Pyrophyllite [Al₂(Si₄O₁₀)(OH₂)]

Structure Monoclinic
Space Group C2/c (No. 15)
a=5.16 Å b=8.90 Å c=18.64 Å
a=90.0, b=100.55, g=90.0
Z=4

Atomic Positional Parameters

Al  8f  0.000 0.333 0.000
Si1 8f -0.239 0.000 0.143
Si2 8f  0.261 0.167 0.143
O1  8f  0.203 0.500 0.058
O2  8f  0.203 0.167 0.058
O3  8f  0.025 0.084 0.176
O4  8f -0.475 0.084 0.176
O5  8f  0.275 0.333 0.176
OH  8f  0.203 -0.167 0.058

Reference
RWG Wyckoff (1960)
Crystal Structures, Volume 4, Wiley, New York.

Mineral Chemistry
In the layered structure of pyrophyllite, sheets of linked tetrahedra have the composition Si₄O₁₀, compared with AlSi₃O₁₀ found in mica. Al³⁺ ions are located in between two sheets. Here, (di)octahedral sites are formed from inward pointing apical oxygens. Hydroxyl ions are coordinated to the aluminium ions, but not to the Si-O sheets.

The structure of pyrhophyllite may be compared with that of talc which has Mg²⁺ ions in (tri)octahedral sites. Interlayer cations are absent in pyrophyllite because of charge neutrality within the sheets.

Pyrophyllite is an alteration product of the aluminosilicates kyanite, andalusite and sillimanite. Hydrothermal alteration of feldspar will form pyrophyllite.

The CHIME figure shows oxygens as red spheres, and hydroxide ions as blue spheres. Aluminiums are shown as grey spheres. The unit cell is outlined.