The Major Components of the Crust
Most of the common rock-forming minerals are combinations of the most abundant materials in the Earth, taking into account the differentiation of the Earth into lighter and denser layers, early in its history. Here is an approximate listing of the eight most abundant elements in the crust, in terms of their overall mass, and relative atomic abundance:
(The total abundances don't add up to exactly 100%, because of round-off errors)
|All others||1.4||< 0.1|
Almost 85% of the atoms in the crust are oxygen or silicon atoms, because the vast majority of common (rock-forming) minerals are silicates -- combinations of oxygen and silicon with, primarily, the other six metals listed above. Pure silica, or quartz, consists of only silicon and oxygen. Aluminosilicates consist of silica in which some of the silicon atoms are replaced by aluminum atoms. This replacement is not chemically correct, and to make up for the difference in chemical reactivity (valence) between silicon and aluminum atoms, additional atoms -- usually sodium or potassium, in a one to one ratio with the replaced silicon atoms, or calcium or magnesium, in a one to two ratio -- are required. Ferromagnesian silicates consist of silica in which some of the silicon atoms are replaced by iron or magnesium atoms and again, additional atoms to make up the difference in valence between silicon and iron. Continental rocks consist mostly of aluminosilicates, but quartz and ferromagnesian silicates are a substantial minority. Oceanic rocks consist mostly of ferromagnesian silicates, but substantial amounts of aluminosilicates and minor amounts of quartz are also usually present. The Earth's mantle consists almost entirely of ferromagnesian silicates.
Rocks which contain a lot of aluminosilicates are sometimes referred to as sialic, using the chemical symbols for silicon and aluminum as the root of the word. Similarly, rocks which contain a lot of ferromagnesian silicates are referred to as mafic (for magnesium and iron). Rocks which are almost entirely ferromagnesian silicates are referred to as ultramafic. Hence, in the various divisions of the Earth listed above, continental rocks tend to be sialic, oceanic rocks are usually mafic, and mantle rocks are ultramafic.
Pure Silica -- Quartz and Its Numerous Varieties
Quartz crystals (USDA)
Quartz is a transparent or translucent form of pure silica, or silicon dioxide (SiO2). It is a major component of granite, and makes up about 12% of the crust of the earth, but is less abundant in oceanic crust and the interior of the earth. Most sands are grains of quartz released by the erosion and weathering of granitic rocks. Gem forms of quartz include rock crystal, and when colored by various impurities a host of other gemstones, such as amethyst, citrine, and smoky quartz.
Varieties of Quartz / Silica:
The structure of quartz consists of SiO4 tetrahedra, in which a silicon atom is surrounded by four oxygen atoms. The valence (chemical ability to bond to other atoms) of silicon is 4, which means it shares one bond with each of the surrounding oxygen atoms. However, each oxygen atom has a valence of 2, and wants to share two bonds with other atoms. Since the silicon atom only occupies one of those bonds, the oxygen atom binds with one other atom. In pure silica, the other atom is another silicon atom; but in most silicates (minerals built on the silica tetrahedron), the other atom is some other metal atom. If most of the other metal atoms are aluminum atoms, the resulting mineral is called an aluminosilicate. If most of the other metal atoms are iron or magnesium atoms, the resulting mineral is called a ferromagnesian silicate.
Amethyst, Ametrine, and Citrine
Amethyst, ametrine and citrine are all technically quartz; but although pure silica is colorless and transparent, the addition of minute amounts of various impurities can change its color, and transparency.
Amethyst's striking purple color is produced by the presence of manganese atoms, plus a sprinkling of iron atoms, which absorb certain wavelengths of light, changing the overall color that passes through the silica. Deeper, darker colors are more valuable, but if there are too many impurities, and the material becomes too dark to see through, the value goes down. If amethyst were rare, it would be extremely valuable; but since quartz is common, amethyst is also common, and is relatively inexpensive for such a colorful gemstone.
Amethyst's color is unstable at high temperatures, and exposure to temperatures in excess of 500 degrees Fahrenheit (250 Celsius) will cause noticeable fading of the purplish color. Still higher temperatures (close to 1000 Fahrenheit, or 500 Celsius degrees) change the color to yellow, producing citrine, or if the heating is uneven, ametrine -- which is amethyst at one end, and citrine at the other. Naturally occuring citrine and ametrine result from volcanic heating of amethyst-bearing rock; but ametrine, in particular, is often artificially produced by deliberately heating amethyst. This is usually done, however, only with paler, less valuable specimens, because the darker amethysts are more valuable than their heated byproducts.
Smoky and Rose Quartz
Tigers Eye and Rutilated Quartz
Banded and Moss Agates
Ferromagnesian Silicates -- Olivines
Left: Olivine crystals on basalt (USGS)Right: Green olivine sand from Mauna Loa (NASA)
Olivine is a glassy, greenish ferromagnesian silicate ((Mg,Fe)2SiO4). Its gem form, peridot, lends its name to a rock made primarily of olivine and other minerals, peridotite. Olivine is one of the most common minerals in the upper portion of the mantle, and dunite -- rocks containing almost pure olivine -- are believed to originate in the mantle. Mauna Loa, in Hawaii, has so much olivine in its lavas that erosion of those lavas results in green sand beaches, in which the sand grains are not quartz, but olivine.
The chemical formula for olivine includes the parenthetical form (Mg,Fe). This means that there is a whole suite of olivine minerals, or olivines, with variable proportions of magnesium (Mg) and iron (Fe). Varying amounts of the two metals produces olivines of different appearance, and slightly different physical properties. Pure magnesium olivine is called forsterite, and is a colorless mineral. Pure iron olivine is called fayalite, and is a brown mineral. The greenish form of olivine usually averages around eight magnesium atoms for every iron atom.
In olivine, each silica tetrahedron is bound to two iron atoms, two magnesium atoms, or an iron atom and a magnesium atom. None of the tetrahedra are bound to other tetrahedrons. This is the exact opposite of quartz, in which every tetrahedron is bound to another tetrahedron. Minerals such as olivine, in which none of the tetrahedra are bound together, are sometimes called orthosilicates.
Ferromagnesian Silicates -- Augites and Hornblendes
Aluminosilicates -- Albite