Abstract

Larsenite, PbZnSiO4, has been synthesized both by heating a precipitated gel of the same composition in air at 1 atm. to temperatures from 500° to 950°C, and by hydrothermal crystallization of the gel at temperatures from 300° to S50°C and a pressure of 2000 bars. Larsenite melts incongruently at 1000°C and 1 atm. to willemite and liquid lead silicate; under hydrothermal conditions larsenite becomes unstable between 550° and 650°C. Efforts failed to synthesize compounds isostructural with larsenite in which Zn is replaced by Mn, Fe, Co, Ni, Mg, Cd or Be. Indexed X-ray powder data are given for synthetic PbZnSiO4.

Barysilite, MnPb5(Si2O7)2, and analogues of this compound in which Mn is wholly replaced by Fe, Co, Ni, Zn, Cd, Mg or Ca have been synthesized either hydro thermally by crystallization of precipitated gels at temperatures from 200° to 550°C and pressures of 1500 to 3000 bars, or by heating gels in air at 1 atm. and temperatures of 600° to 800°C. The idealized formula Pb3Si2O7 has been generally attributed to barysilite, but efforts to synthesize this compound failed. The presence of Mn or of another ion in substitution for it, in addition to Pb, appears to be requisite for the formation of the barysilite structure type. X-ray powder spacing data are given for synthetic barysilite and its seven analogues.

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