Seven manganese minerals have been observed in geodes from Chihuahua, Mexico. Ramsdellite occurs in a number of different habits, but the morphology and precession X-ray patterns suggest that it is pseudomorphous after groutite. Pyrolusite is found only in association with ramsdellite and is believed to be an alteration product of that mineral. Todorokite generally occurs as a late-stage fibrou...

The origin of certain alluvial platinum-group mineral (PGM) grains is controversial, with two major proposed models. One involves primary formation within ultramafic intrusions, followed by weathering into laterites with no further alteration prior to alluvial concentration. The other involves in addition, solution of the platinum-group elements (PGE) during weathering of the ultramafic source ...

A model to explain the high degree of polymorphism at the major histocompatibility complex (MHC) is described. The model incorporates domain transfer between the different loci in a supergene family by either gene conversion or double unequal crossing-over. Population genetics theory is used to formulate changes in the probabilities of allelic and nonallelic gene identities and equilibrium valu...