HOW ARE MINERALS FORMED?
MINERALS are naturally occurring, inorganic solids, with a characteristic chemical composition, having a regular atomic structure throughout. Man-made industrial diamonds are not considered true minerals but, having said that, there are also some Organic Minerals, such as Amber, which purists are reluctant to call minerals, but they satisfy at least three of the criteria for mineral makeup, and therefore the point is open to debate.
How Many Minerals are there? There are about 4000 listed minerals on this planet but only a relatively few have gained popularity due to their pleasing appearance, often bizarre shapes, exquisite spectrum of colours, and trading value. Minerals are generally composed of more than one element or compound. Those which are made up of only one element are called Native Elements e.g. gold, silver, mercury, copper. The Conquistadores fell in love with New World gold and “liberated” it by the ship-load to fill the coffers of Government and Church in Spain. Silver too, along with Topaz, Tourmaline, Agate, Ruby, Diamond and many other precious rocks and minerals, have been highly prized for as long as Man was first enchanted by their beauty, commercial value and status it imparted to the wearer.
So, just how are minerals formed? Minerals can be formed in a wide variety of geological environments; deep inside volcanos, at the bottoms of deep oceans, deserts, salt lakes and cooling deep deposits of molten rock. Also, under the influence of heat and or pressure, when solutions and gasses holding concentrations of specific elements cool or evaporate, minerals growths are deposited inside rock fissures or voids. These minerals are sometimes forced through as a suspension in water, the water then evaporating leaving the mineral deposits as crystals (e.g. Amethyst) when the rock void is not fully filled or as Agates, when the rock is completely filled. These minerals are only visible when the rock is split open; it takes some skill to decide which rock to crack open! More about that later. An example of such crystal and mineral specimens for collectors are Geodes which are round rocks containing precipitated mineral salts, these being commonly Agates (caused by mineral salt crystals forming inside Basalt rocks), Amethysts, Quartz, Jasper.
Minerals don’t necessarily need heat or pressure to be formed; water saturated with mineral salts can leave deposits (Stalactites) as it drips down from the roof of a cave, forming corresponding Stalagmites directly below, over the centuries, where the drips hit. Eventually both grow to meet each other and thicken over the years. Sometimes, as a novelty, tourists can hang an item on a line at a cave, over which calcium carbonate saturated water runs; over a period of a few months it becomes encased in a hardening deposit of Calcium Carbonate.
THE THREE TYPES OF ROCK.
Ok, the basic scene has been set, now let’s examine in more detail the different mechanisms involved in mineral formation. All minerals are formed from ROCKS, which are an aggregate or mixture of various minerals and are the basic materials from which a mineral is formed.
Rocks can be either:
Igneous – formed due to volcanic activity from the Earth’s core.
Metamorphic – formed because of pressure or heat (e.g. tectonic plates colliding) on existing rocks, changing them into another type of rock.
Sedimentary – resulting from the layered compaction of weathered rock materials and/or shells.
Let us examine these rocks a little further:-
IGNEOUS ROCKS(from the Latin ignis – fire) can be further categorised as being or Extrusive.
Mineral crystals formed from Intrusive igneous rocks have a coarse structure because the cooling effect was slow and the crystals could grow for a long time, sometimes to a large size, especially when molten rock (magma) is trapped under ground and cools very slowly. Granite is an example of a commonly found intrusive rock. Other examples are:-
Pegmatite (Pegmatites are known to contain aquamarine, tourmaline, beryl, topaz, cassiterite, fluorite, apatite, tin and tungsten plus a host of other minerals.
These rocks are generally only exposed after mountain-forming upheavals, when rocks deep down are thrust to the surface due to Tectonic Plate Convergence. The Himalayas, for example, are currently still being pushed up by convergence forces.
Extrusive rocks are magma ejected from volcanos and cooling rapidly on the Earth’s surface. This means that their crystal structure is generally very small to microscopically small, as the crystals did not have sufficient time to develop. Obsidian (a glass-like black rock still used today by some surgeons because it keeps its sharp edge, down to one molecule!) & Basalt (the Giant’s Causway at the northern tip of Irleand) are two commonly found extrusive rocks, as are:-
METAMORPHIC ROCK MINERALS (from the Greek meta after, morphe form) are formed when sufficient heat and pressure change the original rock into a completely new rock. The original rock can be sedimentary, igneous or metamorphic. The most important Metamorphic mechanism is heat, changing the rock’s chemical structure at temperatures above 200 Celsius, breaking down the crystalline structures in the rock and converting them into new minerals. If the temperature becomes too high, the metamorphic action stops and the rocks become igneous. Marble is a common example of metamorphised limestone.
(hot water with dissolved ions) can also be responsible for changing parent rocks and is responsible for producing Sulphide minerals (e.g. Pyrite & Galena) and also Copper on the sea floor when the hot mineral-enriched water contacts the sea water.
Metamorphic Minerals – most of the minerals in following list are found exclusively in Metamorphic rock:-
SEDIMENTARY ROCK MINERALS. (from the Latin sedimentum – a settling). Sedimentary rocks are the basic building blocks from which nearly all metamorphic rocks are formed; it is the sedimentary rocks which are drawn down or pushed up by tectonic activity. While igneous & metamorphic rocks produce some of the finest mineral/gemstone specimens, the minerals in sedimentary rocks are not quite so impressive in form or range, though some sedimentary rock can also include igneous & metamorhic minerals as these rocks crumble due to weathering & being dissolved in water. Sedimentary rocks are also a mirror of the past, the different layers telling a story of what the Earth’s climate was like over the millions of years the rocks were forming, especially because of the fossil content as, unlike the other two types of rock, the heat & pressures produced in sedimentary rock formation is not enough to destroy fossil evidence.
CLASSIFICATION OF SEDIMENTARY ROCKS.
CLASTIC Sedimentary Rock is layered or Lithified (from the Greek Lithos – rock) by deposits of erroded debris of other rocks (due to weathering, frost, glacial action, or water). Most of the igneous rock minerals (with the exception of Quartz, which is very hard) are attacked over time by water, acids & alkalis and changed into clay minerals and chemicals in solution. Other minerals in Clastic sedimentary rock, like Zircon, Rutile & Magnetite are inert and highly resistant to mechaincal & chemical breakdown.
Clastic rock also contains the minerals feldspar, quartz, amphiboles & clays.
Minerals found in clastic sedimentary rocks are in the classes of Halides, Sulphates, Borates.
BIOGENIC SEDIMENTARY ROCKS are made up of materials from living organisms, i.e. corals, molluscs & amoeboid organisms, which deposit layers of calcite over the ocean floors, which later form limestone. Other examples of minerals found in biogenic rocks are stromatolites and flint.
PRECIPITATE SEDIMENATRY ROCKS are formed when mineral solutions e.g. sea water, evaporate and deposit minerals such as Halite & Gypsum.
And finally, a list of mineral classes (for those who love lists!)
Classes of Minerals.
All minerals found in igneous, metamorphic & sedimentary rocks can be classified as follows:-
1. Elemental (Diamond, Gold, Silver)
2. Sulphides,Selenides,Telurides,Arsenides,Antimonides, Bismutides (Galena, Pyrite, Chalcoprite)
3. Halides (Fluorite, Carnalite)
4. Oxides & Hydroxides (Corundum, Quartz, Hematite, Magnetite)
5. Sulphates, Chromates, Molybdates, Tungstenates, (Barite, Selenite)
6. Phosphates, Arsenates, Vanadates (Variscite, Mimetite, Vanadinite,)
7. Silicates (Opal, Zircon, Talc, Tourmalines, Topaz)
8. Nitrates, Carbonates, Borates (Calcite, Malachite, Azurite)
9. Organic Minerals (Amber, Whewellite, Oxammite)
I hope the information has been of help to you in your quest for answers as to how are minerals formed. The subject of Mineralogy can be developed to great scientific depths, but I have tried to present the information in a format anyone can understand. I hope this small offering has whetted your appetite for further research! Later, I will be adding some information on where and how to find some of the most popular minerals & gemstones. Happy hunting!