I'm looking for some help identifying this rock. I'm also interested in finding out how it is likely to have formed.

enter image description here

I found it on a black sand beach in Vik, southern Iceland, not far from the basalt column formations. I assumed it was basalt too or at least volcanic, but I have no experience so would love to find out more.

It weighs around 115g so feels fairly heavy for it's size. It's fairly smooth on the surface. What is the white material and also what is the light brown material likely to be? How do these three types of material come together to form this stone? If I could cut it in two, what's it likely to look like inside?

Sorry if these questions are basic, I'm a complete novice and any answers would be much appreciated.

  • $\begingroup$ My guess that the white crystals are either quartz, calcite or zeolite. Try scratching it with metal keys. Does it scratch the surface of the mineral? Try dropping a little drop of vinegar on it. Do you see bubbles? $\endgroup$ – Gimelist May 7 '16 at 11:13
  • $\begingroup$ The smoothness is just erosion - it's not really diagnostic of anything. Can you describe the components in more detail? For the dark groundmass, do you see any individual crystals within it, if so what colour are they, do they have reflective surfaces (cleavage), what shape are the crystals? For the large crystals, Michael's advice is good, try and get a relative hardness and determine whether it is carbonate based. Does it have any notable texture to it? Does it appear glassy and/or slightly translucent? $\endgroup$ – bon May 7 '16 at 11:28
  • $\begingroup$ @bon The large crystals can be scratched but not easily and are reflective. Acid has no effect on any of the surface of the dark groundmass or crystals. There are no individual crystals really on the groundmass. The only crystals I can see look like chunks have been knocked out of the rock and you can see through to more of the pale brown material seen in the lower left hand side of the photo. Almost like the grey is a thin covering. $\endgroup$ – Lindsay May 7 '16 at 13:22
  • $\begingroup$ It weighs around 115g so feels fairly heavy for it's size You could have done some calculations and give us density - that is more relevant. Please read meta.earthscience.stackexchange.com/questions/124/… and edit additional information into your question. $\endgroup$ – Jan Doggen May 7 '16 at 19:12
  • $\begingroup$ Thanks everyone for your very detailed responses. All very helpful and I feel I have a much clearer idea on what my rock is. $\endgroup$ – Lindsay May 8 '16 at 9:49

To be 100% sure one would need to study a thin section of the rock under a polarizing microscope. That said, however, I am 99.9% sure that this is a vesicular basalt in which large bubbles of volcanic gas have been frozen in as the lava rapidly solidified. I think your assumption that it is basalt is well founded. Under the microscope you would most likely find a mass of tiny interlocking crystals. The white laths would be plagioclase feldspar, whist the brownish / greenish or possibly grey crystals would be various types of pyroxenes. In most basalts these are two small to resolve with the naked eye. In addition there is likely interstitial glass, which froze before it had time to crystallize, and some opaque grains of magnetite. Some varieties of basalt will also have traces of other minerals.

The white and brown minerals are late stage precipitates introduced by heavily mineralized ground-waters (common in volcanic environments). You can clearly see the hairline fractures through which the groundwater seeped, probably under considerable pressure. Note that if a bubble is not intersected by a hairline fracture then it is hydraulically isolated, in which case the bubble remains empty - no mineral deposition. You can see a few such examples in the photo.

If the white crystals do not effervesce with acid, and are hard to scratch then they are almost certainly quartz. Otherwise they are most likely carbonates such as calcite, aragonite or dolomite (much more rarely hydroxy-carbonates).

Iceland is well known for a range of zeolites (complex hydroxy-alumino-silicates) which characteristically infill the vesicles (gas bubbles). Most zeolites are rather more coarsely crystalline than the large pinkish precipitate, but one could not rule zeolites out as a possibility.

As for cutting it in two, you would see more of the same. The larger vesicles / bubbles are more likely to intersect a fracture and be filled by any of the above late stage precipitates. There will be many smaller bubbles. If you carefully polish the cut surface, and examine it carefully with a hand lens you should be able to see the component crystals of the basalt.

  • $\begingroup$ Thanks everyone for your very detailed responses. All very helpful and I feel I have a much clearer idea on what my rock is. $\endgroup$ – Lindsay May 8 '16 at 9:49
  • $\begingroup$ I would bet the white crystals filling the vesicles are zeolites, not quartz, given the appearance. $\endgroup$ – haresfur May 8 '16 at 22:28
  • $\begingroup$ The diagnostic test is hardness. zeolites are 3.5 to 5.5 on Moh's scale, with the most common zeolites, like Stilbite and Heulandite tending towards the softer end of the range. Quartz is, by definition, hardness 7. $\endgroup$ – Gordon Stanger May 9 '16 at 18:49

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