I've been reading generic stuff about biomineralization but I still don't actually understand how a seashell forms? Can anyone walk me through this. "The bivalve secretes calcium carbonate" doesn'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'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't beaches all chalky?

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