File:Journal - South African Institute of Mining and Metallurgy (1898) (14776460091).jpg

Identifier: journalsouthafri08sout (find matches)
Title: Journal - South African Institute of Mining and Metallurgy
Year: 1898 (1890s)
Authors: South African Institute of Mining and Metallurgy
Subjects: Mineral industries Mines and mineral resources Metallurgy
Publisher: Johannesburg
Contributing Library: Gerstein - University of Toronto
Digitizing Sponsor: University of Toronto

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tion, is that of a somewhat gritty sandstone.The interstices between the quartz grains arcrilled with clear quartz, and the outlines of thequartz grains are marked by dustlike opaqueinclusions. Polarised light reveals the factthat the interstitial tilling, which usually in aquartzite consists of a mosaic of opticallydifferently arranged minute secondary quartzgrains, disappear-. /.• . the original round quartzgrains have grown at the expense of the secondarycement, which ultimately has been altogetherreplaced by extended portions of the quartz grains,each individual grain having grown until it wasmet and stopped by the extending portion of thenext grain. In polarised light, therefore, nocement is seen, but the whole of the rock consistsof angular elongated crystals of quartz dovetailedand interlocked tightly together on account oftheir serrated edges, and inside which one stillrecognises the outline of the impure originalquartz grain. This structure is shown in photo-graph No. III.,
Text Appearing After Image:
FIG. Ill—LOCAL LINES x85. and it is to this that I attribute largely theexceptional wearing quality of the rock. Duringexamination uniform wavy extinction, due tomolecular rearrangement produced by pressure wasnoticed and suggested that the hardness of the rockmight have altered. On polishing a face I actu-ally found that an ordinary quartz crystal did notmake any incision into the polished face, i.e., thehardness was more than seven, and obviouslyincreases resistance to wear. A lining of this■material was put in and has been used up to the present for 85 day-, behaving quite satisfactorily and appearing likely t la-r 120. The other specimen was a dark coloured chertwith white rims surrounding crystalline quartz.Applying the same test t a polished piece Itumid that the black portii ds were easily scratchedby quartz. The white rims surrounding thecrystalline quartz were found to he harder thanquartz, so that the lace ; this rock in use as aliner will necessarily, alter a short time

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