of the Monthly Mineral Chronicles) includes a bunch whose names start with “hydro”. So where does this come from?
Minerals containing the hydroxyl molecule as a fundamental building block have a defining characteristic, the presence of an oxygen and hydrogen atom bonded together, known as the hydroxyl (OH)⁻ ion.
Hydroxide minerals are defined by the presence of the hydroxyl group as a key anion (a negatively charged ion) within their structure. Anions are essentially atoms or groups of atoms that have gained one or more electrons, giving them a negative electrical charge, and these sit on the right-hand side of a mineral's chemical formula.
Some common hydroxide minerals include brucite (Mg(OH)2), manganite (MnO(OH)), diaspore (α-AlO(OH)), and goethite (α-FeO(OH)).
The presence of the (OH)⁻ group generally leads to weaker bonds than the metal-oxygen bond found in say, oxides. This translates to hydroxide minerals being softer and less dense than their oxide counterparts.
Hydroxide minerals are more often formed at low temperatures and are prevalent as products of weathering, where existing minerals break down due to exposure to air and water. For instance, when iron-bearing minerals weather, they can transform into goethite.
There are more than 2,200 species containing the hydroxyl ion. Sometimes, depending on the current flavour of the month, naming conventions may include something like hydro or hydroxy or hydroxyl as a prefix. Examples include hydrotalcite, hydroxycalciomicrolite, and hydroxylapatite. Rarely, (OH) is added as a suffix (eg: pertsevite-(OH)), or hydro might appear in the middle of a mineral name (eg: alumohydrocalcite).
Of the 60 or so minerals that begin with “hydro”, only 53 actually have (OH) in their formula. This is because hydro can also refer to the water molecule, H2O.
Below: Malachite (green) with hydrozincite (white), formula Zn5(CO3)2(OH)6, Monakoff Mine, Queensland, Australia. Width of view 22mm.
