tetragonal mineral.

This is another shot of the smaller mineral.there are smaller ones at the lower right.

Looks like Pyrite altered to "limonite".
Dave

It still does not look right for pyrite (see also my previous post in the other thread). If you zoom in on the first photo and look at the small broken crystal immediately left of the the thumb, you will notice that the mineral is not metallic (and is also not limonite).

Reminds me more of andalusite than pyrite.

I found a rock recently with two teteragonal minerals in it. The large one (pictured) is 1 inch long and about1/2 inch wide. It appears to be dark brown and has small golg colored band at the top. I cannot find it in any rock and mineral books I have , anyidea what it is? thanks for any help.

You haven't really given us enough information eg where it was from - but it looks to me like a pyrite crystal - these can be distorted into "stretched cubes" that look tetragonal.

Jolyon

I agree with Jolyon. Surely Pyrite.

Uwe Ludwig

Uwe and Jolyon, I've just come across this photo, and I am curious about it. To me, the matrix looks like rhyolite, and the "crystal" seems to be set oddly in it. Are we sure this is natural? Could you have another look? Thanks, Becky

Becky,

I see nothing wrong with it - except that I don't think it's pyrite (and the matrix possibly not rhyolite). There is a damage along one edge of the crystal, and the reflections on it can lead one into thinking that the mineral has a golden colour. This taken aside (since we can't be really sure about it), there's nothing left to indicate that it is pyrite. The iron-staining does not necessarily indicate that the mineral is iron-bearing, because there's obviously plenty of iron in the host rock: note the colour along the cleavage plane, which is different from that of the bulk rock (as seen on top of the chunk). The iron does not appear to originate from the mineral in question, as this would have resulted in the formation of a more or less pronounced halo around the crystal.

Sure looks like a former pyrite with a coating, partial replacement (pseudomorph) or completely altered to "limonite". Almost exactly identical matrix-xl combinations are to be found att a bit over 4000 m altitude no a mountain between Ouray and Silvertopn, just a kilometer towards Ouray from the start of the Coloraduo slope for skiing speed record "kilometro lanciato". No mine, just on the surface and scree of the mountain /ridge top.

Jolyon is right, I have many different forms of pyrite crystals, and two of them look tetragonel, but they are actually cubic, and yours deffinately looks like a pyrite crystal that has canged into limonite.!

Spencer.

If there is plenty of it, I'd be quick to say pyrite, especially if the matrix is limestone, but that matrix is a bit grittier looking than limestone.
Where I used to live, a lot of the blacktopping on parking lots was done with crushed limestone in the mix, and the limestone was loaded with pyrite, so after some few weeks of rain and wear, the oil film was gone from the surface, and gleaming bits of pyrite, a quarter inch square or so, would reflect in the sunlight.. A quick geology lesson to anyone who I went with to Wal-mart. (Sorry; it was a small town, we had no choice!)

I too think this is a slightly "stretched" pyrite, perhaps only superficially oxidised to limonite and still pyrite inside, in a mica schist or chlorite schist matrix. But needs a much sharper picture to be certain.