CompuServe Messages

Hi Jerry, Yes, very common, in fact probably extremely common! You probably haven't seen limonite ps. of pyrite much because most people consider them boring and they aren't exactly highly aesthetic, so not prominently displayed. But they are very common. There are millions (probably many more zeros than that) in siltstones, sandstones and the slightly metamorphosed equivalents of these, and possibly in other geologic environments. In my area (N. Idaho) and in neighboring Montana we have hundreds of square miles of boring 1.4 billion year old metasediments of the Belt supergroup (mostly fine grained silt size to sometimes sand size grains). Many of the layers have these little critters in them, moslty <4 mm or so but up to 4 cm or more. Mostly as cubes, sometimes as pyritohedrons. They are mostly completely altered to "limonite", but vary from completely altered to partially altered to fresh pyrite with only a thin brown skin. As Doug states–black would suggest goethite. That is true, while in college I x-rayed these pseudos from several localities. They were all brown or black on the surface but black inside and all checked out to be goethite. Well known localities include Pelican Point, Great Salt Lake, Utah and Tekoa Mountain, Whitman County, Wash. At both localities they occur as sharp cubes to 4 cm or more, sometimes in clusters. For your non-technical friend: They are common because pyrite easily oxidizes, in the right environment the pyrite is oxidized with the sulfur being released (lost to the immediate system, my guess is it goes into "rockspace" which is a cousin to "cyberspace" which we all know is where lost e-mail messages and data go) and the iron combining with oxygen and hydroxyl ions forming crystalline goethite which replaces the pyrite crystal, retaining the crystal shape of the original pyrite. If the environment is not right for this, then the pyrite oxidizes to the "non-crystalline" form of limonite, generally leaving a rusty lump, not a sharp pseudomorph of the pyrite. Where does all the sulfur go? In nearly all the environments, you have oxidation altering the pyrite to goethite/limonite and the sulfur is gone. I know of only one site where the pyrite is altered, but remains shiny with no rust attached but only elemental sulfur (a reducing environemnt?). Lanny