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During a recent trip to the Portuguese island of Madeira, I found a couple of interesting rocks on a beach. Both the rocks pictured in this question contain the same black mineral. I first guessed this was tourmaline but the luster isn’t quite right as it’s more metallic than vitreous or dull. Madeira is a volcanic island so I’m assuming that the the rocks are igneous.

Info: Black colour, white streak, small pieces attracted to a strong magnet, metallic luster, can be scratched by a steel knife (although takes a bit of effort).

Pictures:

First mineral bearing rock. The first rock containing the mineral.

Closer view of the mineral. Closer view of the mineral.

Second mineral bearing rock. The second rock.

Mineral seen as thin crystals in second rock. Thin crystals of the mineral can be seen.

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The stone is Ignimbrite, the black crystals are either Hornblende or Pyroxene phenocrysts.

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  • $\begingroup$ Do you think that both of these stones are Ignimbrite? $\endgroup$ – The Garage Chemist Aug 16 '18 at 14:14
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    $\begingroup$ @TheGarageChemist Yes, the second one has a finer matrix so it will have been closer to the top of the fusion zone (the part of the deposit hot enough to fuse together as a single mass) but they both appear to be ignimbrite from similar if not the same eruption. $\endgroup$ – Ash Aug 19 '18 at 10:57
  • $\begingroup$ Ok, thanks. The different textures made me wonder if both rocks were as you said. $\endgroup$ – The Garage Chemist Aug 19 '18 at 13:52

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