A few substances that are sometimes included in the Quartz Group because they also contain SiO2 are classified as mineraloids. They are opal, SiO2 - n(H2O) and a very rare pure silica glass called lechatelierite, SiO2. Both of these are amorphous and therefore lack a true crystal structure. Many mineralogists refuse to consider them to be true minerals because of this and the mineraloid class is a good compromise.
An interesting comparison within the quartz group is between the high temperature minerals, high pressure minerals and quartz. Both high temperature minerals, cristobalite and tridymite, have both a lower density and index of refraction than quartz. Since the chemistry is the same, the reason for the differences must be in the increased spacing in the high temperature minerals. As is common with most substances the higher the temperature the farther apart the atoms due to the increased vibration energy.
The high pressure minerals, stishovite and coesite, on the other hand, have a higher density and index of refraction when compared to quartz. This is probably due to the intense squeezing together of the atoms that must occur during their formation and the condensed structure that they are actually forced into forming.
At surface temperatures and pressures, ordinary quartz is the most stable form of silicon dioxide, to no one's surprise. Quartz will remain stable up to 573 degrees Celsius at 1 kilobar of pressure. As the pressure increases the temperature at which quartz will lose stability also increases.
Above 1300 degrees and at a pressure of approximately 35 kilobars, only beta quartz (also known as high quartz) is stable. Beta quartz is not the same as normal quartz, actually referred to as alpha quartz, low quartz or, as is mostly done here, just quartz. Beta quartz has higher symmetry, is less dense and has a slightly lower specific gravity. The conversion, from one solid substance to another solid substance, of quartz to beta quartz is quick, reversible and accompanied with a slight energy absorption. The conversion in fact is so easily accomplished that a crystal of quartz heated to beta quartz, cooled back down, heated again to beta quartz, etc and the crystal when all is done, will be the same as when it started.
The reason that the conversion is so easily accomplished is that the difference between quartz and beta quartz is relatively slight. The bonds between the oxygens and silicons are "kinked" or bent in quartz and are not so "kinked" in beta quartz. At the higher temperatures the atoms move away from each other just enough to allow the bonds to unkink or straighten and produce the higher symmetry. As the temperature is lowered, the atoms close in on each other and the bonds must kink in order to be stable and this lowers the symmetry back down again.
Although all quartz at temperatures lower than 573 degrees Celsius is low quartz, there are a few examples of crystals that obviously started out as beta quartz. Sometimes these are labeled as beta quartz but are actually examples of pseudomorphic or "falsely shaped" crystals more correctly labeled 'quartz after beta quartz'. These crystals are of higher symmetry than low quartz although low quartz can form similar crystals to them. They are composed of hexagonal dipyramids which are a pair of opposing six sided pyramids and the crystals lack prism faces. Quartz's typical termination is composed of two sets of three rhombic faces that can look like a six sided pyramid. Other differences between beta quartz and quartz are shown in the table below.
Keatite is a synthetic substance that has only been produced in a laboratory. It has never been found in nature but it is a possibility that one day a natural specimen might be discovered. Until then, it is not considered a mineral because minerals exclude man-made substances (those substances are studied by metallurgists and chemists, not geologists). Some of keatite's physical characteristics are listed in the table below.
Beta Cristobalite and Beta Tridymite:
Just like quartz and beta quartz, there exists phase relationships with cristobalite and tridymite and higher symmetry phases referred to as betas. They both are higher temperature phases than the lower temperature minerals. They both have higher symmetries and lower densities than their mineral "brothers". The conversions between the alpha phases and the beta phases are as easily accomplished as in the alpha quartz - beta quartz conversions. The differences between cristobalite and beta cristobalite as well as the differences between tridymite and beta tridymite are shown in table below.
Interestingly the structure of beta cristobalite is analogous to the structure of diamond. Diamond is composed of pure carbon, but each carbon atom has four identical bonds that connect to other carbon atoms. These bonds lay as far apart from each other as four bonds can get in three dimensions or in a tetrahedral shape. By replacing the tetrahedrons in the diamond structure with the tetrahedrons of SiO4, we get approximately the structure of beta cristobalite.
The structure of beta tridymite is composed of sheets of SiO4 tetrahedrons linked into six membered rings, hence the hexagonal symmetry. The sheets are unlike the sheets of the phyllosilicates in that these sheets are linked together by strong oxygen to silicon bonds. The tetrahedrons alternately point up then down to connect to the sheets above and below making tridymite's true tectosilicate structure.
These are members of the Quartz Group:
|With Mineral Name:||Crystal System:||Symmetry:||Specific Gravity:||Index of Refraction:|
|Coesite||Monoclinic||2/m||SG= 3.00||IR= 1.59|
|Cristobalite||Tetragonal||4 2 2||SG= 2.32||IR= 1.48|
|Beta Cristobalite||Isometric||4/m Bar 3 2/m||SG= 2.20||IR= 1.48|
|Keatite||Tetragonal||4 2 2||SG= 2.50||IR= 1.52|
|Quartz||Trigonal||3 2||SG= 2.65||IR= 1.55|
|Beta Quartz||Hexagonal||6 2 2||SG= 2.53||IR= 1.54|
|Stishovite||Tetragonal||4/m 2/m 2/m||SG= 4.28||IR= 1.81|
|Tridymite||Monoclinic*||2/m or m||SG= 2.26||IR= 1.47|
|Beta Tridymite||Hexagonal||6/m 2/m 2/m||SG= 2.22||IR= 1.47|
|Opal, SiO2 - n(H2O) and a very rare pure silica glass called lechatelierite, SiO2 are amorphous, do not have symmetry and have variable properties, but they are sometimes considered to be a part of the Quartz Group.|