Birth of minerals
Rocks and crystals are originated from three different environments: volcanic, metamorphic and sedimentary.
Thus, there are at least three different ways to generate crystalline structures in rocks.
- Volcanic rocks are created on the earth surface out of the solidification of lava.
- Sedimentary rocks are issued from the slow accumulation of sediments by the ongoing action of erosion on landscapes (mainly winds and water flows) or shell deposits on the sea floor.
- Metamorphic rocks are created deep in the earth, consequently to orogenesis (formation of mountains) when high pressures and temperatures are involved, but under the melting point of these rocks.
Finally, the word "rock" means a compact conglomerate of a single or several different minerals and it represents the principal component of Earth crust.
Volcanic rocks
The very origin of volcanic rocks is the magma, which resides deep inside the oceanic or continental crust of the Earth.
These magmas are not absolutely considered as melted rocks. Actually, they do behave more like a plastic material with a honey-like consistency and density.
Because of the perpetual continental drifts, some weak areas are created within the solidified crust that stands above the magma.
When a crack happens in the crust, the pressure is dramatically released, consequently increasing temperature levels and driving the magma into a more fluid state suitable for an upwards flowing.
The magma makes its way all along the different cracks and finally reaches the surface of the earth where it flows or bursts out of volcanoes. The solidification of lava at the earth surface generates the volcanic extrusive rocks.
Sometimes, the up going flowing magma is not able to reach the surface. On its way, it may encounter a colder layer of rocks and solidify thereby.
In such a case, it generates the volcanic intrusive rocks (intrusive because these rocks have introduced themselves into another type of rocks).
Birth of minerals
Volcanic rocks :
A) Deep rocks, solidified at high deepness.
B) Rocks in a vein.
C) Effusive rocks.
D) Pyroclastic rocks, volcanic debris, expulsed by a volcano.
Metamorphic rocks :
E) F) G) Metamorphic rocks generated in a highly pressurized and heated
environment
.
H) metamorphic rocks generated by contact with melted volcanic rocks.
Sedimentary rocks :
I) J) K) L) Sedimentary rocks dating from different geologic ages.
Considering that intrusive volcanic rocks do cool down far slower than their
extrusive counterparts (because of
thermal isolation due to hot neighbouring rocks), crystals have much more time ahead to generate. Thus, intrusive rocks generally harbour some very beautiful, large and well designed clusters of crystals. On the other hand, as extrusive rocks tend to loose their inner heat quite rapidly in contact with the atmosphere, they only produce disseminate tiny to small crystals.
The magma can be considered as a very dense solution of its miscellaneous atomic components. Its global composition is a very important factor driving the final types of crystals that will grow into the birthing rock. Oxygen, Silicium, Aluminium, Iron, Magnesium, Calcium, Sodium and Potassium, in this specific order, do represent 99% of the elements of the Earth crust. When the magma cools down, its different minerals start to crystallize, but not all together, as each mineral has a specific degree of solubility. Theoretically, this order of crystallization is well defined, but some local chemical or physical variations inside the magma can modify slightly or dramatically this theoretical schedule. Moreover, small quantities of water, carbon dioxide or hydrogen sulphide may influence the specific crystallization temperatures of each mineral.
Actually, it has been acknowledged that without the presence of water and carbon dioxide, crystals were very unlikely to generate. Some crystals like quartz or feldspar hardly happen to germ within a totally dry fusion state, i.e. in the absence of gaseous additives. In such a dry environment, they lead to the formation of uncrystallised amorphous glass. On the contrary, just a few traces of water vapour will trigger the formation of wonderful crystals.
When the underground magma has cooled down enough to settle in a solid state, a great amount of high temperature liquids and gases are released. These fluids, loaded with mineral materials, make their way towards the surface through any kind of cracks they are able to find on their path. When the physical conditions are adequate (low pressure, low temperature, presence of porous calcite rocks for instance) the mineral fluids tend to crystallize and deposit along the cracks, generating veins of crystals.
A particularly interesting kind of vein that generally harbours good sized and well shaped crystals is the pegmatite.
As the pegmatite is theoretically associated with the formation of granite, it is often loaded with the very components of granite, i.e. quartz, feldspar and mica. These three minerals are generally combined all together, but sometimes quartz and feldspar can be excavated in a pure state.
As the percolating mineralised solution is boiling and highly pressurized, its different components will deposit into the vein following a pattern of successive layers, according to their own specific solubility threshold. That is the reason why a common pegmatite will display a bottom layer of feldspar covered by an intermediate layer of quartz and topped by a final layer of mica.
The presence and concentration of other metals into the percolating solution will determine the formation of other kinds of crystals. Zirconium will trigger the generation of zircons, beryllium will allow the creation of beryl (the blue aqua-marina, the green emerald), fluorine will generate fluorite, boron will give tourmaline, and the different species of garnet will be created through different combinations of calcium, magnesium, iron or manganese.
Metamorphic rocks
Metamorphic rocks are either originally volcanic or sedimentary rocks that have undergone a secondary chemical and physical transformation.
Metamorphism operates generally in very drastic thermodynamic conditions (high pressure and high temperature) in the presence of chemical agents like water vapour or other gases. When volcanic or sedimentary rocks undergo a metamorphism, they may loose some of their previous minerals, gain some other ones or have their resident minerals structure changed. Some crystals like garnet, spinel or corundum can be found embedded withinmetamorphic rocks.
There is another kind of metamorphism, called contact metamorphism. This happens when an underground magma gets into contact with a rock layer then induces a remelting of this rock, hence a secondary release of fluids. When calcite is the bed for contact metamorphism, its secondary crystallization with other components of the magma gives birth to a great variety of calcium-based crystals.
Sedimentary rocks
The erosion of landscapes by the perpetual ongoing action of winds and water flows creates the basic material for sedimentary rocks to form. There is also a biological sedimentary action due to an accumulation of organic debris (shells, skeletons of marine animals) on the seafloor. The crystals found into sedimentary rocks are usually created at very low temperature, hence their softness, and seem to develop quite quickly (sometimes through evaporation of concentrated solutions like on the shores of salt lakes). The most common of the sedimentary rocks is calcite and some beautiful crystalline species are selenite (strontium sulphide), halite (sodium chloride), barite (barium sulphide) and gypsum (calcium sulphide).
- Crystal Mysticism -
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