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Sanidine [KAlSi3O8]

Structure Monoclinic
Space Group : C2/m(No. 12)
a=8.5950 Å, b=13.0280 Å, c=7.1750 Å
a=90.00, b=115.94, g=90.00
Z=4

Atomic Positional Parameters
K1  4i 0.7857 0.0000 0.1379
Si1 8j 0.2113 0.1250 0.3333
Si2 8j 0.5000 0.2084 0.2113
O1  4g 0.0000 0.3333 0.0000
O2  4i 0.2113 0.0000 0.3333

O3  8j 0.8169 0.3333 0.2113
O4  8j 0.0000 0.1667 0.2113
O5  8j 0.8169 0.3333 0.5773

Reference
TA Scambos, JR Smyth, T McCormick (1987) American Mineralogist, 72, 973

Mineral Chemistry
Sanidine, a potassium feldspar, has a silicate structure formed by (Si,Al)O4 tetrahedra linked to form a 'framework'. Potassium ions occupy large interstices resulting from the framework arrangement; each potassium is coordinated by nine oxygens.

Sanidine is characteristic of volcanic rocks rapidly cooled from a high eruption temperature.

The CHIME page has K atoms shown as large unbonded crimson spheres. Use Display-Wireframe, or Display-Sticks in CHIME to show the Si-O feldspar framework. The unit cell is outlined.