![]() ![]() View of trioctahedral mica structure looking along sheetsĬhemically, micas can be given the general formula X 2 Y 4–6 Z 8 O 20( OH, F) 4, However, the original hexahedral symmetry is discernible in the pseudohexagonal character of mica crystals. ![]() This breaks the hexagonal symmetry and reduces it to monoclinic symmetry. īecause the hexagons in the T and O sheets are slightly different in size, the sheets are slightly distorted when they bond into a TOT layer. The remaining negative charge of the TOT layer is neutralized by the interlayer cations (typically sodium, potassium, or calcium ions). The octahedral sheet has a positive charge, since its bulk composition is Al(OH) 2+ (for a dioctahedral sheet with the apical sites vacant) or M 3(OH) 2 4+ (for a trioctahedral site with the apical sites vacant M represents a divalent ion such as ferrous iron or magnesium) The combined TOT layer has a residual negative charge, since its bulk composition is Al 2(AlSi 3O 10)(OH) 2 − or M 3(AlSi 3O 10)(OH) 2 −. Tetrahedral sheets have a strong negative charge, since their bulk composition is AlSi 3O 10 5. Apical oxygens take the place of some of the hydroxyl ions that would be present in a brucite or gibbsite sheet, bonding the tetrahedral sheets tightly to the octahedral sheet. A dioctahedral sheet has the structure and (typically) the composition of a gibbsite sheet, with aluminium being the cation. A trioctahedral sheet has the structure of a sheet of the mineral brucite, with magnesium or ferrous iron being the most common cation. The octahedral sheet can be dioctahedral or trioctahedral. The remaining oxygen ion (the apical oxygen ion) is available to bond with the octahedral sheet. The tetrahedra each share three of their four oxygen ions with neighboring tetrahedra to produce a hexagonal sheet. In most micas, one in four silicon ions is replaced by an aluminium ion, while half the silicon ions are replaced by aluminium ions in brittle micas. The tetrahedral sheets consist of silica tetrahedra, which are silicon ions surrounded by four oxygen ions. It is the relatively weak ionic bonding between TOT layers that gives mica its perfect basal cleavage. The TOT layers in turn consist of two tetrahedral sheets ( T) strongly bonded to the two faces of a single octahedral sheet ( O). The crystal structure of mica is described as TOT-c, meaning that it is composed of parallel TOT layers weakly bonded to each other by cations ( c). Deposits of mica tend to have a flaky or platy appearance. Micas are translucent to opaque with a distinct vitreous or pearly luster, and different mica minerals display colors ranging from white to green or red to black. All crystallize in the monoclinic system, with a tendency towards pseudohexagonal crystals, and are similar in structure but vary in chemical composition. The tutorial linked below uses the free image utility FastStone Image Viewer to reprocess progressive images in batch mode, which quickly converts them to normal JPGs.The mica group is composed of 37 phyllosilicate minerals. Luckily, these files are easy to detect since they show up pixelated or takes a long time to load. While computers have powerful software and hardware to handle progressive JPGs, inexpensive electronics will often experience difficulties displaying them. They are obsolete and should all be converted to normal JPG files. ![]() Progressive JPGs were created during the days of dial-up Internet to allow partial image loading on slow connections. If certain photos show up on the photo frame as pixelated and blurry but looks perfectly fine on a computer, it is most likely due to the photo being a progressive JPG image file. Photos show up as pixelated or takes very long time to load ![]()
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