A group name for phyllosilicates with the general formula of (R2+6-y-zR3+y□z) (Si4-xR3+x)O10(OH)8 where ▫ represents vacancies, x is the number of tetrahedral R3+ cations, y is the number of octahedral R3+ cations, and z is the number of vacancies. The common structure consists of negatively charged trioctahedral 2:1 layers alternating regularly with positively charged trioctahedral interlayer sheets. Ideal composition of the 2:1 layer is (R2+, R3+)3(Si4-xR3+x)O10(OH)2 and that of the interlayer is (R2+, R3+)3(OH)6. Bayliss (1975) defined trioctahedral end members based on the dominant cation, e.g., Fe-rich, chamosite; Mg-rich, clinochlore; Mn-rich, pennantite; Ni-rich, nimite; Zn-rich, baileychlore. The trioctahedral chlorites are commonlyfound in metamorphic rocks and are the diagnostic mineral of the greenschist facies. Chlorite is also a common alteration product. Eggleton and Bailey (1967) combined composition and structure characteristics to define dioctahedral chlorite species. Cookeite is the Li-rich chlorite, whereas sudoite is essentially Li-free. Cookeite and sudoite occur in pegmatite, hydrothermal deposits, and ore deposits. Both have a dioctahedral 2:1 layer and a trioctahedral interlayer (di, trioctahedral chlorite). Donbassite has two dioctahedral sheets (di,dioctahedral chlorite) and is Al-rich. For trioctahedral chlorite, the common polytype is the one-layer, IIb form, although cookeite, commonly forms in the one-layer Ia form (cf., Ia polytype, IIb polytype).
See also Part 1 of the Glossary.
See further explanation under “group names”.
Part 1 of the Glossary:
A group name for platy phyllosilicates of 2:1 layer with a variable layer charge. The interlayer material consists of a hydroxide octahedral sheet, which results in d(001) value of approximately ~ 14.0-14.4 Å. The group is further divided into subgroups that are either trioctahedral or dioctahedral, and these subgroups are further divided into mineral species based on chemical composition.