Abstract

The occurence of lizardite 1T from Val Sissone, Italy, is reported. Electron microprobe analysis leads to the doubled unit cell content  
(Mg5.58Fe0.082+Fe0.203+Al0.14)(Si3.66Al0.34)O10.00(OH)8.00.

The unit cell parameters are a = 5.332(3)Å, c = 7.233(4)Å, space group is P31m.

The crystal structure has been refined, using 209 symmetry independent reflections, to R = 0.031. The refined model is not far from the idealized geometry of the serpentine layer. The ditrigonal distortion of the six-membered tetrahedral ring is small (α = −3.5°). No buckling of the brucite-like sheet is observed.

The occurrence of both negative and positive α values in serpentine minerals is explained on the basis of stacking sequence and hydrogen bonds. Namely, the best hydrogen bond system is attained by rotation of the bridging oxygens belonging to the tetrahedral sheet. The direction of rotation depends on the way in which subsequent layers are stacked one upon the other. The substitution of trivalent atoms for magnesium and silicon leads to stronger hydrogen bonding between adjacent layers, thus promoting the formation of flat-layer structures and increasing thermal stability.

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