Why investors need stoichiometry

I was browsing the internet recently, eyeballing some of the ASX releases of various other exploration companies working in the NT, when I noticed the following release (pdf).

The headline announcement is “MgO values of up to 37.9% after removing LOI.”

A brief aside:
Magnesium is commonly mined from magnesite, MgCO3, and a brief description of several mine types can be found here (pdf). Magnesite decarbonates at a few hundered degrees C to MgO + CO₂. As shown in the article above, ore grades are often reported as %MgO after LOI (so pure magnesite would be 100%).

Magnesium is a very common element, though, and is found in all sorts of other less economic minerals, such as pyroxenes, dolomite, hornblende, talc, etc. But it is generally less economical to extract Mg from these minerals. If, as this AXS release suggests, their exploration model is a Kunwarara-type sedimentary magnesite deposit, then they may want to demonstrate that their magnesium is actually in magnesite, and not some other less economically useful mineral.

So I wondered, is the magnesium content of dolomite after LOI greater or less than 37.9%? Dolomite, CaMg(CO₃)₂, decomposes to CaO + MgO+2CO₂, which is lost. Using masses of 15.9994 for O, 24.3050 for Mg, and 40.078 for Ca (via webelements), we get a LOI-removed MgO content of 41.8%. So any bulk analysis greater than this amount must contain some phase that is more MgO rich than dolomite (like magnesite, the stuff we are looking for).

Trouble is, their highest reported value is 37.9% MgO, from what they describe as “dolomite nodules”. Assuming no other Mg bearing minerals, 38% MgO corresponds to a rock that is about 90% dolomite. Now, finding dolomite could be very exciting in some parts of the world where carbonates and magnesium are rare. But the Georgina basin is full of dolostone.

Of course, if there is no calcium in the rock, then their can't be any dolomite, and it could very well contain magnesite. We don’t know what the other 68% of the rock is made of. But given that they report dolomite nodules, and dolomite has a higher post-LOI MgO content than what they found, there is no reason to believe that they have any magnesite.

Read this part carefully:
I am not an investment advisor. Anyone who buys or sells shares based on my advice will probably end up broke. But I am a geochemist. And based on the calculations above, these reported results are consistent with having found a common rock type in the Georgina Basin, and not magnesite.