Download - Talc Mg 3 Si 4 O 10 (OH) 2
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TalcMg3Si4O10(OH)2
PyrophylliteAl2Si4O10(OH)2
Phyllosilicates: Talc and Pyrophyllite
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Crystallographic properties of talc and pyrophyllitepyrophyllite 1Tc talc 1Tc
space group P-1 P-1
a (Å) 5.160 5.290b (Å) 9.966 9.173c (Å) 9.347 9.460
(º) 91.18 90.46(º) 100.46 98.68(º) 89.64 90.09
V (Å3) 424.84 453.80(g/cm3) 2.72 2.80
Interlayer separation (Å) 2.759 2.832Sheet thickness (T) 2.153 2.176Sheet thickness (O) 2.079 2.168Octahedral angle (º) 57.1 61.5 (M1)
61.5 (M2)(ideal) 54.73 54.73
(Lee and Guggenheim 1981, Perdakatsis and Burzlaff 1981)
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c-axis projection of T-O-T sheet in talc. Light green - Mg1, Dark Green - Mg2, Blue - Si, Red - O, small spheres - H (Perdikatsis and Burzlaff 1981)
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T-O-T layers in 2M talc. Silica tetrahedra are distorted with coplanar basal oxygens.Light green - Mg1, Dark green - Mg2, rlue - Si, red - O, small blue spheres - H(Evans and Guggenheim 1988)
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c-axis projection of a single T-O-T layer in 1Tc pyrophyllite. Al - orange, Si - blue, O - red, OH - pink. (Lee and Guggenheim 1981)
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T-O-T layers in 1Tc pyrophyllite. Note that the basal oxygens are slightly noncoplanar.Al - orange, Si - blue, O - red, OH - pink. (Lee and Guggenheim 1981)
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Layer Stacking
• Not constrained by intelayer cations (as in micas) or hydrogen bonding.
• Empty cation sites between layers are large and irregular, therefore the structure is distinct from a cation-deficient mica
• Stacking disorder is ubiquitous, therefore it is not worthwhile to differentiate between polytypes generally between crystals
• T-O-T layers are shifted by approximately 0.3a to minimize Si - Si repulsion
• Common polytypes are 2M for talc and 1Tc for pyrophyllite, however 22 pyrophyllite polytypes and 10 talc polytypes are known, only some of which can be identified by x-ray analysis
(Durovic and Weiss 1979, Bailey 1984)
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Chemistry of talc and pyrophyllite
• Possible complete series with minnesotaite (Fe3Si4O10(OH)2)• Minor Al substitution for Si, no more than 4% • Trace amounts of Mn, Ti, Ni, Cr, Ca, Na, K• Mssbauer studies indicate in hydrothermal talc all iron is Fe+3, while talc from weathering profiles can contain Fe+2 or Fe+3
• Small amount of Al can substitute for Si• Octahedral sites contain less > 95% Al, remainder being Fe+2, Fe+3, Mg, or Ti
Talc:
Pyrophyllite:
Both talc and pyrophyllite are often found to be very pure in composition.
Evans and Guggenheim 1988)
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Layer stacking in talc and pyrophyllite is not constrained by interlayer cations. Adjacent tetrahedral layers are offset approximately 0.3a due toSi+4 repulsion. (Evans and Guggenheim 1988.)
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Occurrences of talc and pyrophyllite
• High pressure metamorphic rocks rich in Mg but with low amounts of Al (chlorite), Ca (tremolite), and K (phlogopite)
• Stable in surface environments as a precipitate over serpentine + quartz
• Alteration product of olivine and orthopyroxene
• Al-rich metamorphic rocks, much less common than talc
Talc:
Pyrophyllite:
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T = 400ºC
Thermal decomposition of talc and pyrophyllite
• Talc undergoes rapid decomposition to enstatite + cristobalite + water at 600ºC (800ºC above 2kbar)
• Pyrophyllite loses water and becomes pyrophyllite dehydroxylate above 450ºC. The structure contains under- and over-bonded oxygens and 5-coordination Al
Mg3Si4O10(OH)2 3MgSiO3 + SiO2 + H2O
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Uses of talc and pyrophyllite
• Used in the manufacture of ceramics• As a filler material in plastic, rubber, paint, insecticides, paper, and other
materials• Made into acid and heat resistant laboratory countertops• Cosmetics and personal products• Used as an ornamental stone for carvings