To determine clay minerals you can hardly renounce to start with XRD, IR spectroscopy, CEC treatment and apply at an advanced level EMPA and Mössbauer spectroscopy to get an idea of the valence state. Every clay mineral you detect can be allogenic and authigenic in origin. To take a decision which of these types of formation play the most significant part in the clay mineral assemblage you need images to see the grain and X morphologies, the intergrowth with clay and non-clay minerals and the relative ages of formation. This can only be accomplished by SEM EDX/WDX and much practical experience. Learning by doing and a set of comprehensive textbooks will help you to achieve this goal.

I wish you much success H.G.Dill

Thanks as always for your quick and exhaustive reply Prof. Dill!

Best wishes,

Luca

I agree with prof. Dill, however, I would like to point out that to distinguish between authigenic and allogenic components, powder X-ray diffraction analysis is performed on the grain fractions of the sample. Analyzes are usually performed on sedimented preparations, on a fraction below 0.1 μm, 1-2 μm and about 5 μm or more too, and the expandability of the samples is also tested.

Thanks!

Harald G. Dill in the coastal sediment, authigenic clay formation is an important geochemical process but difficult to quantify. Prof. Dill, could you please recmmend us a textbook explaining the way of suing DEX-SEM to determine authigenic clay? Many thanks.

There are common hints to help take a decision whether minerals, including the clay minerals are authigenic or allogenic. It is the structure and texture of a rock which helps to chronologically assign the minerals a precise position within the evolution during lithogenesis and the process of "purification by recrystallization" which means a change in the chemical composition towards a more primitive state and in terms of crystallization lowering in the degree of crystallization.

The "philosopher´s stone" is a precise separation of clay minerals by grain size and composition and subject it to age dating such as K/Ar, Ar/Ar or Rb/Sr. There exists a wealth of fine papers devoted to the system muscovite-illite. It runs the gamut from basin-related studies on provenance and diagenesis to late-stage fracturing and the study of argillaceous fault gauge.

Recently, I found a very nice paper that also addresses the petrographic criteria for such sort of characterization. Strongly suggested:

Rafiei et al., 2020. Quantitative petrographic differentiation of detrital vs diagenetic clay minerals in marine sedimentary sequences: Implications for the rise of biotic soils. Precambrian Research 350.

Best,

Luca

Hi Luca,

yes this is possible

for an example, please take a look at our recent paper in GSA Bulletin, especially Figure 9. Article Authigenic berthierine and incipient chloritization in shall...