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Abstract--Vernad{te (MnO2" nH~O) is a mineral with a poorly ordered structure. Its synthetic analogue is designated 6-MnO2. Birnessite and vernadite are independent mineral species and cannot be described further under the same name. They have similar hexagonal unit-cell parameters, a 0 ~ but different co parameters. Rancieite has a structure similar to that of birnessite. Calcium beanng, 14-A birnessite occurring in nature was first described by the authors. In addition to the todorokite having the parameters a,, = 9.75 A, b0 = 2.84 ,~, and c o = 9.59 ]k, other species of natural todorokite are known having aa parameters that are multiples of 4.88/~ equal to 14.6 and 24.40/~, the b0 and ca parameters being the same.

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