Diopside [CaMg(Si₂O₆)]

Structure Monoclinic
Space Group : C12/c1 (No. 15)
a=9.7500 Å, b=8.9260 Å, c=5.2510 Å
a=90.00, b=105.90, g=90.00
Z=4

Atomic Positional Parameters
Ca 4e 0.0000 0.3011 0.2500
Mg 4e 0.0000 0.9082 0.2500
O1 8f 0.3507 0.0179 -0.0043
O2 8f 0.3615 0.2496 0.3196
O3 8f 0.1159 0.0877 0.1423

Si 8f 0.2866 0.0933 0.2302

Reference
E Bruno, S Carbonin and G Molin (1982)
Tschermaks Mineralogische und Petrographische, 29, 224

Mineral Chemistry
Diopside is a member of the clinopyroxene group of minerals, and forms a complete solid solution with hedenbergite (CaFe[Si₂O₆]). The structure consists of chains of SiO₃ held together by magnesium in octahedral sites, and calcium in eight-coordinated sites. A metamorphic mineral, diopside is characteristic of thermally metamorphosed, siliceous, dolomitic limestones.

Replacement of Si⁴⁺ by Al³⁺ ions within SiO₃ chains permits inclusion of trivalent and tetravalent ions outside the chains. The pyroxene mineral augite, which arises from such replacement, may adopt a variety of compositions within the general formula (Ca,Na,Mg,Fe²⁺, Mn,Fe³⁺,Al,Ti)₂[(Si,Al)₂O₆].

In the CHIME figure, calciums are shown as large grey spheres. Calciums are in an eight-fold coordination environment. Magnesiums are green spheres, silicons are ochre, and oxygens are red. The unit cell is outlined.