Portal de Periódicos da CAPES

Tektites and silica-glass (With Plates XVI and XVII.)

1939; Volume: 25; Issue: 167 Linguagem: Inglês

10.1180/minmag.1939.025.167.01

2515-821X

L. J. Spencer,

Building materials and conservation

The natural glasses recognized as tektites are characterized by their acidic composition, the silica percentage ranging from 68 to 80. Their origin is still a debatable point. Within this range glasses are known which have undoubtedly been produced by the fusion of siliceous terrestrial materials in the intense heat developed when large iron meteorites have struck the earth's surface; while others of the same origin show silica ranging up to 98 per cent., that is, an almost pure silica-glass. From a collection of 84 chemical analyses of tektites taken from the literature, only 36 (many of them due to A. Lacroix) give data for the specific gravity and refractive index which presumably were determined on the same sample of material as that analysed. These are reproduced in table I (arranged in order of silica percentages) and plotted in figs. 1 and 2. Corresponding with the range in SiO 2 from 68·00 to 80·73% there is a nearly uniform decrease in d (2·498–2·339) and n (1·526–1·4867). The specific refractivity, K = ( n −1)/ d , varies from 0·2050 to 0·2107 (0·2258 in the case of no. 29 with the abnormally high n 1.5380).