Determination of the Atomic Weight of Silicon by Physical Measurements on Quartz

Abstract

The value 28.095±0.005 for the atomic weight of silicon has been obtained through precision measurements of the density and unit-cell volume of an unusually pure crystal of natural quartz, SiO2. The value is based on the Avogadro number, 6.02338, and wavelength conversion factor, 1.002030, of Birge (1945). The following experimental data were obtained: density in g/cc in vacuum at 25°, 2.64847±0.0001; unit-cell dimensions in kX at 25° and corrected for refraction, a0 4.90360, c0 5.39417, both ±0.0001; indices of refraction for λ589.29 mμ at 18°, nO 1.544258, nE 1.553380, both ±0.00001; inversion temperature 573.3° on both heating and cooling cycles. Direct analysis gave (weight percent): Li2O 0.0005, Na2O 0.0004, K2O 0.0002, Al2O3 0.0008, TiO2 0.0001, MnO2 0.00002, Fe2O3 0.0000. The major systematic errors in this method stem from compositional variation in the quartz, of the nature of a coupled substitution of Al for Si with interstitial (Li,Na), and probably lead to a high value for the atomic weight. For purposes of comparison, data are also given for smoky quartz, rose quartz, and a synthetic quartz containing much Al and Li in solid solution.