Canadian Mineralogist Vol. 13, pp. 178-180 (1975) NOMENCLATURE OF THE TRIOCTAHEDRAL CHLORITES PETER BAYLISS Mineralogisch-Kristallographisches Institut der Universitiit Gbttingen, West Germany* Abstract proximate chemical composition of a trioctahe Trioctahedral mineral species within the chlo dral chlorite, one must use cell constants or re rite group have the end-member compositions as flection intensities rather than simple compari follows: clinochlore (MgsAl)(Si3Al)O,0(OH)8, cha- son. For example, the curves of Shirozu (1958) mosite (Fe.,+ +Al)(Si3Al)O,0(OH)8, nimite (NisAl) relate the (001) spacing to the MgVISilv^±AlVIAl,v (Si3Al)O10(OH)8, and pennantite (Mn,Al)6(Si,Al)4O10 substitution, and the 6-axis length to the Mg^± (OH)8. The varietal names of brunsvigite, corundo- Fe++ substitution. Also, the intensity ratio of philite, daphnite, delessite, diabantite, grovesite, (002 + 004)/(003) reflections has been related kammererite, kotchubeite, leuchtenbergite, ortho- to Mg^±Fe++ substitution by Petruk (1964). The chamosite, pennine, pseudothuringite, pycnochlo- accuracy of these methods is discussed by Bai rite, ripidolite, sheridanite, talc-chlorite and thurin- gite should be discarded. The varieties between ley (1972). In addition, Brown & Bailey (1962) these end-member compositions should be de recognized four different one-layer poly-types scribed by chemical element adjectives. At present (lib, lb orthorhombic, lb monoclinic, and la), the polytype symbols of Brown & Bailey (Amer. which cut across all the arbitrary nomenclature Mineral. 47, 819) should be used. divisions established for different compositions. Therefore the nomenclature needs re-evaluation Introduction and simplification. Compilations and classifications of the tri octahedral chlorites have been made by Orcel Discussion et al. (1950), Serdyuchenko (1953), Hey (1954), Both Foster (1962) and Brown & Bailey (1962) Brindley & Gillery (1956), Lapham (1958), Fos have plotted a diagram from numerous natural ter (1962), Phillips (1964), Strunz (1970), and samples to indicate the extent of substitution Fleischer (1971). Problems have arisen because with respect to Mg^±Fe and (Mg, Fe)VISiIv^± different names have been given in the original A1VIA1IV in the chlorite group. The number of descriptions for similar compositions, and then samples recorded that are either manganese- or the same name has been used for different com nickel-rich is limited. Therefore it appears un positional ranges in various classifications. Each necessary to give tabulated compilations or com of these classifications is based upon different positional diagrams in this paper. arbitrary divisions within a complex solid-solu Initially the chlorite series was described with tion series. Some authors avoid nomenclature serpentine Mg3Si2Os(OH)4, and amesite (Mg*Al) problems by the use of the group name chlorite (SiAl)Os(OH)4 as the end-members, but later instead of a species name, which is also undesir both of these minerals were found to belong to able as there are a number of valid species. the kaolinite-serpentine group (7A or 1:1 layer) There seems to be no adequate reason, as sug rather than the chlorite group (14A or 2:1 layer gested by Albee (1962), to continue the use of plus an interlayer sheet). In order to fit together a large number of ill-defined varietal names, es the tetrahedral and octahedral sheets in chlorite pecially as only the end-member compositions to produce a stable structure, some Al substitu are accepted as species by the Commission on tion is essential to produce tetrahedral and octa New Minerals and Mineral Names, Internation hedral sheets of similar a- and Z>-axis dimensions. al Mineralogical Association. This limits the composition to approximately the The powder diffraction data of trioctahedral center of this series. This composition of (MgsAl) chlorites are similar, as shown by a comparison (Si3Al)Oio(OH)8 is most commonly used to de of data in Selected Powder Diffraction Data for scribe the mineral species clinochlore. The te Minerals (1974). Therefore, to ascertain the ap- trahedral composition of (Si3Al) has been used for the end-member, as it represents the mid point of the series and is fairly simple, although *On leave from Department of Geology, Univer the average tetrahedral composition of natural sity of Calgary, Alberta T2N 1N4 samples is near (Su.tAIi.s). A common type of 178 Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/13/2/178/3445639/178.pdf by guest on 28 September 2021 nomenclature of the trioctahedral chlorites 179 substitution in clinochlore is MgVISirvr^±AlvlAlIv. chromium is very large compared to aluminium Varietal names that should be discarded include (Burns 1970), which means that chromium leuchtenbergite which is a clinochlore, pennine strongly prefers octahedral sites. Therefore until (often misspelled as penninite) which is a mag- conclusive evidence is presented for tetrahedral nesian siliconian clinochlore, and sheridanite chromium, it should be taken to occur only in which is an aluminian clinochlore. The varietal octahedral sites. name of talc-chlorite with a composition similar The endrmember composition of (NisAl)(Si3« to serpentine should be discarded because it Al)Oio(OH)8 is produced by complete substitu falls considerably outside the compositional tion of magnesium in clinochlore by nickel. A range of the trioctahedral chlorite group. nickel-rich trioctahedral chlorite has been de Complete substitution of magnesium by fer scribed by De Waal (1970) with the name of rous iron in clinochlore produces the end-mem nimite. ber composition (Fe5++Al)(Si3Al)Oi0(OH)8. For Complete substitution of magnesium in clino this composition, chamosite appears to be the chlore by manganese produces the end-member most logical name as it is the most often used (Mn,Al)6(Si,Al)«Oio(OH)8. The first occurrence name in the literature for approximately this of this manganese-rich trioctahedral chlorite was composition. The substitution of Fe++Si by described by Smith et al. (1946), who called it A1V,A1IV is frequent; the reverse substitution is pennantite. Initially grovesite was described by rare, although it frequently occurs in clino Bannister et al. (1955) as a member of the kao- chlore. In some chamosites, ferric iron results linite-serpentine group, but later it was decided from oxidation of ferrous iron with a loss of by Peacor et al. (1974) to be a one-layer poly hydrogen, but Foster (1962) considers ferric iron type Mn trioctahedral chlorite, whereas pennan as a normal constituent unless there is other tite is a two-layer polytype Mn trioctahedral evidence of secondary oxidation. The classifica chlorite. Because a polytype has only varietal tion of Hey (1954) into orthochlorites (unoxi- status, the name of grovesite should be dis dized ferrous) and leptochlorites (oxidized fer carded and replaced by pennantite plus poly ric) on the arbitrary division of 4% Fe*Os is type stacking symbol. rejected by Foster (1962), because ferric iron Frondel (1955) has described the mineral commonly substitutes for aluminum in many gonyerite (Mm.ssMgi.ssFe^Zn.MPb.ojCu.oOs.oifSia.Ts chamosites. Varietal names that should be dis Fe.i7Al.o8)401o.2o(OH)7.8o as a trioctahedral chlo carded include daphnite which is a chamosite, rite. However the ;t-ray diffraction powder data orthochamosite which is the polytype chamosite of this mineral shows that it is not a chlorite. lb, pseudothuringite which is an aluminian cha This has been confirmed with single crystals mosite, and thuringite which is a ferric alumi from the type material of Frondel by Bailey nian chamosite. (personal communication). Varietal names have been given to composi tions in the magnesium-ferrous iron solid-solu Conclusions tion series between clinochlore and chamosite. Varietal names that should be discarded include There are descriptions at present for four dis brunsvigite which is a magnesian chamosite, co- tinct chemical species within the trioctahedral rundophilite (often misspelled as corundophyl- chlorite group. With the use of chemical ele lite) which is an iron aluminian clinochlore, de ment adjectives, the complete range of chemical lessite which is an iron clinochlore, diabantite varieties may be described, so that all special which is a magnesian siliconian chamosite, pyc- varietal names should be discarded. In addition, nochlorite which is an iron clinochlore, and ri- the polytype symbol should be stated if it can be pidolite which is either an iron-aluminian clino determined. Until the symbol nomenclature is chlore or a magnesium-aluminian chamosite. agreed upon by the joint committee of the Inter Minor substitution of aluminum by chromium national Mineralogical Association and the In in clinochlore occurs in the varieties kammere- ternational Union of Crystallography, the sym rite and kotchubeite. These names should be dis bols of Brown & Bailey (1962) may be used. The carded in favour of chromian clinochlore. A use of prefixes such as ortho in front of cha proposal to redefine these varieties on an arbi mosite instead of suffixed symbols to indicate trary division of 2% Crj03 minimum was made the polytype must be avoided. The general for by Lapham (1958), who suggested that the mula for the trioctahedral chlorite group is chromium occupies octahedral positions in kam- [(Mg,Fe+ +,Mn,Ni)..x-yfAl,Fe+ ++,CrJr] [(Si^fAl) mererite but tetrahedral positions in kotchu z]Oio(OH)8. Foster (1962) gives the range of Z beite. This subdivision is extremely unlikely, as 1.6 > Z > 0.6, and states that the number because the crystal field stability energy of of octahedral cations which is normally 6.0 may Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/13/2/178/3445639/178.pdf by guest on 28 September 2021 180 THE CANADIAN MINERALOGIST drop to as low as 5.6 with X number of vacant Frondel, C. (1955): Two chlorites: gonyerite and octahedral sites. melanolite. Amer. Mineral. 40, 1090-1094. Hey, M. H. (1954): A new review of the chlorites.