Posted January 14, 2014
Everyone did a great job guessing at this one. Mica, halite, quartz, and calcite all have a more or less transparent variety; and Iceland spar is a transparent variety of calcite. But the best guess is gypsum, and this Freak of the Week is actually selenite – which is a crystalline form of gypsum. Though I picked this mineral because it resembles ice, gypsum actually feels warm to touch due to its thermal insulating properties! Gypsum is also a very soft mineral, ranking 2 on the Mohs scale (a way to rank mineral hardness)- as our astute reader mentioned, and can easily be scratched with a fingernail. Gypsum is found on every continent and surprisingly one of the world’s best localities for selenite is right here in Ohio, in Mahoning Co. in northeastern Ohio!
Selenodont premolars and molars
The name selenite can be a bit confusing. It sounds like the chemical element selenium but the mineral selenite contains no significant amount of selenium. Both substances are named after the Greek word “selene” meaning Moon. One theory for the name is that the pearly luster of selenite resembles the light from the moon.
But those of you who took (survived) my osteology class might say “Hey, I remember that ‘selene’ and ‘selenodont’ were terms used to describe the crown appearance of teeth of certain large mammals!” and you’d be right! The term selene is used in regards to the “crescent-moon” appearance of the chewing surface of premolars and molars of large ungulates such as the deer family, cattle family and pronghorn. Note in the photo how the cusps of the gazelle are elongated in an anterior-posterior direction and resemble the shape of a crescent moon. You can see this same pattern in common Ohio mammals such as deer, sheep, and cattle and also in mammals that are now extirpated from the state, but may still show up from historic or Ice Age deposits, such as elk, bison, caribou, and muskox.