Above is shown a specimen of travertine, also
called Mexican onyx. This rock consists mostly of calcium carbonate but
may have various impurities which can affect its coloration.
Whereas limestone originates from ancient marine life, travertine is the product of purely inorganic chemical action. Travertine comes from calcium carbonate that has precipitated out of groundwaters that were rich in calcium ions. This process can happen quite close to the surface of the earth and does not require the intense heat and pressure associated with forming other rocks. Mineralogically speaking, travertine is made of calcite or aragonite (both are calcium carbonate).
Some calcite and aragonite specimens are fluorescent in ultraviolet light, and as you'd guess, some travertine also has this property.
As a side note, here's a question for you... Can a travertine deposit undergo metamorphism and turn into marble? Probably! If it gets buried deeply enough in the earth or is near an intrusive magma body, then why not?? Starting with a marble specimen, it's hard to trace backward and know what the source rock was-- aside from the fact that its chemical composition was calcium carbonate. I suppose there are clues-- radioisotopic or chemical-- that help identify the source of a particular rock. However, with sufficient metamorphism, I'm sure that becomes obscured.
I think limestone beds are much, much more abundant than travertine deposits, so chances are that a given marble formation came from limestone. Still, think of the "rock cycle"... if enough geologic time goes by, any rock type can undergo sufficient change to turn into any other rock type! Even the dullest shale or the waviest gneiss can be re-melted at sufficient depth to form magma again, and this in turn can find its way back to the surface as basalt or something else.
Somewhere there's got to be an ancient travertine bed that's become deeply buried back into the earth's crust. Maybe this travertine will fracture into angular pieces that will be re-cemented by groundwaters carrying silica or more calcium carbonate... if the travertine is buried deeply enough, maybe it will distort and undergo plastic flow... maybe superheated fluids will carry new ions into it... maybe it will encounter magma... so many possibilities, so many rocks.
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