FMF - Friends of Minerals Forum, discussion and message board

[monizerimar]

Hello, i found this mineral in an andesite rock in thin section, the photo is in 2.5x with cross polarized light, unfortunately i dont have the plane polarized light photo. The mineral im talking about is the acicular one with high birrefrigence index. Can you tell me what it is or what do you think it is? thanks!

[Bob Carnein]

Hi, First, a couple of comments/questions. Is this a standard thin section (i.e. 0.03 mm thick)? In plane light, when you rotate the stage, does the relief of the mineral in question change significantly (it should, if it's a carbonate)? The mineral doesn't appear to have a high birefringence, to me, though it's hard to be sure because the grains are a bit out of focus. Do you have access to dilute hydrochloric acid? If you do, can you apply a droplet to the rock (do you have access to the rock that the thin section was made from?)? Are you sure the rock is andesite (it might be a partly devitrified glass with plagioclase phenocrysts--a vitrophyre--in which case, the minerals in question may be crystallites that formed as the glass crystallized). The mineral doesn't look like a carbonate to me.

[monizerimar]

You are right, its not an andesite its a Trondhjemite, with a a standard thin section, so then i would think that those minerals are from alteration, and no i don't have access to the rock from which the thin section was made from, but i really appreciate your help, thank you!

[Roger Warin]

Hi,
Without criticizing the previous answers, I recall that for calcite for example, the birefringence is so high that the shades of interferences are of the 6th or 7th order. They become so bland that they look like the color of an aged newspaper.
Better to use HCl.
Roger.

[Bob Carnein]

Roger is right, though the interference colors are usually pastels in carbonates; the colors shown resemble, to me, sericite (fine grained muscovite) or, possibly, a zeolite. Again, I would check whether the relief changes significantly on rotation. Unfortunately, there are many minerals that have interference colors like those shown in the photo. Carbonates, from my experience, would not be expected to occur in rosettes; this would be more typical of some alteration mineral.

The plagioclase phenocryst in the lower right of the photo (gray, with oscillatory zoning) looks more like something I would expect to see in a volcanic rock than in an intrusive (like trondhjemite). Also, it doesn't show any obvious alteration, which isn't really consistent with what one would expect if the rock has extensive carbonate (or other) alteration. That's one reason why I suggested vitrophyre. I still don't know what the mineral is, but I'd be surprised if it turned out to be a carbonate (but, I've been surprised many times when confronting unknowns with inadequate information).