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Anorthite [Ca(Al2Si2O8)]
Structure Triclinic
Space Group : P-1 (No. 2)
a=8.1730 Å b=12.8690 Å c=14.165
Å
a=93.113,
b=115.913,
g=91.261
Z=8
Atomic Positional Parameters
see list below
Reference
JE Wainwright and J Starkey (1971) Zeitschrift fuer Kristallographie, Kristallgeometrie,
Kristallphysik, Kristallchemie, 133, 75
Mineral Chemistry
Pure anorthite is the calcium end-member of the plagioclase feldspar series: Na(AlSi3O8) - Ca(Al2Si2O8). The triclinic crystal structures of plagioclase feldspars comprise a framework of linked (Si,Al)-O tetrahedra, with interstices occupied by calcium or sodium ions. See the Bravais lattice page for a description of a triclinic lattice.
In pure anorthite, AlO4 and SiO4 tetrahedra alternate regularly forming a fully ordered structure, and calciums are located within an irregular cavity bounded by about10 oxygens. Partial replacement of calcium (2+) by sodium (1+) in anorthite requires equivalent replacement of aluminium (3+) by silicon (4+) - regular alternation of aluminiums and silicons is no longer possible.
At elevated temperatures, the plagioclase series forms an essentially solid solution from pure albite [Na(AlSi3O8)] to pure anorthite. Six divisions in this series give the mineral names albite, oligoclase, andesine, labradorite, bytownite and anorthite. As ubiquitous rock forming minerals, plagioclase feldspars are the commonest mineral series, and are found as igneous, metamorphic and sedimentary rocks. Pure anorthite itself is rare in igneous rocks. It occurs as ejected blocks amongst old lavas (Mt. Somma, Vesuvius) and in basic plutonic rocks and lavas. Calcium-rich plagioclase may also be found in contact-altered limestones.
The CHIME figure shows Ca ions as large grey spheres. The aluminosilicate framework is shown with oxygens red, silicons ochre, and aluminium ions yellow spheres. The unit cell is
outlined.
Atomic Positional Parameters
O 1 2(i) 0.0269 0.1242 0.9960
O 2 2(i) 0.9812 0.1257 0.4835
O 3 2(i) 0.4875 0.6241 0.4868
O 4 2(i) 0.5169 0.6247 0.9966
O 5 2(i) 0.5744 0.9913 0.1434
O 6 2(i) 0.5720 0.9897 0.6379
O 7 2(i) 0.0731 0.4875 0.6354
O 8 2(i) 0.0734 0.4932 0.1386
O 9 2(i) 0.8154 0.1018 0.0806
O10 2(i) 0.8124 0.0968 0.6057
O11 2(i) 0.3325 0.5957 0.6047
O12 2(i) 0.2854 0.6034 0.0798
O13 2(i) 0.8175 0.8554 0.1443
O14 2(i) 0.8113 0.8518 0.6034
O15 2(i) 0.2987 0.3559 0.6115
O16 2(i) 0.3419 0.3587 0.1333
O17 2(i) 0.0141 0.2796 0.1351
O18 2(i) 0.0205 0.2909 0.6474
O19 2(i) 0.5094 0.7769 0.6344
O20 2(i) 0.5092 0.7965 0.1510
O21 2(i) 0.0008 0.6806 0.1044
O22 2(i) 0.0089 0.6899 0.6013
O23 2(i) 0.5165 0.1788 0.6101
O24 2(i) 0.5071 0.1963 0.0975
O25 2(i) 0.1826 0.1059 0.1917
O26 2(i) 0.2155 0.1025 0.6847
O27 2(i) 0.6989 0.6079 0.6790
O28 2(i) 0.6908 0.6043 0.2019
O29 2(i) 0.2038 0.8740 0.2107
O30 2(i) 0.1709 0.8564 0.7197
O31 2(i) 0.6884 0.3628 0.7332
O32 2(i) 0.7006 0.3697 0.1970
Ca1 2(i) 0.2651 0.9864 0.0867
Ca2 2(i) 0.2692 0.0312 0.5435
Ca3 2(i) 0.7737 0.5359 0.5412
Ca4 2(i) 0.7634 0.5052 0.0747
Si1 2(i) 0.0092 0.1592 0.1044
Al1 2(i) 0.0066 0.1610 0.6112
Si2 2(i) 0.5062 0.6560 0.6042
Al2 2(i) 0.4984 0.6658 0.1128
Al3 2(i) 0.9912 0.8152 0.1176
Si3 2(i) 0.0061 0.8154 0.6135
Al4 2(i) 0.5073 0.3145 0.6212
Si4 2(i) 0.5041 0.3204 0.1099
Al5 2(i) 0.6845 0.1130 0.1519
Si5 2(i) 0.6814 0.1034 0.6646
Al6 2(i) 0.1907 0.6110 0.6674
Si6 2(i) 0.1713 0.6067 0.1495
Si7 2(i) 0.6742 0.8829 0.1876
Al7 2(i) 0.6809 0.8719 0.6725
Si8 2(i) 0.1762 0.3789 0.6734
Al8 2(i) 0.1852 0.3775 0.1816