98 Magnetite

The Geology and Petrography of Crater Lake National Park, 1902






Magnetite is not present in very great abundance. It occurs in the customary grains and small octahedral crystals scattered throughout the rock and inclosed in all the other ingredients. It varies considerably both in amount and in the size of the individual crystals, but it never assumes the importance that is ordinarily expected in basalts. It also occurs, as described below, in the form of fine dust impregnating the interstitial glass.

As already mentioned, the glass base of these basalts varies somewhat in amount. It is never very conspicuous, and it vanishes almost entirely at times. In a number of cases the glass can not actually be seen, but its presence is probably indicated by the occurrence of black dust, which is probably magnetite and globulitic matter, and which occurs either in separate but very minute grains that are too small to measure or in very minute rod-like or trichitic-like growths. Apparently these opaque particles, are inclosed in the feldspars, but as they are to be seen mostly at the junction of the plagioclase laths it is more likely that they are really embedded in a very thin film of glass. None of the specimens above described is entirely free from this black powder.

This interstitial structure is brought out in fig. C of Pl. XIX (p. 138).

The chemical analysis of No. l58 will be found with the analyses of other basalts from Crater Lake on page 161. Although this is as typical an example of basalt as may be found around Crater Lake, the analysis indicates a rock closely allied to the andesites. It is distinctly more acid than is the olivine-bearing basalt (173), a description of which may be found on page 155.

A partial analysis of another interstitial basalt (162) was carried out in the chemical laboratory of the Colorado State School of Mines by Prof. R. N. Hartman. This analysis gave SiO2=55.18 per cent. It would seem, therefore, from these two analyses that the interstitial basalts of Crater Lake have unusually high percentages of silica.

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