Atomic Radius Si = 1.46 Angstrom, O = 0.65 A
SiO2 and H2O [water] are so closly related structurally that they go through the exact same crystal structures in the same order as pressure and heat are applied or removed, with only a 2,000 degree temperature variance between them. That is, as water freezes and passes through the crystaline phases of Ice I to Ice VIII, liquid silica is doing the same thing with low temperature quartz, high temperature quartz, etc. Whoever takes time to closely examine this phenomena will make some great advances in our understanding of crystalography.
On heating low quartz, it converts to high quartz, low mid and high tridymite, low and mid christobalite, lussatite [silica W?] then it melts and eventually turns to a gas.
On cooling, T.P.s are sluggish, sometimes requiring physical shock for the transition to complete. This is particularly noticable in chrystobalite, retaining its structure far below its T.P., but the presence of a small amount of Keatite rapidly converts the whole mass into quartz.
Hardness = N/A ; Refractive Index = N/A ;Cell Parameters = N/A
Silica boils at 2503 K [at 0 KBars] and 3773 K [at 50 KBars] and condenses on cooling back to those same temperatures. It can be super-cooled to 237 - 385 K and crystalize directly into the crystaline state. See Melanophlogite below.
Hardness = ; Refractive Index = ; Cell Parameters =
Silica melts at 1,743 K [at 0 KBars] and 2,503 K [at 50 KBars].
Hardness = ?; Refractive Index = ?; Cell Parameters = ?
Mentioned in the literature as fiberous silica, may be chalcedony.
Hardness = 6 - 7; Refractive Index = 1.424; Cell Parameters: a = 13.402 Angstroms; Volume = 611.047 cubic Angstroms.
Melanophlogite is a cubic silica mineral with a gas-hydrate structure containing many large voids that are often filled with 6 to 12 % by weight of compounds of Hydrogen, Carbon, and Sulfur that may be necessary for crystal growth. If these minerals are driven off by heating, the crystals may be darkened by carbon, but do not collapse. Crystals form at temperatures below 112 degrees Centegrade [234 degrees F] and have been collected along with calcite and celestite on opaline silica crusts on sulfur at Girgenti, Sicily
Hardness = 5.5 - 6.5; Refractive Index = 1.43
Common Opal [potch] is formed by precipitation of random sized Cristobalite/Tridymite spheres and contains 4 to 9% water.
When nearly equal sized spheres are formed, they may be arranged in either hexagonal or cubic close packing that form diffraction gratings providing the brilliant colors of Precious Opal.
Hardness = 5.5; Refractive Index = 1.43; Cell Parameters = ?
Usually found in puff ball like spheres.
Hardness = 6 - 7; Refractive Index = 1.486; Cell Parameters: a = 7.15 A at 1,300 C. Volume = 365.529 Cubic A.
Temperature of Initial Crystalization = 1,743 K [at 0 KBar] and 1986 K [at 5 KBar]
High Crystobalite is deposited in cavities and cracks of many volcanic rocks from hot hydrous gases. It has been found in Lunar basalts, meteorites, and is common in silica refractories. It is the stable phase of silica at one atmosphere of pressure from about 1,470 degrees Centigrade [2,678 F] to the melting point [1,790 C]. It can be super-cooled to 175 C - 270 C [350 F - 520 F] where it changes reversibly to Low Crystobalite. High Crystobalite consists of sheets of silica tetrahedra stacked in sheets of three sheets normal to a three fold axis: ABC-ABC-ABC in six member rings. This provides cubic closest packing for the oxygen atoms.
Hardness = 7[?]; Refractive Index = ?; Cell Parameters [at 200 C]: a = 8.74 A, b = 5.05 A, c = 8.24 A, Cell Volume = 362.89 Cu. A.
Temperature of Initial Crystalization = 1,143 K [0KBar] and 1743 K [3 KBar]
Tridymite is an often unrecognized silica mineral that may form as a direct crystalization from a silica magmatic melt, but it is most often found in volcanic rock voids where it probably was deposited from hydrous gases. It also forms in contact Metamorphic rocks, has been found in lunar basalts, and meteorites. It forms in quantity in Fire Bricks in steel mills and other refractories. It is the stable form at one atmosphere pressure from about 876 C to 1,470 C [1,593 F to 2,678 F] containing two hexagonal sheets of cross-linked tetrahedra stacked AB-AB-AB in six member rings perpendicular to the c-axis. Although it is dimensionally hexagonal, it has orhorhombic symmetry due to three mutually perpendicular crystallographic axis of unequal length. As the temperature drops through it's Transition Point [about 163 C] tridymite retains an orthorhombic sub-cell, but it may contain as many as twenty sheets.
Hardness = 6; Refractive Index = ?; Cell Parameters = N/A
Temperature of Initial Crystalization 0 K to 1986 K
Vitrious Silica, also known as "Lechatelierrite" is soper-cooled liquid silica. It is formed in nature by lightening strikes in silica soils, or by shock from large meteorite impacts where it may approch pure man made silica glass in composition and properties. It may devitrify into tiny crystallites of crystobalite.
Hardness = ?; Refractive Index = ?; Cell Parameters = ?
190 K to 436 K
Hardness = 7; Refractive Index = 1.437; Cell Parameters: a = 18.54 A, b = 4.99 A, c = 23.83 A, Cell Volume = 2,158.5 C. A. b = 105.5 deg. Temperature of Crystalization = 0 to 390 K.
Tridymite usually found in small [up to 1/16 inch] thin hexagonal terminated plates, appears to have six fold symmetry [Psuedo Hexagonal], but careful study shows it to be formed of thin plates made up of three crystals in triplet relationship, thus the name that means "triplet.". There are two ways spheres can be stacked with the greatest possible compaction and still have a periodic arrangement, cubic and hexagonal. Tridymite uses the latter. The structure is tetrahedra cross linked into hexagonal sheets stacked in regular, irregular, or random sequences, and the stacking may be different in other places in the same cystal. Non-terrestrial tridymite exibits a monoclinic supercell structure not found on earth, probably caused by our unique atmosphere. Tridymite transforms slowly to Quartz when enough energy is added to the crystal to disrupt and re-link tetrahedra.
Hardness = 6 - 7; Refractive Index = 1.486; Cell Parameters: a = 4.97 A, c = 6.93 A, Cell Volume = 171.171 Cubic Angstroms
Temperature of Crystalization = 0 to 523 K
Low Cristobalite show complex complex polysenthetic twinning [twins of three or more individual crystals conforming to the same twin law] and transforms reversably into High Cristobalite in the range 175 C to 270 C [350 F to 520 F].
Hardness = ?; Refractive Index = ?; Cell Parameters: a = 7.456 A, c = 8.604 A, Cell Volume = 478.59 Cubic Angstroms.
Keatite is only known to form in the presence of steam in the temperature range of 300 to 600 degrees C [[600 - 1,200 F, or 573 K [0.4 KBars] to 873 K [40 KBars]. It forms 5 and 7 sided tetrahedral rings that have the ability to attach to Cristobalite and convert it to Keatite while the original ring converts to High Quartz, starting a reaction that quickly changes the whole Cristobalite mass onto Quartz!
Hardness = 7; Refractive Index = ; Cell Parameters: a = 4.999 A, c = 5.457 A [at 575 C] Cell V =136.875 cubic Angstroms.
Temperature of Crystalization = 846 K [0.4 KBar] to 1143 K [40 KBar]
High Quartz is easily recognized by the hexagonal termination points. It can form directly from silicate magmas, high temperature gases, or from silica rich hot solutions. It crystalizes at about 573 C [1,063 F]. As the pressure increases, temperature must also increase to maintain the process. High Quartz has six fold symmetry along the c axis and may be right or left handed. This prevents most of the twining effects of Low quartz, so a typical High Quartz twin will show inclined sets of axis.
On cooling below the transiton point of 573 C it invariably transforms into Low Quartz, but maintains its outer form so it is in reality, a psuedomorph. All Low Quartz changes into High Quartz on being heated beyond 573 C. Crystals sometimes crack or shatter in the change as the linkage between tetrahedra are broken and re-linked.
Hardness = 7; Refractive Index = ?; Cell Parameters: a = 4.913 A, c = 5.405 A, Cell Volume = 130.976 Cubic Angstroms.
Temperature of Crystalization = 0 K [0 KBar] to 846 K [32 KBars]
Low Quartz is easily recognized by its trigonal termination points. The higher the temperature at the time of crystalization, the sharper the termination point. Crystals may be right or left handed [the tetrahedra may spiral up clockwise or counter clockwise to the tip in three fold symmetry] Twinning is common, even if not easily visible. "Optical Twinning" occurs when two crystals intergrow with opposite handedness and the a- axis has opposite electrical polarity. ""Electrical Twinning" occurs as above, but both crystals have the same handedness and the a - axis still has opposite electrical polarity. "Attachment twins" do not penetrate each other and their axis are inclined to each other.
Hardness = 7.5; Refractive Index = ?; Cell Parameters: a = 7.16 A, b = 12.39 A, c = 7.16 A, b = 120 deg., Cell V = 635.235 Cubic A. Temperature and Pressure of Crystalization = 0 K [19 KBars] to 3000 K [120 KBars]
Coesite is dimensionaly hexagonal, but structurally monoclinic, made up of tetrahedra arranged like those in feldspars. It may exist naturally at depths of 100 kilometers [60 miles] if enough free silica is present there, but is only found on the surface at sites shocked by large meteorite impacts.
Hardness = ?; Refractive Index = ?; Cell Parameters: a = 4.179 A, c = 2.665 A. Cell Volume = 46.47 Cubic Angstroms
Temperature and pressure of crystalization = 0 K [68 KBar] to 3000 K [? KBar]
Stishovite is the densest form of silica known. Only pressures at depths in excess of 200 kilometers [120 miles] are sufficient to form it. It is only found at sites where quartz bearing rocks have been intensly shocked by very large or fast meteorite impacts. Such pressures force silica to bond with six oxygen atoms in an octahedral structure.
