Franz Michael Meyer

Comparison of methods for the quantification of montmorillonite in bentonites

In this paper, we describe and evaluate three methods for measuring the amount of montmorillonite in 10 bentonite samples. Each of these methods is based on certain assumptions that usually result in analyses of low accuracy. In order to assess the attainable accuracy, two procedures for the determination of montmorillonite contents are tested. The first is based on the quantitative separation of the <2-μm fraction in combination with XRD-analysis. The second considers cation exchange capacity (CEC), layer charge (LC) and variable charge. In addition, a third and new method based on the comparison of the CEC of a bentonite with the CEC of the respective pure montmorillonite fraction is proposed. This new method provides accurate values for montmorillonite contents, but is restricted to bentonites, which are free of X-ray amorphous fine-grained constituents. By combining the results of all methods, the layer charge (charges/half unit cell (HUC)) was calculated. Obtained results are comparable with the well-known n-alkylammonium method.

True elemental imaging of pyrites from Witwatersrand reefs

Abstract Dynamic analysis imaging was used on-line to obtain true elemental maps of round, compact pyrite from Witwatersrand reefs. Zones of high and low As concentrations were detected, which are spatially related to zones of Au accumulation. Thus, elemental maps provide key information for the discussion of the origin of the gold-pyrite association and suggest a correlation between zones of high As and gold. This is interpreted in terms of a model based on electrochemical reduction of gold complexes driven by the galvanic cell produced by zones of varying As concentration.

K-Ar dating of white micas from the Ventersdorp Contact Reef of the Witwatersrand Basin, South Africa: timing of post-depositional alteration

SummaryWhite micas from metamorphosed and hydrothermally-altered basaltic lavas, conglomerates, quartzites and shales in and around the Ventersdorp Contact Reef of the Witwatersrand Basin, South Africa, were dated by the K-Ar method to constrain post-depositional thermal and mineralization processes. The minerals were separated into various grain sizes between < 0.4 and 10μm, and characterized in terms of composition, paragenetic sequence and texture, by XRD, SEM and TEM techniques. The K-Ar isochron of all white micas in the basaltic lavas suggests an age of 1994 ± 60 Ma, and that of the smaller mica particles (< 2μm) in the quartzites defines a younger age of 1917 ± 66 Ma. This range is considered to reflect the timing of long-lived hydrothermal alteration in the Ventersdorp Contact Reef. The older age is slightly younger than the intrusion of the Bushveld Complex (2060–2054 Ma) and the Vredefort catastrophism (2020 Ma), which are well-documented events that were superimposed onto the Witwatersrand Basin. The younger age may be associated with the Eburnian orogenesis along the western edge of the Kaapvaal Craton resulting in continental-scale fluid migration and hydrothermal activity that extended throughout the Griqualand Basin and even into the Witwatersrand Basin. The K-Ar ages obtained here for the white mica fractions of the Ventersdorp Contact Reef in the Witwatersrand Basin confirm that the period between 2.0 and 1.9 Ga was significant, as far as alteration, and possibly also gold mobilization, was concerned.ZusammenfassungHellglimmer von metamorph und hydrothermal überprägtem Basalt, Konglomerat, Quarzit und Pelit aus dem Ventersdorp Contact Reef im Witwatersrand Becken, Südafrika, wurden mittels der K-Ar Methode datiert, um das Alter der postsedimentären thermischen Überprägung einzugrenzen. Mineralkörner wurden in einzelne Fraktionen zwischen < 0.4 und 10μm separiert und auf deren Zusammensetzung, Paragenese und Textur mittels Röntgendiffraktometrie und Elektronenmikroskopie untersucht. Das Isochronenalter für alle Hellglimmerproben aus dem Metabasalt liegt bei 1994 ± 60 Ma, jenes für die Glimmer mit einer Korngröße < 2μm im Quarzit bei 1917 ± 66 Ma. Diese Altersspanne wird als Ausdruck einer lang anhaltenden hydrothermalen Veränderung des Ventersdorp Contact Reef interpretiert. Das ältere Alter ist etwas jünger als die Intrusion des Bushveld Komplex (2060–2054 Ma) und die Vredefort Katastrophe (2020 Ma) - zwei gut dokumentierte Ereignisse, die das Witwatersrand Becken erfaßten. Das jüngere Alter könnte mit der eburnischen Orogenese entlang des Westrandes des Kaapvaal Kratons in Zusammenhang stehen, während der es zu Fluid Migration und hydrothermaler Aktivität quer über den Kontinent vom Griqualand Becken bis in das Witwatersrand Becken kam. Die neuen K-Ar Alter bestätigen somit, daß die Zeitspanne von 2.0–1.9 Ga wesentlich für die Alteration und möglicherweise auch für die Mobilisierung von Gold im Witwatersrand Becken war.

Tetrataenite in terrestrial rock

Abstract Tetrataenite is an equiatomic and highly ordered, non-cubic Fe-Ni alloy mineral that forms in meteorites from the distortion of fcc taenite due to extremely slow cooling. The mineral has drawn much attention of the scientific community because of its superb magnetic properties, which may make the phase an alternative to the REE-based permanent magnets. Barring only a few passing mentions, the mineral has never been described from any terrestrial rock. Here we report the characteristics of terrestrial tetrataenite from an ophiolite-hosted Ni-bearing magnetite body from the Indo-Myanmar ranges, northeast India. Although the mineral assemblage surrounding it is very similar to that found in the meteorites, the postulated cooling regimes cannot be similar. The mineral is formed as a consequence of hydrothermal alteration of ferromagnesian minerals of the olivine and pyroxene groups. Iron and nickel were released from the silicates and precipitated in the form of Fe-Ni alloy at low temperature in extremely reducing conditions with a lack of sulfur. Our findings suggest a low-temperature hydrothermal origin of tetrataenite warrants a re-examination of the Fe-Ni phase diagram at low temperatures and puts a question mark on the age-old concept of tetrataenite formation as due solely to extremely slow cooling of fcc taenite in meteorites. It also opens up a new vista for adoption of a hydrothermal route to synthesize this rare material.