Aragonite is a calcium carbonate like Calcite and indeed shares the same exact chemical composition with it arranged subtly different causing an entirely different crystal structure.
http://www.galleries.com/minerals/C.....T/aragonit.htm
the name comes from the location it was originally discovered from which is Aragon Spain.

this cluster i bought recently at an AWESOME ROCK SHOP in near Zelionople PA called the Appalachian rock shop and museum. It gets the name star cluster because it has not attaching point for matrix rock. Most crystals grow on things. like other rock. sometimes they grow in mineral muds or in the sand or other places that allow the crystals to grow out from a single point in all directions. so every side is just more crystals. very beautiful.

I really have to take my camera next time so i can take some shots of the fossils they have there. they have a huge Crinoid fossil that is awesome beyond belief and some truly interesting trilobites.