25 July, 2015

Let's XRD something unusual: What's in my ghassoul?

X-ray diffraction is an incredibly versatile method not restricted to rocks and minerals. While the material scientist and engineers among you might use it to examine concrete or thin-films in photovoltaics, I tried it on cosmetics from the organic supermarket. Actually, I picked a ready-to-use mixture of ghassoul that can be used as a shampoo. I will not disclose the brand or producer. But as far as I can tell from my very quick-and-dirty analysis: what is written on it - it is actually inside! But I make no guarantee!

Ghassoul is a natural clay rock found in the Atlas mountains of Morocco. Depending on the sources and literature it is either a bentonite, a stevensite-rich or a hectorite-rich clay rock. The clay has been mixed with oils, alcohol, and perfumes. So I was sceptical about identifying anything. I put the clay into a cup of water, stirred, and pipetted a few drops onto a glass slide to make an orientated texture mount. That is usually done for the clay fraction less than 2 microns and therefore probably only qualifies as playing around in the lab. 

The XRD trace was recorded from 2° to 70° 2θ and CuKα radiation. Measuring time is rather short with about 50 minutes. The characteristic peaks for several minerals are visible in the XRD trace. There is a large smectite 001 peak at about 14 Å and a small peak at about 1.52 Å characteristic for trioctahedral smectites. So this very likely contains a trioctahedral smectite such as hectorite, stevensite or saponite. I do not know if and how oils and perfumes intercalate into the smectite interlayer but assuming that they have not, the 001 position indicates an interlayer dominated by divalent cations, most likely Ca and Mg. If there would be Na or even K it would rather be around 10 to 12 Å.

XRD trace of ghassoul smear slide.
A lot of other peaks are visible at higher angles. I only marked the most important main peaks of easily recognisable phases, such as quartz, feldspar, calcite, mica, dolomite, and some kaolinite or chlorite. The later two are hard to distinguish without further work. There also are some peaks that would require more work, perhaps they are pyrite/hematite, more feldspar, etc. 

So, the ghassoul really does look like it contains natural clays and minerals! I cannot, however, determine if this is the natural composition or if minerals have been added.

Finally, as the stuff is very expensive I am putting it to practical trials on head and beard. :-)

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