Density, Refractivity, and Composition Relations of Some Natural Glasses

C. E. Tilley
Research Student, Emmanuel College, Cambridge

Summary: (1) The natural glasses investigated fall into two groups: (a) Tektite glasses, embracing moldavites and australites, and (b) Volcanic glasses, including rhyolite-, trachyte-, and basalt-obsidians.

(2) When their physical properties, density, and refringence are graphically plotted, these glasses are seen to group themselves into distinct fields. For comparison, the glasses of the albite-anorthite felspars and the glasses of composition CaSiO3—MgSiO3 are similarly treated. The field of the tektites is sharply demarcated from those of the rhyolite- and trachyte-obsidians. The basalt-glasses including tachylytic types form a field which is enclosed by the CaSiO3—MgSiO3 curve and the prolongation of the Ab-An curve.

(3) In the same way the graphical plot of the relations, density and specific refractivity (K = (n − 1)/d), is broken up into fields separating the tektite and volcanic glasses. The mean specific refractivities of the various groups of glasses is given by—moldavites 0·2089, australites 0·2109, rhyolite-obsidians 0·2065, marekanites 0·2060, trachyte-obsidians 0·2076, and basalt-obsidians 0·2136.

The characteristics of the tektite glasses amply confirm their divergence from terrestrial glasses, established on other grounds, and support the contention of Suess and Summers with regard to their meteoric origin.

(4) The specific refractivities of five analysed glasses are compared with the values calculated from the specific refractivities of the component oxides, or ‘normative’ minerals. A notable correspondence is revealed, when the imperfection of the optical data with regard to the K values of a number of these oxides is considered.

The presence of water in a glass has a marked effect on the K value of the glass, as is shown by the Newry pitchstone, which with a water content of 7·04 per cent. has a specific refractivity of 0·2146. Water has a further influence, however, in that addition of water to certain glasses not only raises the K value of the glass, bat has a marked effect on the refringence. This is to be observed in three analysed rhyolite-glasses from Lipari. It is shown that together with an increase in the refringence, a notable degree of contraction is involved. The three analysed glasses can be regarded as mixtures of one type, with varying amounts of water, and the departure from a mechanical mixture relation is indicated by their specific gravities (1) 2·370, (2) 2·363 instead of 2·347, (3) 2·320 instead of 2·276.

Lastly, in certain cases, an approximate estimate of the volume change accompanying the passage from the vitreous to the crystalline state is given by a comparison of the actual vitreous density and the density calculated from the ‘norm’. For the three rhyolite-obsidiaus mentioned above the expansional volume changes (on fusion) are 10·6, 9·7, and 7·1 per cent. respectively.

Mineralogical Magazine; March 1922 v. 19; no. 96; p. 275-294; DOI: 10.1180/minmag.1922.019.96.01
© 1922, The Mineralogical Society
Mineralogical Society (