Transcript Muscovite
Muscovite By Tiffany Yesavage muscovite chemical composition: KAl2(AlSi3O10)(OH,F,Cl)2 Muscovite is a dioctahedral mica. Only two octahedra are occupied and the third octahedron is vacant. However, in most micas, the number of octahedra is not always exactly 2.0 or 3.0 Substitutions: For K: Na, Rb, Cs, Ca, Ba For octahedral Al: Mg, Fe2+, Fe3+, Mn, Li, Cr, Ti, V For OH: F, Cl Each tetrahedral sheet is of T2O5 composition. In muscovite, Si and Al can occupy the T site The two common planes of junction between the octahedral and tetrahedral layers are pointed to. Muscovite KAl2(AlSi3O10)(OH)2 Paragonite NaAl2(AlSi3O10)(OH)2 A limited solid solution forms between muscovite and paragonite. Above temperatures of 600o the following reactions occur: KAl(Al2Si3O10)(OH)2 > > KAlSi3O8 + Al2O3 + H2O muscovite K-feldspar corundum NaAl(Al2Si3O10)(OH)2 > > NaAlSi3O8 + Al2O3 + H2O paragonite albite corundum Crystallography Monoclinic for 2M1 polytype Point group for 2M1: 2m Polytype 2M1 is the most common. 1M, 3T and 1Md also occur. The polytypes found for muscovite can be distinguished from each other using x-ray diffraction. Each polytype has its own distinctive diffraction lines Distortion in muscovite Because the aluminum atoms in the octhedral sites have a high charge of 3+, they tend to repel each other. In order to shield the aluminum atoms from each other, apical oxygen atoms of the tetrahedral sheets above and below the octahedral chains will change their positions. The octahedra shown are attached to the upper and lower tetrahedral sheets through the apical oxygens. The diagram shows the movement of the apical oxygens in a dioctaheral mica in order to stabilize the repulsion beween the Al3+ atoms. Compared to trioctahedral micas, the hexagonal geometry of the dioctahedral micas is more distorted. In order to have a stable junction between the tetrahedral and octahedral layers, the dimensions of the octahedral and tetahedral sheets must be similar. The tetrahedral sheets usually extend further laterally than the octahedral sheets. The tetrahedral sheets reduce their lateral dimensions through tetrahedral rotation. Because every other cation is moved towards the center of each 6-fold ring, the tetrahedral sheets actually have a ditrigonal geometry rather than a hexagonal symmetry. The tetrahedral and octradehral sheets adjust themselves in order to attain a similar thickness.The tetrahedral t and octahedral angley may Be adjusted in order to thicken or thin the sheets. Tetrahedral: Octahedral: t ideal y ideal 109.28o 54.44o t experimental 110.6o y experimental 63.2o (Gatineau, 1964) These data correspond to a thickening of the tetrahedral sheets and a thinning of the octahedral sheets. Location of the H+ The position of the H+can be determined with neutron diffraction. The H+ points away from the octahedral sheet and towards the tetrahedral sheet. In dioctahedral micas the H+ is directed towards the vacant octahedral site. In trioctadral micas the H+ is directed towards the interlayer, weakening the interlayer bonding. This may explain the difference in weathering patterns between dioctahedral and trioctahdral micas. Occurrences Muscovite is a commonly occurring mineral in igneous, metamorphic and sedimentary rocks. In igneous rocks, muscovite is common in pegmatite, granites and granodiorites. Muscovite is found in a wide variety of metamorphic rocks including slate, schist, gneiss, hornfels and quartzite. Use Was originally used as glass Used in electronics to make insulators, capacitors and transistors Used as a filler in paints and makeup such as lip stick, nail polish and eye shadow.