Quartz crystals fascinate me. I wanted to know more about them and thought that I would share my findings as I research this wonderful crystal. This is the first part of a multi-part report. Most of the information is summarized from the website www.quartzpage.de which also has much more detail and scientific data if you wish to know more.
Quartz has many forms and comes in many colors. It is also the foundation for many other forms of lapidary materials such as agate, jasper and petrified wood. It is a very simple compound of silicon and oxygen called silicon dioxide, SiO2. The earth’s crust is about 12% quartz with most of it contained in granite. Pure quartz is used in the manufacturing of glass, ceramics, and computer components. It has a hardness of 6.5-7.
If the quartz crystal is pure it is colorless. If it contains impurities it can be purple (amethyst), yellow (citrine), pink (rose), blue, milky or smoky. These crystals are called macrocrystalline since the crystals can be seen by the naked eye. These can also come in many forms such as scepter, artichoke, cactus, etc.
If the crystals are too small and dense to be seen by the naked eye it is called microcrystalline or cryptocrystalline. These are divided into 2 types: fibrous such as agate, carnelian, onyx and chrysoprase or grainy such as chert, flint, and jasper. The differences between these two types can only be seen in thin slices under a microscope.
Whether the crystals formed are macro or micro sized depends on temperature and concentration of the silica molecules. Temperatures above 150C and low concentrations of silica with favor the formation of the macro crystals.
Quartz can be found in veins filling a crack in a host rock that was formed during fold, shattering or cooling of the host rock. The silica “brine” will fill the crack and on cooling form the crystals. If the brine cools quickly milky quartz will form and as the cooling slows the crystals will be less milky or even clear. During cooling and shrinkage of the quartz vein pockets may occur where individual crystals can form.
Sometimes during volcanic eruptions solidifying magma releases hot fluids containing dissolved minerals flow into fracture cracks and cool forming quarts pockets with other crystallized minerals grown in with them. During metamorphosis rocks can release fluids which will cool into veins of crystals. Veins formed by solidifying magma or by metamorphosis are called pegmatites. During the cooling process pockets can form where individual crystals may grow.
Sedimentary rocks such as limestone or dolomite can dissolve or form cracks where pockets of quartz crystals can grow. Gas cavities in magma can also become the home for quartz crystal growths. Mexican coconuts and thunder eggs are two kinds of these forms.
In the next installment we will look at quartz as a “rock forming” mineral.
** All images courtesy of R.Weller/Cochise College