Question
I recently found a chunk of rock while walking around and I can't figure out what it is. Would you mind helping out with the identification of it please? It kind of looks like pumice but it greyish in color with goldish or copper color metallic flakes and white and yellow banding on part of it. Carl
Answer
Hi Carl, Thanks for the pictures, one of which is shown here.
The whitish to opaline material looks like a secondary silica gel that commonly occurs in hydrothermal environments where silica (SiO2) is saturated in the liquid phase, causing it to precipitate on a host rock. Take the blade of a pocket knife and drag it across the white material. If the knife blade leaves a metal gray streak it is softer than the white material. That would be characteristic of a silica gel, which has a hardness of seven, like’s its crystalline relative quartz. Unfortunately, magnifying the images just cause them to blur. I’m hard put to describe the rock minerals. I do suspect that this rock occurred in an area of highly altered rock, perhaps associated with copper ore. I can barely make out platy looking minerals. If they are really platy, again try the knife blade, if they are soft and flaky they are probably a form of mica – muscovite (silvery) or (biotite); there are other micas as well. If instead of being platy they are rectangular prisms, they are probably altered pyrite. I hope this helps. Mike
