CLAY MINERALS 361 Table Clay classification by layer type Tr trioctahedral, Di dioctahedral, x layer charge, Note the list of species is not exhaustive, and interstratified mixed layer clays abound (see below) (Adapted from Brindley and Brown, 1980.) Layer type Group Sub group Species (clays only) 2:1 Vermiculite (x $ 0.6 0.9) 2:1 Mica illite (x $ 0.8) 2:1 Chlorite (x variable) 2:1 Sepiolite palygorskite (x variable) Serpentines (Tr) Kandites (di) Talc (Tr) pyrophyllite (Di) Tr smectites Di smectites Tr vermiculites Di vermiculites Tr illite? Di illite Tr Tr chlorites Di Di chlorites Tr Di chlorites Di Tr chlorites Sepiolites palygorskites Berthierine, odinite Kaolinite, dickite, A˚ & 10 A˚ halloysite, nacrite 2:1 2:1 Serpentine kaolin (x $ 0) Talc pyrophyllite (x $ 0) Smectite (x $ 0.2 0.6) 1:1 Clays (Smectite) (2:1) The 2:1 clays with the lowest interlayer charge are the smectites This group have the capacity to expand and contract with the addition or loss (through heating) of water and some organic molecules It is this property that is used to identify smectites by glycol or glycerol solvation and heat treatments in XRD studies This swelling is believed to be due to the greater attraction of the interlayer cations to water than to the weakly charged layer Montmorillonite is a predominantly dioctahedral smectite with the charge primarily in the octahedral sheet (Rỵ(Al3Mg.33)Si4O10(OH)2), while beidellite (RỵAl2(Si3.67Al.33)O10(OH)2) and nontronite (Rỵ Fe(III)2(Si3.67Al.33)O10(OH)2) are dioctahedral with the charge mainly in the tetrahedral sheet (where R ¼ mono or divalent interlayer cations) Hectorite is a rare clay, similar to montmorillonite but it is predominantly trioctahedral with Mg2ỵ and Li2ỵ in the octahedral layer Saponite is another uncommon smectite with a positive charge on the octahedral sheet and a negative one on the tetrahedral sheet In smectite the interlayer cations are typically Ca2ỵ, Mg2ỵ, or Naỵ; in high charge smectites Kỵ may be present When smectite expands, the interlayer cation can be replaced by some other cation Hence the cation exchange capacities of smectite is high compared with nonexpanding clay minerals CEC can therefore be used to identify smectite Clays (Vermiculite) (2:1) The second group of clays with exchangeable cations is vermiculite Vermiculite has a talc-like structure in Montmorillonite, hectorite, saponite beidellite, nontronite Illite, glauconite Chamosite, clinochlore, ripidolite etc donbassite Sudoite, cookeite Sepiolite palygorskite (syn attapulgite) which some Fe3ỵ has been substituted for Mg2ỵ and some Al3ỵ for Si4ỵ, with the resulting charge balanced by hydrated interlayer cations, most commonly Mg2ỵ The layer charge typically ranges from 0.6 to 0.9 Vermiculite is distinguished from smectite by XRD after saturating with MgCl2 and solvation with glycerol This results in expansion of the interlayer to ˚ , rather than the 18 A˚ characteristic of smectite 14.5 A (though there may be exceptions to this rule) Vermiculite is much less often encountered in sedimentary rocks than is smectite, probably because it is most commonly a soil-formed clay, while coarsely crystalline vermiculite deposits are formed from alteration of igneous rocks Clays (Mica and Illite) (2:1) Substitution of one Al3ỵ for one Si4ỵ results in a layer charge of 1, which in true mica is balanced by one monovalent interlayer cation (denoted R) In mica the cation is usually Kỵ, less often Naỵ or Ca2ỵ and rarely NHỵ The term clay grade mica is sometimes used to describe mica which has been weathered resulting in loss of interlayer cations or formation of expandable smectite layers 2:1 clays with layer charge $0.8 are illite (Rỵ(Al2-xMg, Fe II, Fe III)xSi4-yAlyO10(OH)2) and glauconite (the ferric iron-rich equivalent of illite) Like mica, illite and glauconite are characterised by a basal lattice spacing of d(001) ¼ 10 A˚ which is unaffected by glycol or glycerol solvation, nor by heating Illite is used as both a specific mineral term and as a term for a group of similar clays, including some with a small degree of mixed layering Illite has been described as detrital clay-grade muscovite of the M