AboutDr Thomas Bell Expertise I can answer questions regarding surface earth processes and the chemical transformations that sediments and rocks undergo with burial. I can also answer questions regarding deep time, the evolution of the elements, and the last 4.5 billion years of earth history. I specialize in metallic ore forming processes, the major geologic time periods when they were produced and what they tell us about the evolution of our planet. Learn more about my professional interests at Stratamodel.com.
Experience I am a professional consulting geologist with a background in the petroleum, mining, environmental, and geotechnical industries with over 25 years of experience.
Education/Credentials Ph.D., Geology, University of California at Berkeley, 1984
M.A., Geology, University of California at Berkeley, 1980
B.S., Geology, San Jose State University, 1978
I'm hoping you can help me determine the composition and age of this specimen. I found this rock in a creek in South-Eastern Missouri, near Bonne Terre. I'm no geologist, but I would guess that the Mohs hardness scale would be between a 3 to a 5. The "crystal" portion of the rock will scratch my fingernail, and I can scuff the shiny surface of the crystal with a metal pick. The porous portion of the rock will crumble into sand if picked. This rock is about four inches in diameter and weighs a couple of pounds. The back of the rock shows more striae of the "crystal" portion through the base rock. Thanks for your help!"
Answer Brad,
I can't be sure from the picture and description you sent but it looks like a fragment of mudstone coated with calcite (calcium carbonate). Grind some of the crystals up with a file or sandpaper and pour some swimming pool acid on them. If they bubbled furiously, it is calcite. Or, grind them up and put them in a glass of lemon juice or white vinegar and let them stand overnight. If there is a coat of bubbles on them in the morning, it is calcite.
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