Little School of Mineralogy 7
What are oxide and hydroxide minerals, and what are their properties?
Oxide minerals are compounds of metals or metalloids with oxygen, while hydroxide minerals contain the OH– group.
Oxide minerals are mostly characterized by relatively high hardness and density, and they are resistant to weathering. Unlike oxides, hydroxides are softer and have lower hardness.
What is the most common occurrence of oxide and hydroxide minerals in nature?
Oxide minerals often occur as accessory minerals in igneous and metamorphic rocks, as well as resistant transported, or detrital, grains in sediments.
Hydroxide minerals mostly occur secondarily, as products of weathering.
What is the significance of oxide and hydroxide minerals?
Numerous oxide and hydroxide minerals are components of well-known ores from which economically important ferrous and non-ferrous metals are obtained.
Thus, there are magnetite, hematite and limonite iron ores. Cuprite is an ore mineral of copper, chromite of chromium, cassiterite of tin and brucite of magnesium, while ilmenite and rutile are ore minerals of titanium, and pyrolusite, psilomelane and manganite are ore minerals of manganese.
Bauxite, a mixture of several minerals among which the most important are gibbsite, diaspore and boehmite, is an ore of aluminum.
What makes the mineral corundum special?
Corundum also belongs to the oxide class. It is a mineral of high hardness and, when transparent and beautifully colored, is a highly valued gemstone.
Its well-known varieties are red ruby and blue sapphire.
CORUNDUM – Al2O3
The name probably comes from kuruvinda, the Sanskrit name for ruby. It crystallizes in the hexagonal system. It is relatively often found as crystals with columnar, tabular or barrel-shaped habits, polysynthetic twins, or granular aggregates.
It is transparent to translucent, colorless to variously colored depending on impurities, with a white streak, vitreous to diamond-like luster, without cleavage but with parting along the rhombohedral plane, splintery fracture, hardness 9, meaning the 9th mineral on the Mohs hardness scale, and density 4.0 g/cm3.
Do quartz and other minerals of the SiO2 group belong to oxides?
Although in much mineralogical literature, based on chemical composition, quartz and other minerals of the SiO2 group, tridymite, cristobalite and opal, are classified as oxides, it is more correct to classify them as silicates.
The reason for this is their structure, which corresponds to the structure of tectosilicates, whose crystal lattice is built so that SiO4 tetrahedra are connected to each other through all four vertices into a spatial structural unit, or framework.
Moreover, we can say that quartz and other minerals of the SiO2 group are silicates above all silicates.
Prepared by Biserka Radanović-Gužvica.