No abstract

A B S T R A C T:The weathering of biotite in Quaternary lahars from Monti Ernici (Central Italy) via interstratification stages has been studied by X-ray diffraction of both the interstratified minerals and the initial (biotite) and final (vermiculite) end-members. The structure of the biotite and vermiculite is described by means of monodimensional Fourier series. The interstratified phases have been studied using the INTER program which permits an analysis of two interstratified components by Fourier transform methods. It is shown that in the alteration range for biotite to vermiculite even small samples have different spatial concentrations of biotite, vermiculite and an intermediate biotite-vermiculite-type phase. Hydrobiotite and a rare biotite-hydrobiotite with a tendency to regularity have also been documented. Chemically, the process involves considerable oxidation of Fe 2+ and the removal of most of the elements, corresponding to a 34% loss in matter.

A computer program for analysing two-component interstratified structures by two different Fourier transform methods is described. The first method consists of the calculation of the X-ray diffraction intensity function, and in the second one (direct method) the distribution function of interlayer distances is calculated. The programs have been written in compiled GWBASIC for an Olivetti M-24 microcomputer (MS-DOS operating system), and they may also be run on any IBM compatible personal computer. As an example, this program is applied for the analysis of a chlorite-smectite interstratified mineral.

The dehydration-rehydration of Mg-vermiculite, as large flakes (7 x 5 x 0.1 mm3) and powder (< 80 pm), has been studied. Samples were treated in a sealed bomb at 140 and 100 "C for several days. After dehydration, they were rehydrated at room temperature. The samples were examined by X-ray diffraction, and the results are analysed here for the first time by the direct Fourier transform method. The resu!ts indicated the formation of two-water layer hydration states (14.4, 14.2, and 13.8 A phases), and one-layer Mgvermiculite ( 11.5 A phase) segregated phases, besides binary interstratified one-two water layer hydration states of Mg-vermiculite phases. The existence of interstratified phases was confirmed in Mg-vermiculite flakes in the conversion from two-one and one-two water layer hydration states in Mg-vermiculite flakes by dehydrati0n;rehydration treatments. The dehydration takes place through interstratified phases consisting of alternations of 13.8 and 11.5 A components (25.3 A, regular interstratified phase). The transformation towards the 11.5 A phase at 140 "C is more rapid than at 100 "C. The rehydration produces mixed layers consisting of alternating 11.5 and 14.2 A components (random interstratified phases). Using powdered Mg-vermiculite, the changes occur faster than in the flakes, and are produced only at 140 "C. Under these conditions, no interstratification can be detected, and the dehydration-rehydration takes place spontaneously.The presence of the interstratified phase in the conversion of two-one and one-two water layer hydration states arises from the mobility of the water molecules in the crystals. The results suggest that the large particles permit a low mobility of water molecules and the thermodynamic equilibrium between the original and final segregated phases gives rise to a regular phase, characterized by the loss of an equal number of water molecules in the interlayer space. In the case of rehydration, the process is reversed. The transformation from two-to one-water layer Mg-vermiculite throughout interstratified phases depends on the temperature, time of treatment and particle size.

Alrstract--The conversion of Malawi vermiculite into K-vermiculite by treatment with bi-ionic K-Mg solutions of 1 N total ion concentration (KC1 and MgCI2 mixed solutions of ionic strength equal to 0.5) was studied by following the 00l X-ray powder diffraction (XRD) reflections. Flakes of Mg-saturated samples were treated at 160~ during 24 hr with bi-ionic solutions, with the K concentration varying from zero to pure 1 N KC1 solution. The K-Mg interlayer exchange began at a critical value xK = .0196 (K/Mg = 1/100) of the molar fraction of K in the solution. Above the critical concentration and extending to pure 1 N KCi, the XRD diagrams were characteristic of a 10-,~/14-~ interstratification that had a marked tendency towards regularity. Experiments with KCI and MgC12 mixed solutions of ionic strength equal to 0.75 and 1.0 showed that the exchange began at the same critical value xg as the experiments with ionic strength equal to 0.5, if the K added was equivalent. X-ray fluorescence analysis further showed that the amount of K adsorbed was proportional to the molar fraction xK and to the proportion of K-saturated layers (10 A) in the interstratification. To explain the mechanism of this quasi-regular interstratification, a crystallochemical rather than a thermodynamic mechanism is proposed.