Creedite (#22)

Lustrous thin plate of gemmy amethyst-colored crystals to ½" x ¼" in a compact aesthetic aggregate.
North America Hall Moly mine, Tonopah, Nye County, Nevada, USA 1 ½" x 1 ¼" x ½"

 

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Creedite, named for its discovery locality in Colorado, has a rather complex looking chemical formula: Ca3(SO4)Al2F8(OH)2 ·2H2O . Let's take that formula apart and see what it means: Ca is calcium; as always, it is a positive atom, Ca2+. S, sulfur, and O , oxygen, are included in parentheses to indicate that these form a tight little group of four O2- ions (charged atoms) around one S6+ ion; the parentheses are not necessary and are often omitted. Al is aluminum, F is fluorine. (OH) is the hydroxyl group - an O2- ion and an H+ ion bonded tightly together. H20 is water, of course, and there are two water molecules bonded within each formula unit of the crystal structure. Some minerals have such complex chemical formulas that classification is not straightforward. The So4 group would make it a sulfate, except that the F would make it a halide. So, sulfohalide? Halosulfide? Sulfohalide hydrate? Most "official" classification schemes begin with "complex halide" and then make a subclass that includes the su lfate group.

Creedite is formed when fluorite-bearing ore deposits are strongly oxidized. Fluorite is CaF2, which provides the fluorine. Tonopah, Nevada, has been an active mining town since around 1900, although little mining is now being done there. The production began with silver, which lasted until the middle of the twentieth century. Thereafter various mining companies acquired properties that produced molybdenum, a very important element in producing hard steel alloys. Molybdenum is combined with sulfur in the mineral molybdenite, MoS2. Molybdenite is a common mineral in many mining areas, such as Bingham Copper Mine in Salt Lake Valley.