talc- talc

详情

 Cleavage
Perfect on {**1} basal cleavage

Fracture
Flat surfaces (not cleavage), fracture in an uneven pattern
 
Mohs scale hardness 1 (defining mineral)
Luster
Waxlike or pearly
Streak
White to pearl green
Diaphaneity
Translucent
Specific gravity
2.*8 to 2.*3
 
 
 
Pleochroism
Weak in dark varieties
Ultraviolet fluorescence
Short UV=orange yellow, long UV=yellow

Analysis
No. 1 of talc
SiO2  MgO  Fe2O3  Al2O3  L.O.I  Bright
*2.*6  *1.*4  0.*5  0.*2  4.*8  *6
Grade No. 2
SiO2  MgO  Fe2O3  Al2O3  L.O.I  Bright
*2.7  *0.*4  0.*9  0.*6  4.*1  *5

Grade No. 3
SiO2  MgO  Fe2O3  Al2O3  L.O.I  Bright
*2.*1  *0.*4  0.*1  0.*8  5.*2  *7

general

Talc is a metamorphic mineral resulting from the metamorphism of magnesian minerals such as serpentine, pyroxene, amphibole, olivine, in the presence of carbon dioxide and water. This is known as talc carbonation or steatization and produces a suite of rocks known as talc carbonates.
Talc is primarily formed via hydration and carbonation via the following reaction;
serpentine + carbon dioxide → talc + magnesite + water
2 Mg3Si2O5(OH)4 + 3CO2 → Mg3Si4O*0(OH)2 + 3 MgCO3 + 3 H2O
Talc can also be formed via a reaction between dolomite and silica, which is typical of skarnification of dolomites via silica-flooding in contact metamorphic aureoles;
dolomite + silica + water → talc + calcite + carbon dioxide
3 CaMg(CO3)2 + 4 SiO2 + H2O → Mg3Si4O*0(OH)2 + 3 CaCO3 + 3 CO2