Origins of minerals

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There are, in principle, three types of rocks and minerals are part of those.

Igneous (Magmatic) rocks

Basic

By the cooling down of magma, atoms are linked into crystalline patterns and subsequently different minerals are formed.
When the formation takes place in the depths of the earthcrust (approx. 33km deep) quite large rocks may be formed (for instance granites).

Igneous rocks are formed and created by magmatic processes in the earth. To form very large crystals of rare minerals, exceptional conditions are needed.

For instance a rock called pegmatite, which is formed by the crystallisation of magma enriched with water in the veins of other rocks. They may contain Beryl, Tourmaline and Topaz.

The stages of the igneous or magmatic cycle are as follows:

1. Early Magmatic Phase (early crystalization) from the firey hot magma:

  • Chromite
  • Magnetite
  • Titanium magnetite

2. Liquid Magmatic Phase: (main crystallization) 1500-600 degrees C

  • Spinel
  • Zircon
  • Apatite
  • Peridot
  • Diamond

3. Pegmatite Phase (rest crystllization) 700-400 degrees C
The residual part of the magma which is rich in fluxes is known as the pegmatite stage. The melt becomes a watery solution as the solidification proceeds. Because of this fluidity, the liquids can penetrate fissures and cracks in the surrounding rocks. Under the concentrated pressure and temperatures, individual crytals form that can measure several centimeters and occasionally, several meters! The prismatic crystals grow perpendicular to the walls of the vein. Pegmatite veins are some of the best examples of gemstone formation.

  • Tourmaline
  • Beryl
  • Quartz
  • Feldspar
  • Zircon
  • Apatite
  • Brazillianite
  • Graphite
  • Muscovite
  • Lepidolite

4. Pneumatolytic Phase: 500-300 degrees C
Minerals formed in this phase, form at lower temperatures and rising pressure. Superheated volatile components are involved. The most prominent of these components is water vapor, boron and fluorine gasses. Under the influence of these vapors, other minerals are often formed in the contact zone of limestone.

  • Topaz
  • Euclase
  • Vesuvianite
  • Fluorite
  • Cassiterite
  • Sheelite
  • Wolframite

5. Hydrothermal Phase: 400- 50 degrees C
This is a process associated with igneous activity that involves heated or super-heated water. Water at very high temperature and pressure is an exceedingly active substance, capable of breaking down silicates and dissolving many substances normally thought to be insoluble. This is the last stage of minerals that can be considered to be formed directly from magma.

  • Gold
  • Silver
  • Emerald (Colombian)
  • Beryl
  • Quartz
  • Baryte
  • Pyrite
  • Dolomite
  • Calcite


Igneous rocks can further be divided in two types, depending on where the magma cools:

Volcanic or extrusive rock

This is rock that is formed on the surface of the earth.
In contact with air, or seawater, molten rock cools rapidly and either quenches to a glass (like Obsidian) or it forms small crystals (basalt).

Volcanic rocks are usually finely grained or glassy in structure.
Basalt is an extrusive rock and is finely grained due to its rapid cooling. It largely consists of tiny Feldspar and pyroxene crystals (like Diopside and Enstatite).
Some basalts contain gemstones like Corundum, Zircon and Garnets.

Another volcanic rock is called Kimberlite and some of the kimberlite pipes are the major source of Diamond.

Occasionally, varieties of volcanic glass, Obsidian, are cut and fashioned as gemstones. It is an amorphous mineraloid with the hardness of approximately 5.5. Varieties include:

  • Snowflake Obsidian ( with inclusions of the mineral cristobalite)
  • Rainbow Obsidian
  • Red Mahogany Obsidian
  • Silver Sheen Obsidian
  • Midnight Lace Obsidian
  • Pumpkin Obsidian

Plutonic or intrusive rock

When the molten rock solidifies below the earth's surface, it cools slowly and it forms plutonic rocks with larger crystals. They tend to be coarser grained.

Granite is a coarse grained intrusive rock containg the minerals Quartz and Feldspar, usually with Mica or Hornblade. In some circumstances granite undergoes "fractional crystallisation": during slow cooling different minerals may form at different temperatures. Among the last to be formed, often in veins penetrating the surrounding, are minerals of the Pegmatite group.

Associated minerals that find their origin in igneous rocks:

  • Beryl
  • Chrysoberyl
  • Corundum
  • Diamond
  • Garnet
  • Feldspar
  • Peridot
  • Quartz
  • Spinel
  • Topaz
  • Tourmaline
  • Zircon

Sedimentary rocks

Basic

The Sedimentary cycle is the second largest cycle in mineral and rock formation. These are formed by erosion, transport in rivers, ice etc. and involve the decay and disintegration of a pre-existing rock mass.
Usually there are no new minerals formed, only found.

When these particels eventually settle, they form alluvial gravels, sands or clays. When they are either cemented or compressed, then they form sedimentary rocks such a conglomerations, sandstones and limestones.

Chemical action in the environment leads to some material dissolving in water. Eventually the water may evaporate and desposits of Borax and other salt "evaporites" may form this way.

Plant and animal remains are commonly incorporated among the rock fragments and these may be preserved as fossils.

Many gemstones are found in "alluvial deposits".
These deposits have their origin in the destruction of the origional rocks and the resulting materials by rivers, floods and glacial movement. During this movement the heavier minerals tend to remain relatively close to the source, whilst lighter minerals are carried further away.
The heavier and harder materials do not wear as much as the lighter ones and tend to retain more of their crystal shape.
Stones as Sapphire and Topaz do not show as much abrasion as softer minerals like Quartz.
However due to the continious grinding and tumbling over a period of time a large number of gem minerals are found a rounded "water-worn" pebbles. The gem gravels in Sri Lanka contain a wide variety of such minerals.
Because of their supreme hardness and density many diamonds survive the sedimentary processes and are frequently found in alluvial deposits.

Minerals found in sedimentary rocks:

  • Beryl
  • Opal
  • Quartz
  • Turquoise
  • Malachite
  • Azurite
  • Chrysoprase
  • Chrysocolla

Metamorphic rocks

Basic

The metamorphic cycle is the third largest cycle in mineral and rock formation. Metamorphism is the alteration of mineral parageneses after their deposition, by external action such as contact with magmetic rocks, regional changes in the pressure and temperature (e.g. contact metamorphosed limestones, crystalline schists, etc.). The consolidated rocks are altered in composition, texture or internal structure through pressure, heat and new chemical substances.
There are two kinds of metamorphism:
Regional and Contact
Regional metamorphism is caused due to a rise in temperature and directed pressure, effecting the earth's crust.
Contact metamorphism is caused when magma is intruded into a preexisting rock mass. The heat and pressure of this magma, causes a metamorphic change in the rock it intrudes.


Both igneous and sedimentary rocks can change in texture or chemical composition as the result of either contact or regional metamorphism. Thus excisting rocks change into new types of rocks.
They are usually harder and denser than the origional material.

For example shale may alter into slateand by further metamorphose into schist. Limestone is converted into marble.
Sometimes schists contain gem minerals like Garnet, Emerald and Corundum.

Some minerals that grow in metamorhic rocks are:

  • Beryl
  • Corundum
  • Danburite
  • Garnets
  • Iolite
  • Lapis Lazuli
  • Jadeite
  • Nephrite
  • Spinel
  • Alexandrite
  • Andaluscite

External links

FUNDAMENTALS OF PHYSICAL GEOGRAPHY