A dioctahedral member of the true mica group. The end-member formula is KV₂vAlSi₃O₁₀(OH)₂, where v = vacancy. Roscoelite occurs as a 1M polytype and more rarely as 2M₁. Roscoelite is found in sedimentary rocks where V has been remobilized from black shales and mafic rocks. Roscoelite is known from the Colorado Plateau district (Colorado, also Utah, Arizona, New Mexico, USA) in U-bearing sandstones. It is also known from Gabon, Western Australia, and Ontario, in the latter occurrence, the V was introduced from metasomatic fluids.

Symmetry involving a repetition of features about an axis. The axis type may be either a one-fold, two-fold, three-fold, four-fold, or six-fold axis, where the angle of repetition may be determined by 360/n, where n is equal to the axis type (1, 2, 3, 4, or 6). A one-fold rotation axis is the identity operation. Another name for this type of axis is “proper rotation axis”.
Cf., symmetry, rotoinversion axis

Phyllosilicates commonly show rotations between adjacent layers involving n60° (where n is an integer from 1 to 6), and with registry or partial registry between layers. Stacking disorder or partial disorder involves the lack of regularity in the n value. Equivalence to this type of stacking order/disorder may be achieved also by random or partially random translations along the pseudohexagonal a or b axes (commonly along a, and very rarely along b) of the layer silicate.
See turbostratic stacking

A general type of symmetry axes that involves a rotation followed by an inversion operation, with a repeating set of rotation/inversion operations until “closure” (returning to the starting position in the rotation/inversion or repetition process) is reached. The one-fold rotoinversion axis (͞1) is equivalent to a center of symmetry, and the two-fold rotoinversion axis (͞2) corresponds to a mirror plane; only the latter are used, respectively. In addition, the ͞3 implies that the object possesses a center of symmetry and a 3-fold axis and a ͞6 is equivalent to a 3-fold and a mirror plane perpendicular to it. Thus, only the ͞4 is a unique symmetry operation.
Cf., symmetry

A discredited name for material primarily containing pimelite.

A poorly defined material, possibly altered biotite or interlayer-deficient biotite, and/or vermiculite.

A weathering process wherein minerals in a soil release iron which subsequently precipitates to form free iron oxides, such as hematite (red) and goethite (yellow brown), to produce reddening of a soil horizon.

A poorly defined term, possibly for a Si-poor chlorite.