I& #39;ve been reading generic stuff about biomineralization but I still don& #39;t actually understand how a seashell forms? Can anyone walk me through this. "The bivalve secretes calcium carbonate" doesn& #39;t really help

So along with a bunch of other stuff seawater has:

- Ca+ (mostly from weathered limestone? elsewhere?)
- Co3 2- (is this from carbonic acid from atmospheric co2, or also from weathering?)

then these... touch biology somehow... and end up as mineral CaCo3? which wouldn& #39;t have naturally precipitated otherwise?

and how is this different than what happens when hard water limes up a pipe? if the ocean has all these calcium ions sitting around ready to be biomineralized, why aren& #39;t beaches all chalky?

(sorry, CO3 2- and CaCO3, obviously not cobalt)