that formed on the exterior of the rocks. In the upper right of the above posted image, also in the lower left center, nested rims may be seen apparently formed on the same rock. If these were due to alteration, as at Mazatzal, they could only form if there was accretion to the rocks after one phase of alteration. These rims are not uniformly concentric, thus the accretion would have had to be non uniform too, which is very unlikely. That would have been followed by later alteration to form the outer rim. More likely scenarios include precipitation in intertidal zones of "rimstone" like that from carbonate springs. I believe that the forms observed so far, which have some sharp corners, and lack the generally lobate structure of rimstone, more closely resemble coral formations - on the macro, as well as micro level.