dolomite - display specimen of white crystalline Upper Ordovician dolomite
This particular metamorphic rock can look like marble or it can look like limestone. In this case it looks like a fine-grained white marble. It differs from marble in having some or all of the calcium in calcium carbonate replaced by magnesium. This replacement can occur with or without metamorphic heating. Before metamorphism it was a limestone. If not heated and recrystallized, it would have looked just like the original limestone though some of the calcium had been replaced by magnesium.
If heated by deep burial or by a nearby intrusive body, limestone will recrystallize and may take up magnesium as in this specimen, which has the appearance of marble.
Marble and limestone are calcium carbonate and effervesce freely with dilute hydrochloric acid. Dolomite, calcium-magnesium carbonate, will effervesce weakly and may need to be powdered as when lightly tapped by a geologist's hammer to encourage effervescence.
Students should compare this with marble, with limestone and with dolomite that looks like limestone. It will surprise them that dolomite can look both like crystalline marble and like sedimentary limestone.
This dolomite comes from the Lower Ordovician Ely Springs Formation in the Inyo Range in California.
If you are a science or earth science teacher purchasing this as a teaching specimen for your class, your students should compare this with marble, with limestone and with dolomite that looks like limestone. It will surprise them that dolomite can look both like crystalline marble and like sedimentary limestone.
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