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Minerals form in a variety of different ways. Some form when salt water evaporates or when chemicals come out of hot fluids. Others are made when hot gases or molten rock cool or when heat or pressure change pre-existing minerals.
Minerals start as atoms moving randomly in a fluid. When the fluid cools, the atoms link together in regular shapes and form a crystal lattice. How large a mineral grows, and its final crystal shape, is determined by temperature, pressure, chemical conditions and available space.
How minerals form naturally
Water is a liquid mineral that both destroys and creates other minerals. It dissolves minerals to form solutions that can, through evaporation and chemical changes, form new minerals. Limestone caves are an example of this.
On its own, water cannot easily dissolve limestone. However, as it moves through the atmosphere and soil it takes on carbon dioxide and becomes a weak acid. This acid eats away the calcium carbonate of the limestone to form caves. The water, now rich in dissolved limestone, can create crystalline formations in the cave. As water seeps into chambers, carbon dioxide escapes from the solution and calcium carbonate crystallises to form decorations such as stalagmites, stalactites, columns, flowstone and helictites.
A variety of minerals can form in caves. Calcium carbonate minerals such as calcite and aragonite are common in limestone caves, but other types of caves often contain more unusual minerals. For instance, calcium phosphate and rare ammonium phosphate minerals have been found in caves used by bats. This is because decomposing bat droppings form a sticky substance called guano. Mineralised groundwaters combine with the phosphates and other organic materials in the guano to create new minerals.
Salt lakes, seas and inland drainage basins contain large amounts of dissolved minerals, leached from surrounding rocks, older evaporite deposits or ocean salts blown inland. Evaporation by the sun concentrates these salty waters until crystals grow.
Gypsum is usually the first mineral to crystallise because of its lower solubility, and the remaining sodium, potassium and chlorine form common salt (halite) and sylvite (potassium chloride). Salt can also be grown in artificial ponds in lakes and seas. Many of the world’s chemical industries depend on these mineral salts.
Sulphides and sulpho-salts
Fine particles of metallic sulphides (sulphur combined with one or more metals) are dispersed into oceans from underwater volcanic vents and settle in layers. When parts of the ocean floor are subducted under continents, the sulphides are heated, recrystallised and concentrated into economic ore deposits. When rocks containing the sulphides rise into the Earth’s crust, the ore deposits are also brought up. Later, erosion leaves them at or near the surface.
The world’s metal resources are mostly found in sulphide deposits. The metal sulphides, and accompanying sulpho-salt minerals, are found in primary ores that lie below the secondary zone of weathered ore. We mine lead, zinc, copper, antimony and bismuth from these minerals.
Sulpho-salts are often found around extinct volcanoes in young mountain belts. They are related to the sulphides but have sulphur combined in more complex arrangements with metals like copper, silver and lead, and semi-metals like arsenic, antimony and bismuth.
Gossans are dark, rusty-looking rocks. They form on top of sulphides when acidic groundwater, oxygen and carbon dioxide react with the sulphides and leach out soluble minerals. Less soluble iron oxides and silica are left behind. New and often beautiful secondary minerals crystallise in the porous gossan from these metal-rich solutions. Insoluble minerals like quartz, and iron and manganese oxides and hydroxides are left to form the gossan cap.
The weathered zone that forms gossans can extend for hundreds of metres below the surface.
Gossans are easy to mine and their rich secondary minerals are becoming scarce. These secondary minerals, with often large and perfect crystals, are among the most precious of our natural history heritage.
Volcanoes are vents where molten rocks and hot vapours escape Earth’s interior. They are also where most of the rocks and minerals that make up the Earth’s crust are formed. The minerals formed depend on temperature and the chemical make-up of the lava and gases. Some form in the early hottest stage or as lava becomes solid. Others form from escaping gases and vapours that react with the colder rims to leave minerals behind. Some minerals also grow in cavities left by escaping steam.
Sulphur is a smelly mineral that forms around volcanoes. It starts as a hot gas, sulphur dioxide, that is blasted through small holes called fumaroles. The gas reacts with the colder rim and condenses as sulphur.
Thunder eggs and agates are found in rhyolite and basalt in many volcanic areas of eastern Australia.
In rhyolite rocks, thunder eggs are formed when nodules of minerals grow in flowing lava. The nodule starts as a gas bubble, which expands and later fills with silica minerals from solutions left inside it. Sometimes a second set of silica minerals forms as plates of chalcedony and small quartz crystals. Finally, a third set of minerals can fill the cavities when groundwaters bring in dissolved minerals after the lava solidified.
Zeolites are water-bearing alumino-silicate minerals that form during the late stages of volcanic activity in basalts and dolerites. They are common in volcanic regions in eastern Australia.
These minerals grew from solutions in cavities left by escaping gases in volcanic rocks. Zeolites have an internal cage-like framework structure of open channels of specific diameter. These channels give the minerals remarkable filtering and absorbent properties that allow some atoms and molecules to pass through but block larger ones. This ‘molecular sieve’ property gives zeolites important industrial and environmental applications, such as making hard water soft through ion exchange, soaking up pollutants, absorbing odours in kitty litter or as a base for slow-release fertilisers.
Hydrothermals (‘hot fluids’)
Magma beneath volcanoes can heat nearby underground water. The heated water carries metals, gases and sulphur leached from the magma and surrounding rocks. As the water moves through cracks below the Earth's surface, it reacts with rocks and dissolves and concentrates chemicals, including metallic and non-metallic elements, and gases. As the fluids cool, these solutions form ore deposits in cavities and veins, or irregular patches scattered through rocks.
As the metal-rich fluids travel further, they cool and deposit minerals in zones of decreasing temperature, each with their own distinctive suite of minerals, often with calcite and quartz. Many of the world’s gold deposits are hydrothermal.
Deep below the surface, two of Earth’s ‘cooking agents’, pressure and heat, work together to change minerals in sedimentary and igneous rocks. Sometimes volcanic eruptions bring up these rocks and minerals from the dense deeps.
When a heat source bakes rocks, larger crystals or new minerals may grow in newly formed rocks such as hornfels, quartzite and marble. This is called contact metamorphism. When high pressures squeeze and fold rocks over a large area (which also generates heat), new minerals can grow and often line up in bands in new rocks such as slate, schist and gneiss. This is known as regional metamorphism.
Metasomatism is special form of metamorphism. Instead of heat and pressure, it is hot fluids and gases (along with dissolved elements) escaping from molten masses that change the surrounding rocks. For instance, an impure limestone (calcite, clay, mica, quartz) will be bathed in hot fluids with fluorine, chlorine or boron. The carbon dioxide from limestone is driven off and the calcium combines with silicon and other elements to make new minerals such as vesuvianite, axinite, diopside and epidote.
Pegmatites are veins or pockets of extra-large crystals, usually made of the same minerals as the parent rock. They are mostly found in granites and are formed from corrosive gases and liquids left over from late cooling of molten granite. Large, well-formed crystals grow because of the rapid liquid and gas transport of chemicals to where the crystals are growing. The liquids are occasionally enriched in elements such as boron, beryllium, lithium and fluorine. This allows more unusual minerals to form, including gem minerals.
Simple pegmatites are just overgrown minerals of the same type as those in the parent granite rock, such as feldspar, mica and quartz. Exotic pegmatites can carry minerals with rare earth elements, or zirconium, caesium, beryllium, boron, lithium and fluorine.