  Ooids are typically spherical sediment grains characterised by concentric
layers encapsulating a core. There is no universally accepted explanation for
ooid genesis, though factors such as agitation, abiotic and/or microbial
mineralisation and size limitation have been variously invoked. We develop a
mathematical model for ooid growth, inspired by work on avascular brain
tumours, that assumes mineralisation in a biofilm to form a central core and
concentric growth of laminations. The model predicts a limiting size with the
sequential width variation of growth rings comparing favourably with those
observed in experimentally grown ooids generated from biomicrospheres. In
reality, this model pattern may be complicated during growth by syngenetic
aggrading neomorphism of the unstable mineral phase, followed by diagenetic
recrystallisation that further complicates the structure. Our model provides a
potential key to understanding the genetic archive preserved in the internal
structures of naturally occurring ooids.
