I'm pretty sure most of the mineral grains filling the vesicles (gas bubbles in igneous rocks) here are calcite. This rock has undergone quite a bit of weathering, so whether the voids were originally full of calcite is impossible to say. They may have been, and the mineral simply dissolved out. This is just behind the Green Peter Dam, and is a nice stop to look for zeolites and calcite. I don't have an off-the-cuff list of all the varieties of zeolite I've found here, but it would be pretty lengthy. Natrolite and stilbite are the most common, and are easy to find on any visit. Others just kind of show up now and then, and you can't really count on finding them. That list would include chabazite, laumontite, heulandite, and analcime/analcite. (The latter is one I learned as a zeolite, and commonly occurs with them. I've recently learned it can also be classified as a feldspathoid.) There are likely others I've spotted on occasion, but they don't come to mind right now. The crystals tend to be smallish, and this is a good spot to sharpen your hand lens skills. Four pound hand sledge in back for scale.
Photo unmodified. March 9, 2012.
FlashEarth location.
No comments:
Post a Comment