The mystery of the tram line cobblestones in Halifax
Dr. Sandra Barr was approached by Mick O'Neill, former graduate student at Acadia, now a Nova Scotia Dept of Energy and Mines geologist, to identify the source of the paving stones used for the old Halifax tram lines. Mick had acquired some samples of the stones and needed to identify them for storage. You can see the appearance of these brick-shaped cobblestones in photographs at https://skyrisecities.com/news/2016/06/once-upon-tram-halifax-street-railway. They are pale brown and fine-grained, and were mostly removed when the trams were discontinued in 1949.
We made some thin sections of them:
(field of view about 2 mm across)
In thin section, the rocks are very fine-grained and made of colourless grains with distinctive hollow cores ("belt-buckles") or long tapering corners ("swallow-tails"). Students in igneous petrology will recall those textures are distinctive of plagioclase in quickly cooled sea-floor basalt.
Under crossed polars the belt-buckles show low first-order interference colours, and near extinctions display the distinctive Prussian blue and dark chocolate brown tones of dispersion, again both distinctive features of plagioclase:
The more brightly coloured minerals are likely pyroxene. Although plagioclase + pyroxene are the minerals that make basalt, it turns out this material was most likely artificial. It's not possible to say if it was specially made for the manufacture of the paving stones, or if it was recycled steel foundry slag. One clue is that during sawing of the rock to make the thin section, it emitted the powerful smell of H2S, a hint that there must be significant sulphur in it too.