Wulfenite & Mimetite (#14)

Lustrous tabular yellow wulfenites to 1" with yellow mimetite, ex. James Lewis.
North America San Francisco mine, near Magdalena, Sonora, Mexico 2 ¼" x 1" x 1"

 

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Here we have two minerals containing lead (Pb): Mimetite, Pbs(AsO.)CI, and wulfenite, PbMoO .. The symbol As is arsenic, and Mo is molybdenum - not an abundant element in the earth's crust, but an economically important one. You already know that O is oxygen, and Cl is chlorine. The flat, squarish crystals are wulfenite, and the smaller, compact crystals are mimetite.

Two words used in the paragraph above may leave you wondering about the difference: mineral and crystal. We defined a mineral in the "Learn More" on the introduction screen: a natu rally-occurring solid substance, usually inorganically-formed, characterized by a definite (but not necessarily fixed) chemical composition and an orderly arrangement of atoms (called the "crystal structure"). Crystals are solid, have a definite chemical composition, a crystal structure, and are bounded by flat sides, but they need not be inorganically formed (or inorganic, for that matter), or naturally occurring. Many inorganic substances can be crystallized readily, but they are not minerals unless they also occur naturally. The quartz crystal in your watch is likely synthetic, but because the same material occurs naturally, most mineralogists would be comfortable calling what is in your watch a mineral. In addition, organic crystals are routinely synthesized in medical or other research, but they are not minerals.

By the description above, crystals have nice, flat sides making attractive shapes. They are in no fundamental way different, however, from the same material that has grown in a restricted environment and does not have flat sides and an attractive "crystalline" shape. Mineralogists would call the latter crystals, as well, in any context that did not imply or require the existence of a nice, external crystal shape.