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Lawsonite [CaAl2Si2O7(OH)2.H2O]
Structure Orthorhombic
Space Group : Ccmm (No. 63)
a=8.795 Å, b=5.847 Å, c=13.142 Å
a=b=g=90.0
Z=4
Atomic Positional Parameters
Al 8d 0.2500 0.2500 0.0000
Ca 4c 0.3331 0.0000 0.2500
Si 8f 0.9804 0.0000 0.1330
H 8f 0.6731 0.0000 0.1905
H 8f 0.5329 0.0000 0.0704
O 16h 0.3788 0.2726 0.1169
O 8f 0.1378 0.0000 0.0650
O 8f 0.6391 0.0000 0.0479
O 4c 0.6097 0.0000 0.2500
O 4c 0.0495 0.0000 0.2500
References
WH Baur (1978) Crystal structure refinement of lawsonite. American Mineralogist 63:311-315.
AR Pawley (1994) The pressure and temperature stability limits of lawsonite: implications for H2O recycling in subduction zones. Contrib. Mineral. Petrol., 118, 99-108.
Mineral Chemistry
Lawsonite, one of several dense hydrous silicates found in the Franciscan formation, California, crystallises at low temperatures and high pressures in metamorphic rocks. The crystal structure consists of a framework of edge-sharing Al(O,OH) octahedra bridged by Si2O7 groups. Large cavities in this framework contain calcium ions and water molecules resulting in the overall composition of CaAl2Si2O7(OH)2.H2O. Coordination of calcium by the six nearest oxygens forms a distorted octahedron.
Scientific interest in lawsonite stems from the possibility that this phase may release water during subduction of hydrated oceanic lithosphere into the upper mantle. Here, dehydration reactions lower rock melting temperatures, inducing volcanism. Lawsonite is an important water reservoir in subducting oceanic crust below the amphibole dehydration depth; amphibole breaks down at pressures of between 2 to 3 GPa. A recent study (Pawley, 1994) investigating high-pressure and high-temperature stability of lawsonite has identified several possible breakdown pathways for this mineral, which may be stable up to 13.5 GPa. Water released from lawsonite may be incorporated into other hydrous phases in the descending slab, or the overlying mantle wedge. Alternatively, partial melting caused by water may occur in either the descending slab or the mantle. Water released from lawsonite may be incorporated into other hydrous aluminosilicates, and transported further down into the mantle transition zone by subduction.
The CHIME figure shows calciums as large grey spheres, and aluminiums as small blue spheres. Oxygens are shown red except those comprising water (H2O) contained within the crystal. These are dark red. Silicons are ochre, hydrogens small white spheres. The unit cell is outlined.