Almtraet.--Polytype diversity of hydrotalcite-like minerals is mainly a function of the nature of the interlayer anion. Among the varieties with COl-anions, only two-and three-layer polylypes having the same structure as manasseite and hydrotalcite have been confirmed. Stichtite and reevesite, which have been previously identified as six-layer polytypes, are in fact three-layer polytypes.Among SO4 ~ varieties, one-layer and three-layer polytypes have been identified, but the one-layer types are only present in more hydrated minerals with larger interlayer spacings. The three-layer varieties are of three different polytypes, with both P-and O-types ofinterlayers. Both rhombohedral and hexagonal varieties exist. Interlayer type may change during hydration-dehydration or anion exchange. Thus, in contrast with the COs2--bearing minerals, a complete description of the polytype of the SO42--bearing minerals cannot be made by simply indicating the number of the brucite-like layers in the unit cell.The two-layer unit cell seen in refined crystal structures of some minerals with SO42-interlayers is not due to a doubled periodicity of alternation of brucite-like layers but to periodicity of interlayer anions, or layer cations.

Abstract--Several new minerals, structurally and chemically similar to the hydrotalcite-manasseite group, have been found in the saline deposits of the central pre-Caspian depression and in those of Middle Asia. Their structures consist of layers of (Mgl_xAlx)(OH)2 plus interlayers of anions and molecular water. In addition to manasseite itself, [(Mg4A12)(OH)~2][(CO3)" 3H20], having unit-cell parameters of a = 3.042, c = 2 x 7.56 = 15.12 ~ and a calculated density of De = 2.15 g/cm 3, other varieties were identified whose interlayers contain alternately SOn 2-or SO42-and CO32-anions. The following new phases were recognized: (1) For this diverse family of minerals whose structures are based on brucite-like layers of the composition (RZ+~_~R3+~)(OH)2, a unifying system of nomenclature is offered for consideration. In describing these brucite-like structures intercalated with anion-and cation-containing interlayers, the composition of the hydroxide sheet and the interlayers, the periodicity of the layers, and the mixed-layer nature of the material should be described. By means of symbols, a single term might be used to describe the crystal chemistry of any member of the group having a given cation composition for the brucite-like layers, for example, manasseite, a 2H polytype ofhydrotalcite that has a repeat distance of 7.56 ]~ and CO3-containing interlayers, might be designated as 7.56-fik CO3-hydrotalcite-2H.Key Words--Carbonate, Hydrotalcite, Interlayer, Nomenclature, Sulfate, X-ray powder diffraction.Pe3~oMe--B CO.TIeHOCHblX OTJIOZKeHH~IX ILeHTpaJIbHOkI qaCTlt Ilpni~acnnftcKofi BIIa~HHbI H CpeaHefi A3HH OrHapyxceH p~ HOBbIX MHHepa.r/OB, cTpyKTypHo 6dlH3KHX K MnHepa.qaM rpynnbI FH,~poTad/bKHT-MaHaCCeHTa. OCHOBy cTpyKTyp aTrlX MriHepa.rloB COCTaB;IarOT noJIOmrITeJIbHO 3ap~Ix~eHs1,le 6pyCrlTOnO)IOrHbie CJIOtI coTaBa (Mg~ _xAlx)(OH)2, MezKay roTopbliH pacnoymraloTc~i arlraOHbl n MO~eKy~bI BOabl. FIOMrlMO CO6CTBeHHO MaHaccenTa COCTaBa [(Mg4A12)(OH)~j[(CO3)" 3H20] c napaMeTpaMa aJIeMeHTapHo~I a~IefiKa a = 3.042, c = 2 x 7.56 = 15.12/~ a pacc,triTaHno~ naOTHOCTI, FO (Dp) = 2.15 r/CM 3, ycTasoB~IeS~l c~e~yIomne pa3noBna-rlOCTH, Me~KCJIOeBLIe IIpOMe~KyTKrl KOTOpI,IX coaep~KaT aHVlOHI, I SO42 nmI SO42-n CO32 : (1) Ilpea.~araeTc~t yrmqbi,~tmpoBamtaa cricTeMa HOieHK.rtaTypr~ a.aa Bcero MHOrOo6pa3Horo ceie~cTBa MnHepa2Io~, OCHO~y CTpyKTyp KOTOpr~IX COCTaB:IaIOT CZlOri cocTaBa (R:+~_xR3+x)(OH)v ,~IJIa o6o3Haqenrla aTrlX 6pyc}iTOnOaO6m,lx cTpyrTyp, riHTepKa2IapOBaHHb~X MexcYIoeBblMtt aHaOHaMrI tt KaTnOHaMrI, Heo6xoaHMO yKaaaTb COCTaB rH~pooKHcm,IX c~IoeB n Me~KC.aOeB, rlepHoa nOBTOpaeMOCTa C:~OeB, cMemaHoc~IO~Hy~O npripony MHHepa2Ia. Ha ocnoBaHnri npemlaraeMoff CHMBO~IHKH MOXrIO acnoab3o~aT~, TOm, KO oano rtar~Melto-sa~He, qTO6bl orlncaTb .rIlo60~I qaen rpynnbl, nMeiOttlH~ ,!~aHHbI~l COCTaB 6pycnTonoao6n~xx c.rloeB. HanpnMep, MaHacCeHT-2H, nOMaMO raapoTaaSKHTa, rorop~fft riMeeT nepnozt nOSTopneMOCTn 7.56 ~, n COacoaep~amHe Me~rc.ao~, Mo>KeT 6bITS o6oanaqeH Kar 7.56-~ CO3-rnapoTa_a~rnT-2H.

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