Just to clarify, my understanding is that Pectinidae inner shell microstructure is foliated calcite (vs. foliated aragonite, thus far only confirmed in Monoplacophora).
Forgive my fractured science, that would be my understanding too, where calcite forms in spiral patterns within a matrix of aragonite. The structure gives these pearls clarity with great depth between the leaves.
Also, did I read something about sand being the root cause of pearls? Anything to add to
this thread?
I'm back at Clayoquot today (and for the holidays) and have some weathervane scallops, barnacles, turban snails and mussels there, with sandy blisters. I'll post a collection of pics. Most Pectinidae around here often have something stuck to the inner surface.
Intitally, I set out to help debunk the grain of sand thing by destroying a lot of natural pearls. Can't bring myself to it. Started looking closer at species that are heavily exposed to shifting sand. I also have a few loose pearls, harvested in close proximity to blisters and relatively the same growth. I sent some cut shells to a lab for polarography, but the cutting tool contaminated the samples. This time around I'm going to crush two or three and use acids.
Anything lodged in between the mantle and shell can be biomineralized in place, but rarely (and I emphasize, really rarely) these can become dislodged and form loose pearls. This experiment only applies to narrow margin of naturally formed extrapallial pearls.
I stand by Douglas' demonstration. Modern pearling as we know it is in complete absence of grains of sand as nuclei and the incidence in naturals is one in thousands.