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All minerals which consist of silicon dioxde (SiO2) belongs to this category. There are many modifications of silicon dioxide. Quartz is a very huge mineral group. Quartz consists of a net of [SiO4]4--tetrahedrons. The relation is that for one silicon atom there are two oxygen atoms and so the formula is SiO2. To make a quartz crystal a chemical solution with ortho silicic acid has to be supersaturated. This solutions crystallizes and water is splitted off. Ortho silicic acid has got the chemical formula H4SiO2. In geodes often quartz can be found which grew on a chalcedone layer. On the picture on the top there is the form of an ideal crystal. Through disturbances and interruptions of the growth real crystals looks a little bit different than ideal crystals. When the surfaces which form the spike (rhombohedron surfaces) has got all the same size then the crystal is mostly twinned. Many crystals are twinned. Except distorted crystals there are more forms of quartz: thread quartz, sceptre quartz, window quartz, rung quartz, star quartz. When in a material the atom or the molecule only influences the neighbour, but not the neighbour of this neighbour, the whole material would not be symmetrical (near order), but if it can also influences the neighbour of this neighbour then it is very symmetrical. In the structure of quartz we find the symmetrical possibility. This order is called far order. In most quartzes there is this far order and this is the reason that they look like they were sharpened. The perfect quartz should have also two ends and so two spikes. Most crystals cannot have two spikes, because they have got starting points where they grow. When they are lying during they grow or when they grow in water then they can have two ends. These quartz crystals are called double enders. They are rare and so they are more expensive than normal crystals. Quartz crystallizes in the trigonal crystal system and it has got the hardness 7 on the Mohs' scale. Silicon dioxide has got a melting point of 1710 degrees and a boiling point of 2230 degrees. Because of this temperature resistance quartz glass is used to keep very hot materials. The other advantage is the high resistance against acids. All acids can be kept in quartz glass except hydrofluoric acid (HF). Hydrofluoric acid is a hydrogen halide and so it is very similiar to hydrochloric acid (HCl). Silicon dioxide has got a very small coefficient of expansion. This means that it expands very little when the temperature grows. The advantage is that a quartz glass would not break when the temperature grows like normal glass. Pure quartz is colorless and transparent and is called rock crystal. Colored, crystalline quartzes are for example amethyst (violet), citrine (yellow), smoke quartz (brown, black) and rose quartz (rose, big crystals are very rare). Because of admixtures of other substances or radioactive radiation these quartzes have got theirs colors. There are also blue quartz (blue), milk quartz (white and not transparent), aventurine (green) and ametrin (mixture of amethyst and citrine).
Except crystalline quartzes there are also crypto and micro crystalline quartzes. These quartzes has not got huge crystals. Chalcedone and agate belong to them. Agate belongs to the chalcedones, but agate contains also some varieties. When we talk about chalcedone in general then we mean a bluish or white mineral. Except agate chalcedone contains jasper (red, yellow, green), carneol (brown-red), chrysoprase (green), heliotrope (green), flint (grey-white) and many more. Petrified wood and petrified fossils also belong to chalcedone. The wood or fossils are replaced through silicon dioxide, but their structure remains. In agate there are different color layers which are arranged in a circle around each other. Agate contains onyx, sardonyx, eye agate, moss agate, dendrite agate, flame agate, fire agate and many more. At the end we have got the mineral opal. It also consists of silicon dioxide, but it is amorph. Because of water in opals they gleam in many different colors. There are also petrified wood and petrified fossils which consists of opal. Because of admixed substances chalcedone and opal have go so many colors.
Author and Webmaster: Lukas Czarnecki
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Last Update: 03.11.2004