15 April, 2016

April reading: isotopes, bentonite, dolomite, clay minerals and zeolites.

Several interesting articles appeared that I find worth and fun to read. The articles are focused on radiogenic and stable isotope compositions of bentonites as well as on dolomite formation associated with clays and zeolites in volcanic ash and soils. The research is nicely situated in the vicinity of my doctoral studies and own interests. So it is a good practise for reading and writing! I hope you enjoy the short and personal list!

Warr et al. (2016) Constraining the alteration history of a Late Cretaceous Patagonian volcaniclastic bentonite - ash - mudstone sequence using K-Ar and 40Ar/39Ar isotopes. Link 

Smectites are often considered unsuitable for radiometric age dating because they only incorporate tiny amounts dateable elements, most of which are easily exchangeable. Warr et al. explore the K-Ar and 40Ar/39Ar dating of smectite from sodium bentonite of Lago Pellegrini in Northern Patagonia, Argentina. They demonstrate that it is possible to date smectites if radiogenic Ar is retained in low-permeability rocks such as bentonites. Results tentatively indicate that a fairly long time (13-17 Ma) was needed for complete alteration. I try something similar using the Rb-Sr system using smectites from bentonites in Southern Germany. Maybe this can be combined...

Bauer et al. (2016) Stable isotope composition of bentonites from the Swiss and Bavarian Freshwater molasse as a proxy for paleoprecipitation. Link

Smectites might be problematic to use in radiometric dating but they are good for reconstructing the stable isotope composition of e.g. the water they formed in. This is especially true for smectite formed in-situ from the alteration of volcanic ash and preserved as bentonite. The isotope fractionation of oxygen and hydrogen is easily affected by temperature, evaporation, or other factors. Bauer et al. utilise this for paleoclimate and -topography reconstructions in the Swiss and German Foreland basin using bentonites from Switzerland and Germany, including some from my own research area in Bavaria. Again an exciting read in my opinion!

Alonso-Zarza et al. (2016) Chabazite and dolomite formation in a dolocrete profile: An example of a complex alkaline paragenesis in Lanzarote, Canary Islands. Link

Zeolites and dolomite are rare minerals in ancient and modern soils that require special conditions of formation. Alonso-Zarza et al. investigated a chabazite- and dolomite-bearing soil profile formed on basaltic rocks of Lanzarote Island with to elucidate its formation conditions, e.g. vadose vs phreatic, Mg distribution, and water sources. The lithostratigraphic distribution of dolomite in the lower and calcite in the upper parts of the profile together with the stable (C and O) and radiogenic (Sr) isotope composition nicely illustrate how carbonate formation was brought about by Mg from local basalt and meteoric water; with chabazite formed during drier, more alkaline periods.

Cuadros et al. (2016) Chemical and textural controls on the formation of sepiolite, palygorskite and dolomite in volcanic soils. Link

This study is about the formation of dolomite and Mg-bearing clay minerals such as palygorskite and sepiolite, and some smectite, in volcanic soil on Gran Canaria Island. Cuadros et al. examined a profile with predominantly sepiolite and calcite towards the top, and palygorskite, dolomite and some smectite towards the base. The dolomite formed related to sepiolite/palygorksite and volcanic ash particles while calcite is located in inter-particle pore-spaces. The study emphasises, among other issues, the role of Mg and Si transported by solutions within the profile but also within volcanic grains. It is a nice addition to the growing number of papers on dolomite formation in soils and terrestrial settings.

Note: Summaries are neither complete article reviews nor guaranteed to be free of misunderstandings. 

07 April, 2016

Online clay science resources: Glossary & Images

As stated in a previous post I like to share freely accessible clay science resources. Today, I want to point out two of them that are immensely useful for figuring out how to call that thing correctly when writing, studying, illustrating, or teaching.

One is the Clay Minerals Society Glossary of Clay Science Project. An ongoing effort to compile a glossary of terms "as used in clay science". The small site also has word- and PDF-file versions for download. I find this very useful for non-native speakers (like me) because not all terms are easily translated.

The other one is more eye-pleasing. It is the Images of Clay archive of the Mineralogical Society of Great Britain and Irland and The Clay Minerals Society. This is a collection of images of (clay) minerals available for non-profit purposes, such as teaching - and I suppose blogging. I especially like the time lapse video of exfoliating vermiculite.

video
Image (Video) reproduced from the 'Images of Clay Archive' of the Mineralogical Society of Great Britain & Ireland and The Clay Minerals Society (www.minersoc.org/gallery.php?id=2.")

12 March, 2016

Old plastic to clay science: super-size Atterberg cylinder

Size fractionation for sediment and clay minerals analysis is done using a centrifuge or Atterberg sedimentation cylinder. Both methods rely on the Stokes' Law to determine the settling time. The usual size fraction for clay analysis is < 2.0 microns. The settling time is determined by the viscosity of the medium - usually a suspension prepared with demineralised water - as well as its density and that of the material (e.g. clay, sand). Densities of quartz and water may be used for general purpose analysis but should be adjusted if high-quality results are needed. The viscosity is temperature dependent. Size separation must be done at constant temperature (e.g. 20°C). Usual set-ups consists of multiple Atterberg glass cylinders of 25 to 30 cm settling height. A typical set-up is shown below.
A dozen of Atterberg cylinders and glass jars to catch the < 2.0 micron suspensions.
Ordinary Atterberg cylinders have a diameter of roughly 5 to 6 cm. This is great for daily operations. But sometimes you need more and want to produce a concentrate of the sand/silt/heavy mineral fraction, as well. Sadly, swelling clays (e.g. bentonites) are awful to sieve either wet or dry. Sieves will be clogged fast. So, with the help of the department technician and using some old, left-over plastic scrap we built a super-size Atterberg cylinder. It has the same settling height but a diameter of 16 cm! This cylinder can hold half a bucket (~ 5 litres) of clay suspension. You can see the super-sized cylinders and the results of the test run in the image below.
Test run of super-sized Atterberg cylinder using 500 g of bentonite. The bentonite was dispersed in several litres of water.
It is great for separating and keeping tens of grams of sand, silt or heavy mineral fractions that can afterwards be sieved without hassle. The clay fractions (in the bucket) can be re-used in ordinary Atterberg cylinders for a more controlled setting. Admittedly, it has a few downsides. Controlling the settling speed of fine fractions is problematic. The suspension must be poured in and cannot be shaken. The internal locomotion of currents  hinders sedimentation. The prototype has the markings on the inside. We have made a replacement from transparent Plexiglas that I will try to get a picture of in action soon.

Costs: A few hours of playing around.

29 February, 2016

Swelling clays and X-ray diffraction

Introduction

Recently, I have been asked by a friend to guide him through a crash course in X-ray diffraction and the identification of swelling clay minerals. Swelling clay minerals are a major issue in formation damage for petroleum/natural gas production but also for natural buildings stones and construction. Since I am ever so often asked how to identify swelling clays by friends or colleagues, I figured that it would make a great blog!

Warning: I keep this very simple. No exceptions, no cutting edge tricks, no rare minerals. Every lab will do things a tiny bit different. So this might not be the exact way you saw it elsewhere. If you are unexperienced and want to do it yourself, you should consult the sources listed at the end of the blog post before proceeding.

Clay minerals

Geoscientists use the term clay in two different contexts that may cause confusion. First, clay sensu lato is any material with a particle diameter < 2.0 µm. So, anything with a grain size smaller 2.0 µm is clay in a physical sense. Second, and more important for the clay scientists, clay sensu strictu is a group of phyllosilicates (Fig. 1) composed of tetrahedral and octahedral sheets. The sheets are the building blocks of layers. There are 1:1 layers and 2:1 layers: meaning a tetrahedral sheet + an octahedral sheet, and an octahedral sheet sandwiched in between two tetrahedral sheets.
Fig. 1: An overview of the classification of clay minerals. Source: Bergaya and Lagaly (2006) General introduction: Clays, clay minerals, and clay science. In: Handbook of Clay Science, Edited by F. Bergaya, B.K.G. Theng and G. Lagaly, Developments in Clay Science, Vol. 1
Based on the site occupancy of the octahedral sheet clay minerals can also be subdivided into di- and trioctahedral. While the first have 2 out of 3 octahedral positions occupied with a cation, the later have all 3 positions occupied with a cation, e.g. Al, Mg, Fe, Li or others. Natural swelling clay minerals are hydrated 2:1 clays with an expandable interlayer such as smectite, interstratified smectite in "another mineral", and vermiculite.

Certain clay minerals have an electrical charge due to substitutions in the octahedral sheet. The charge is neutralised by additional cations (e.g. Na, Ca, Mg) located in the interlayer space between each 2:1 layer stack (Fig. 2). Water (and other useful substances) can enter into the interlayer. The clay swells.
Fig. 2: Basic structure of Montmorillonite, a dioctahedral smectite and hydrous 2:1 swelling clay mineral. Source: USGS Laboratory for X-ray powder diffraction
Determining swelling clay minerals

The clay minerals have to be separated from the rest of the rock. This is usually done by gentle crushing, followed by sedimentation in Atterberg cylinders or by centrifugation. When there is no need to determine the trace element contents or exchangeable cation composition of the clay interlayer the dispersion can be enhanced by adding ammonium (pH control) or sodium pyrophosphate. It is important to keep the suspension in a neutral pH range to prevent flocculation. The < 2.0 µm or any other size fraction can be separated based upon the Stokes Law. So, we are actually working with an equivalent diameter. Doing this we obtain a clay suspension.

A few drops of the suspension are pipetted onto a glass slide and allowed to dry. This is the orientated, air-dried sample. It is measured using X-ray diffraction from 2° to 25° or to 35°2theta. Afterwards, the glass slide is placed into a small container to saturate the interlayer with ethylene glycol vapour - or other substances. The intercalation of EG in the interlayer space (Fig. 2) expands the clay lattice. This can be detected by repeating the XRD measurement. This step must be done fast (~ half an hour to one hour) because the EG will easily escape. The third step is calcination above 500°C but several other temperature steps may be used to gain more detailed information. This will cause the interlayer space to collapse. We repeat the XRD measurement a final time.

How does it looks like in the end?

The best method to immediately see and compare the results is by putting all three measurements into one image. Most XRD devices will have their own internal computer program for data comparison. Alternatively, this can be done using commercial or non-commercial software packages. For illustration purposes I drew three XRD traces (Fig. 3) of one of my own samples (a smectite) treated according to the above explanations and measured as air-dried, EG-solvated and calcined sample. The measurement is shown from 4 to 24° 2theta. There is a clear expansion of the d001 peak from roughly 14.3 to 17.3 Å due to the intercalation of the ethylene-glycol. The calcined sample shows the characteristic collapse of the interlayer. A careful review of the peak positions will confirm that there are no other minerals.
Fig. 3: XRD traces of an orientated, air-dried, EG-solvated, and calcined sample of a smectite. Own sample and data.

References:
Bergaya and Lagaly (2006) General introduction: Clays, clay minerals, and clay science. In: Handbook of Clay Science, Edited by F. Bergaya, B.K.G. Theng and G. Lagaly, Developments in Clay Science, Vol. 1.
Hillier (2003) Quantitative Analysis of Clay and other Minerals in Sandstones by X-Ray Powder Diffraction (XRPD)
Moore and Reynolds (1997) X-Ray Diffraction and the Identification and Analysis of Clay Minerals, 2nd edition.

Internet resources for more detailed instructions: 
USGS - A Laboratory Manual for X-Ray Powder Diffraction
The Cutting Edge - Teaching Clay Science 

16 February, 2016

Amazing geology in Peru: How I found a mythical boiling river in the Amazon - Andrés Ruzo - Ted Talks

My good Peruvian friend and geologist (thanks Jenny!) sent me a link today about a Ted Talk featuring Andrés Ruzo, another geologist, and his search for a mythical boiling river in the Amazon far from any volcanic centre. I liked the talk a lot. Reminds me of my own but not quite that adventureous times in Peru... :-)