How Rocks and Minerals Are Formed
Summary: Rocks and minerals are formed over millions of years through geological processes. The three main types of rocks are igneous (cooled from magma), sedimentary (compressed from sediment), and metamorphic (altered by heat and pressure). Crystals grow when specific combinations of temperature and pressure cause atoms to arrange in regular patterns.
During the formation of the planet, cataclysmic forces kept it in almost constant turmoil, with continents splitting open, then crashing shut once again. Volcanoes erupted, blasting the inner molten contents of the birthing globe to the heavens with a roar that must have been, if there had been ears to hear, truly deafening. Lakes-yes, even oceans of rock, so superhot as to be liquid, seethed and boiled to the surface, cooling gradually, then breaking up into soil.
Out of this cooling magma, crystals of the different minerals congealed and grew, some or most of them deeply buried, either in the solidified rock inside which they formed, or in the rock ground into soil. Deposits formed to remain hidden until man finally discovered how to seek them out and mine them.
When we speak of a crystal "growing" in the earth, we are speaking of a period of millions of years.
On the other hand, the formation of certain kinds of crystals may take place in an extremely short period of time.
Since it takes many years for most crystals to form, the pressures they are subjected to are also spread out over the same period of time.
Isn't it a mystery that a rock-a hard, solid, intact rock-could be seen through. The agates, if you ever noticed, when still wet, are transparent-some of them as clear as glass. When they dry off, the surface is opaque from the abrasion of the stream bed or beach.
Rocks are divided into three main classifications, although some geologists use four classifications: igneous rocks, sedimentary rocks, metamorphic rocks, and the fourth classification, plutonic rocks. Plutonic and igneous rocks are really the same classification, as we shall see, but occur in different ways.
Igneous Rocks
Those are rocks that were cast up during volcanic eruption and have cooled rapidly. Plutonic rocks are, equally simply, rocks which, while molten, were trapped beneath the surface, therefore, were molten in their beginning.
Pumice is an igneous lava that, while in the molten stage, has gases trapped in it to make it flow as a froth. Obsidian, when very old, will become porous and absorb water; then, when this mass is heated it swells up into a similar froth. This substance is sold as perlite. Obsidian itself is an igneous rock that cooled so rapidly that the atoms had no time to arrange themselves into their regular crystalline habit. Consequently, obsidian is grainless and has a conchoidal fracture, a hard, lustrous surface, and polishes to an extremely glossy, liquid finish. Some obsidians are translucent, others opaque.
Sedimentary Rocks
Sedimentary rocks are, as the name implies, rocks formed from sediments. When sedimentary rock is found, that region was at one time a lake bottom or an ocean bed. The silts and mud, settling to the bottom, gradually hardened into rock in layers, sometimes of different colors and hardness, depending upon what minerals and foreign substances were present during that age when a layer was deposited. Shale and slate are common forms of sedimentary rocks.
Sedimentary rocks are almost entirely on the surface of the earth. Streams, rivers and oceans have been depositing sedimentary layers throughout time. The Mississippi River, for instance, has been flowing for over half a million years, and has been depositing silts and sediments for that entire period. The sedimentary layer from this river is now said to be over four miles thick!
Besides shale and slate, sedimentary rocks are limestone, sandstone, chalk, coal, gypsum, and conglomerate. Conglomerate results when rocks are tumbled down and left as a layer of rounded, water-worn pieces which later are cemented together into rock. Another form of rock made up of fragments cemented together is breccia. Breccia is exactly like conglomerate, except that the small rocks cemented together are broken fragments with sharp edges instead of water-worn rocks. Puddingstone is a form of breccia.
Halite is another form of sedimentary rock, better known as common salt. Deposits of halite miles thick are to be found in several places on earth. This is mined, purified, ground into fine particles, and used for table salt and for industrial purposes.
Metamorphic Rocks
Those are rocks that have been changed or altered in composition. The changes which take place in metamorphic rocks are related to pressure, temperature, and chemical attack. This is to say that if the rock is changed due to pressure and temperature, it is remelted, permitting the atoms to move about and migrate to form new crystals. Sometimes these are merely the same minerals, but in higher concentration, thus making larger crystals. Sometimes they are entirely new minerals.
Thus, if granite, a slow-cooling igneous rock, became a metamorphic rock through remelt, the diminutive crystals of quartz could coalesce and form huge quartz crystals weighing many tons each.
Chemical attack can also cause a mineral to metamorphose into another mineral.
Earth's Elements and Rapid Crystal Growth
The earth's matter, as we know, is composed of over one hundred elements. It is a source of some amazement to many persons when they discover that 99.75% of the earth's crust is made up of only fifteen of these elements. leaving a mere 0.25% for all other elements. Within this tiny fraction of the crust are such things as gold, silver, copper, and other metals, as well as everything that is not included in the fifteen elements, which are:
Oxygen, Silicon, Aluminum, Calcium, Sodium, Potassium, Magnesium, Titanium, Phosphorus, Carbon, Hydrogen, Manganese, Sulphur, Chlorine, Iron.
The crust of the earth's lithosphere is very much lighter than the core. This is partly due to the fact that three of the elements which make up the crust are gases-namely, oxygen, hydrogen, and chlorine. The heavier elements, such as nickel and iron, are mainly in the core.
As mentioned earlier, sometimes a crystal could form in a very short period of time. When the right combination of temperature, pressure, and saturation of a mineral is present, some trigger can and does start crystalline growth, which, once started, progresses very rapidly. In matter of minutes, hours, or days, crystals can form, sometimes so rapidly that you can actually see them forming! An excellent example of this is the following: Make a smear of an alcoholic solution of iodine on a slide, then view it under a microscope, watching closely until the fluid begins to evaporate. Suddenly a number of tiny specks will appear in the puddle. These are the "seedling" crystals of iodine. A minute or two more for the alcohol to evaporate, and suddenly the crystals will seem to shoot up right into the lens of the microscope! They grow with amazing speed into wonderful and beautiful clusters of perfect iodine crystals. In exactly the same way clusters of larger crystals, such as quartz, grow in nature.
Frequently Asked Questions
What are the three main classifications of rocks?
Rocks are primarily divided into three classifications: igneous rocks, sedimentary rocks, and metamorphic rocks.
How fast do crystals grow?
While most crystals grow over a period of millions of years deep within the earth, certain types of crystals can form very rapidly, in a matter of minutes, hours, or days, when the right combination of temperature, pressure, and saturation is present.
What makes up the earth's crust?
Amazingly, 99.75% of the earth's crust is made up of just fifteen elements, including oxygen, silicon, aluminum, calcium, iron, and others. The remaining 0.25% contains all other elements, including precious metals like gold and silver.
Conclusion
There are a great number of rocks which are not suitable for cutting gemstones, due either to their lack of distinguishing characteristics, color, hardness, or because of some other deficiency. The deficiency is only from the point of view of the gem cutter, however. The rock itself may make an outstanding cabinet specimen.
By: P. Villiard