Noncentric Layer Silicates: An Optical Second Harmonic Generation, Chemical, and X-Ray Study

Abstract

Abstract--Twenty-nine chlorites, seventeen lithium-free micas, and twenty-two lithium-bearing micas from diverse localities have been studied by X-ray diffraction, microprobe, and optical second harmonic generation (SHG) techniques to determine which are noncentric and the cause ofacentricity. Manandonite (B-rich chlorite) and cookeite, both crystallizing in the Ia form, are acentric. Sudoite-IIb gave a questionable SHG signal, possibly indicating acentricity. All other chlorites gave null signals. Bityite, a Li,Be mica similar to margarite, was determined from the positive SHG response to be acentric. It crystallizes in the 2Mj form and, by analogy to margarite, is ordered tetrahedrally in subgroup symmetry, Cc. Masutomilite-1M and "cryophyllite"-1M (a zinnwaldite high in A1 and Si, but low in Fe) are acentric and most probably crystallize in space group C2, thus allowing a noncentric octahedral ordering pattern. Lepidolites showed a diversity in SHG response with only a few being acentric; a lepidolite-lM from Mesagrande, California, a lepidolite-3T from Windhuk, South West Africa, and a lepidolite-2Mz from Nagatare, Japan, gave positive SHG responses. The Japanese material is complexly intergrown and twinned, and intergrain reflection or refraction may have produced spurious signals.Most lithium-free micas showed diversity in SHG response and, although most were non-emitters, clintonite from Amity, New York, showed a positive response, but this sample is twinned and shows stacking disorder. A "manganophyllite"-1M (manganoan phlogopite) from Langban, Sweden, showed a very weak positive response; however, the presence of Mn alone is not suffcient to produce octahedral cation ordering in noncentric subgroup symmetry. Two manganoan phlogopites from Japan were refined in subgroup symmetry, and the higher order and ideal symmetry of C2/m was confirmed.

Structures of Intergrown Triclinic and Monoclinic IIb Chlorites from Kenya

“…Thus, the ideal space groups appear to be correct for both specimens. These space groups are both in accord with the observation by Guggenheim et al (1983) that this sample did not produce a positive optical second harmonic generation response, suggesting, but not proving, that the sample is centrosymmetric.…”

The Dehydroxylation of Chlorite and the Formation of Topotactic Product Phases

“…Approximately 6 mg of cookeite, Londonderry, Western Australia (Guggenheim et al 1983, Bailey 1980 in air, examined as single crystals in the powder diffractometer, and then re-heated at 600~ for an additional 16 hours to assure dehydroxylation. The structural formula (Guggenheim et al 1983) is approximately Nil.22 Mgl.9S Fel.75 Alo.3o (Si2.93All.o7)Olo(OH)s. Crystals were crushed after heating for final powder X-ray examination (diffractometer conditions same as above). The X-ray pattern was similar to that of heattreated sudoite (Figure 2): there was no evidence of a 27-A phase, the (001) peak increased in intensity, and the higher-order peaks generally decreased in intensity.…”

“…3 Analysis from Lin and Bailey (1985) and Fransolet and Bourguignon (1978). 4 Analysis (partial) from Guggenheim et al (1983). 5 Analysis from Miser and Milton (1964) as reported (averaged) in Cern~ (1970).…”

Mössbauer and Infrared Study of Heat-Treated Nontronite

“…In a centrosymmetric environment, the undesired triads of three opposing superexchange interactions are not possible and therefore, long-range magnetic ordering can be achieved rather easily, as is the case for ferripyrophyllite with a Ntel temperature (TN) of --18 K (Coey et al, 1984). However, as noted by Guggenheim et aL (1983), the determination for a centtic or acentric structure is better based on piezoelectric or second harmonic generation (SHG) data. Lear and Stucki (1990) in a thorough analysis of the magnetic properties and site occupancy of iron in several nontronite samples showed, by computer simulation, that occupancy of the trans site at as low as 13% can prevent long-range magnetic order, They suggested that trivalent iron may be accommodated in trans sites either instead of the cis site or in addition to the cis site to produce a trioctahedral domain.…”