Sulphates
All sulphates have one important feature in common: they all contain sulphur and oxygen. Gypsum is a sulphate. The long gypsum crystal in this display case is particularly interesting because it actually contains water in liquid form, enclosed within the crystal. Desert roses are also frequently composed of gypsum. They are created by the action of groundwater in the desert. The water carries gypsum particles to the surface and then evaporates, leaving the gypsum behind in the sand. When the wind blows the surrounding desert sand away, small structures of gypsum and sand may be left behind.
Halides
Halides are minerals that contain fluorine, iodine, chlorine, and bromine. One of their distinguishing features is the speed with which they react with other chemicals. Some halides are soluble in water. The best known of the halides is halite (sodium chloride). We are all familiar with it as rock salt or common table salt.
Carbonates
Carbonates all contain carbon and oxygen. The best known of them are calcite (calcium carbonate), dolomite (calcium magnesium carbonate) and malachite (copper carbonate).
Apart from quartz, calcite is one of the most commonly occurring minerals in the Earth’s crust. Limestone is composed almost entirely of calcite. Transparent calcite is also known as double spar because, if you look through it, you see two images rather than one.
Sulphides
Sulphides are minerals that contain sulphur. Commonly occurring sulphides are galenite (lead sulphide) and pyrite (iron sulphide). They are frequently mined for the sake of the metals they contain.
Pyrite is also known as fool’s gold, because of its close resemblance to the precious metal. Pyrite is often found in the form of cubes that look manmade. But appearances are deceptive. The cubes are a completely natural phenomenon.
Silicates
More than 90% of the Earth’s crust is composed of silicates. This makes them by far the largest group of minerals. In other words, most of the rocks found on the surface of the Earth consist of silicates. They contain silicon and oxygen. The most commonly occurring silicates are feldspars and quartz. Other common rock-forming silicates are olivine, pyroxene, amphibole, garnet and micas. Most of the silicates contain aluminium, magnesium, iron and calcium.
Vanadates
The small group of minerals known as vanadates contain vanadium oxide, a compound of vanadium and oxygen. The best known mineral in the group is vanadinite.
Oxides
The oxides are a major group of minerals. They all contain oxygen. The commonest oxides are ores like bauxite (aluminium oxide) and goethite (iron oxide). If goethite oxidises, it turns into limonite (rust).
Another aluminium oxide is corundum. Pure, transparent corundum is an important gemstone. The red variety is called ruby and other colours are known as sapphire. Of all the gemstones, only emerald (a silicate) and diamond (pure carbon) are more precious.