High pressure NaAlSiO4: the first silicate calcium ferrite isotype

Nepheline (NaAlSiO4) has been transformed at about 1000degree C and pressures greater than 180 kbar to a new phase possessing the calcium ferrite structure in a diamond-anvil press employing YAG laser heating. The unit-cell dimensions of the high-pressure phase at STP are a/sub o/=8.740 +- 0.007, b/sub o/=2.746 +- 0.002, c0=10.206 +- 0.008 A for indexing as space group Pnma with Z=4. The zero-pressure volume change associated with nepheline → calcium ferrite structure is -31.9%, and that with jadeite+α-NaAlO2 (the intermediate phase assemblage) → calcium ferrite structure is -11.9%. The calcium ferrite-type NaAlSiO4 is the first example of a silicate with this structure and is thus the most closely packed A2SiO4 compound. Mg2SiO4 may possibly adopt this structure in the earth's mantle, but in any case the new phase is the most likely host for sodium in the earth's lower mantle