Vernadite (MnO 2 ·nH 2 O) is a mineral with a poorly ordered structure. Its synthetic analogue is designated δ -MnO 2 . 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 c 0 parameters. Rancieite has a structure similar to that of birnessite. Calcium bearing, 14-Å birnessite occurring in nature was first described by the authors. In addition to the todorokite having the parameters a 0 = 9.75 Å, b 0 = 2.84 Å, and c 0 = 9.59 Å, other species of natural todorokite are known having a 0 parameters that are multiples of 4.88 Å equal to 14.6 and 24.40 Å, the b 0 and c 0 parameters being the same.
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Chukhrov et al. (1980) studied this question.
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