A primary mineral is any mineral formed during the original crystallisation of a host igneous rock. It includes the main minerals used to classify the rock as well as any accessory minerals. Primary minerals contrast with secondary minerals, which form after the primary minerals through weathering, alteration, hydrothermal activity or metamorphism.
In ore-deposit geology, hypogene processes occur deep below the Earth's surface and tend to form primary mineral deposits, whereas supergene processes near the surface tend to form secondary deposits. White gypsum veins near Gotham in Nottinghamshire, England, are an example where primary and secondary sulfate minerals coexist.
The elemental and mineral composition of an igneous rock is determined by the chemistry of the volcanic or magmatic system that formed it. Extrusive rocks such as basalt, rhyolite, andesite and obsidian, and intrusive rocks such as granite, granodiorite, gabbro and peridotite, contain different concentrations of primary minerals including quartz, feldspar, plagioclase, muscovite, biotite, hornblende, pyroxene and olivine.
Primary sulfate minerals also occur in igneous rocks. They may appear in veins and include nosean, sodalite, barite, anhydrite, gypsum, celestine, alunite, creedite and polyhalite. These minerals record the sulfur and sulfate activity of late magmatic fluids and are important for understanding volcanic eruptions and ore formation.
Primary minerals can be used to analyse geochemical dispersion halos and indicator minerals. The dominant primary minerals in soils are silicate minerals, and several silicate groups are recognised on the basis of bonding arrangement and the silica tetrahedron.
Primary ore deposits contain primary ores that may form geochemical dispersion halos known as primary dispersion patterns. These primary patterns are essentially syngenetic and may therefore occur at or near the time of ore formation. Primary ore patterns may show wall-rock alteration, including silicification, pyritisation, sericitisation, chloritisation, carbonatisation, tourmalinisation and greisenisation.
Geochemical dispersion halos are an important exploration guide. Element anomalies in soil, stream sediment and rock can be used to trace the location and extent of primary mineralisation. The shape, size and element association of a halo depend on mineralisation type, wall-rock properties and landscape conditions.
Heavy-mineral indicator minerals provide a good approximation of primary geology and the presence of ore deposits. Primary indicator minerals can be used to identify gold deposits, kimberlites and massive sulfide deposits. Indicator minerals are further used to trace dispersion trains in streams, which may locate primary ore or mineral sources.
Indicator minerals are usually stable, easily identified and genetically linked to mineralisation. Chromian pyrope, magnesian ilmenite and chromian diopside in kimberlite are indicator minerals for diamond exploration, native gold, pyrite and arsenopyrite can indicate gold mineralisation, and pyrite, chalcopyrite and sphalerite can indicate volcanogenic massive sulfide mineralisation.
Two types of minerals occur in soils: primary and secondary. Primary minerals have not undergone chemical alteration since crystallising from cooling magma and are defined as minerals present in soil but not formed in it, whereas secondary minerals form during weathering of primary minerals. Blume of North Carolina State University further notes that primary mineral grains are mainly larger than 2 micrometres and include silt, sand and gravel.
The dominant primary minerals in soils are silicates. Silicate minerals make up more than 90% of crustal minerals. According to bonding arrangement and the silica tetrahedron, they are divided into six groups: nesosilicates, sorosilicates, cyclosilicates, chain silicates, phyllosilicates and framework silicates.
Framework silicates such as quartz and cristobalite are common in soils, phyllosilicates include muscovite, biotite and clay minerals, cyclosilicates include tourmaline, chain silicates include amphibole and pyroxene, sorosilicates contain double silica tetrahedra such as vesuvianite, and nesosilicates contain isolated tetrahedra such as olivine.
The crust and soils are composed mainly of silicic acid combined with sodium, aluminium, potassium, calcium, iron and oxygen ions. These elements are components of primary minerals, and primary minerals are components of parent rock. Primary rock is the source of primary minerals and primary water. Classic discussions of primary ore origin appear in Ore Deposits, 1903 and 1913. According to W. A. Tarr in 1938, primary deposits result directly from magmatic action, with magmatic differentiation producing basic igneous rocks and their associated accessory mineral suite, the first minerals to crystallise, and, on the other hand, acidic igneous rocks and a second accessory suite deposited from residual mother liquor.
Primary sulfate minerals are leached by bioleaching to separate primary sulfide ores. Primary ores are also extracted by dense-medium separation, a technique that removes gangue by density contrast. Dense primary ore minerals sink while lighter gangue floats. Dense-medium separation plants are widely used in mining, for example to beneficiate lithium ores such as spodumene from pegmatite.
Another method is magnetic separation, which separates iron-bearing gangue such as hematite. Hematite cannot be used in the steel industry without beneficiation. Primary low-grade ores such as siderite and hematite are roasted to form magnetite, and once the iron oxides are converted, magnetic separation extracts the magnetic ore. Froth flotation is also used for primary ores, often after roasting, when magnetite or other primary minerals are further separated into concentrate.
The choice of beneficiation method depends on ore mineralogy, grain size, liberation and economics. When valuable minerals are finely intergrown with gangue, fine grinding and multiple separation stages are needed, whereas coarse, easily liberated ores can use simpler gravity or magnetic separation. Understanding the origin and occurrence of primary minerals helps design more efficient beneficiation flowsheets.