Orthoclase pseudomorph after Leucite

Item Description

An impressive near baseball-sized pseudomorph of soft beige colored Orthoclase which has replaced a textbook "floater" trapezohedron of Leucite from this locality in the Kaman District of Turkey. The piece is complete all around and you can feel it has a bit of heft when held in the hand. Personally I think that these are such intriguing pseudomorph for a few reasons. A feldspar species (Orthoclase) is replacing a feldspathoid species (Leucite), which is interesting because feldspathoids are defined as rock forming minerals that are lacking the necessary silica to actually form feldspar. So in this case we have the lower silica bearing Leucite that was replaced by a higher silica bearing Orthoclase to create these wonderful pseudomorphs. The habit of the original leucite crystals is a trapezohedron, as the crystal system for the species is actually cubic at around 900°C. You can look up Leucite and see that it is listed as being tetragonal in habit because the actual crystal structure changes once the species cools below 700-600°C, so these crystals were HOT when they formed, and possibly were still in the process of growing when additional silica was introduced, turning them into Orthoclase in the final stages of their formation.