Talc [Mg₃[Si₄O₁₀](OH)₂]

Structure Triclinic
Space Group C-1
a=5.2900 Å b=9.1730 Å c=9.4600 Å
a=90.460, b=98.680, g=90.090
Z=2

Atomic Positional Parameters

Si1  4* 0.2453 0.5026 0.2909
Si2  4* 0.2459 0.8359 0.2911
Mg1  2* 0.0000 0.0000 0.0000
Mg2  4* 0.5001 0.8333 0.9999
O1   4* 0.1991 0.8344 0.1176

O2   4* 0.6970 0.6674 0.1126
O3   4* 0.1980 0.5012 0.1176
O4   4* 0.0199 0.9287 0.3481
O5   4* 0.5202 0.9109 0.3481
O6   4* 0.2429 0.6699 0.3484

H1   4* 0.7190 0.6690 0.2030

Reference
B Perdikatsis and H Burzlaff (1981) Zeitschrift fuer Kristallographie, 156, 177

Mineral Chemistry
The structure of talc is composed of Si₂O₅ sheets with magnesium sandwiched between sheets in octahedral sites, and in tri-octahedral arrangement (see brucite). Talc is a secondary mineral formed from hydrothermal alteration of magnesium silicates. It is often associated with serpentine occurring in veins with magnesite or quartz. Low grade thermal metamorphism of siliceous dolomites also forms talc.

The Chime figure shows the silicon-oxygen sheets, and the magnesiums - green spheres - coordinated by both oxygens and hydroxide ions (OH^-), shown as blue spheres. Note that the hydroxides do not bond to the silicon-oxygen sheets in the structure. The unit cell is outlined.