In reply to andrewmcleod:
Reading the table and other information I am pretty sure they are claiming circa 75% MgCO3 which means they are saying other chalks are ranging from 20% to 40% MgCO3.
I think it is more complicated than Just MgCO3 = good BTW.
It's all complicated by the fact most forms of magnesium carbonate are hydrated to a lesser or greater degree.
Mammut quite helpfully give a formula for their chalk as MgCO3·Mg(OH)2·4 H2O) which is called hydromagnesite according to the quote below. Strictly speaking the term magnesium carbonate should only be used for the anhydrous form (water free) but it appears not to be the case outside of the chemical world.
From Wikipedia:
The most common magnesium carbonate forms are the anhydrous salt called magnesite (MgCO3) and the di, tri, and pentahydrates known as barringtonite (MgCO3·2 H2O), nesquehonite (MgCO3·3 H2O), and lansfordite (MgCO3·5 H2O), respectively. Some basic forms such as artinite (MgCO3·Mg(OH)2·3 H2O), hydromagnesite (4 MgCO3·Mg(OH)2·4 H2O), and dypingite (4 MgCO3· Mg(OH)2·5 H2O) also occur as minerals.
So you see they can all claim to be 100% magnesium carbonate.
It explains how 'pure magnesium carbonate' can have peculiarly low levels of MgCO3 as, in the Mammut formula, I calculate that this contains 39% MgCO3. (which puts it in the best of the rest camp according to Fricion Lab's figures)
Interestingly if you perform the same calculation for the other formulae then the dihydrate (barringtonite) gives 72% of its mass as MgCO3 which is roughly where the chart claims Friction Labs chalk to be.
From Wikipedia again:
"The barringtonite is a mineral that takes its name from the place of discovery, Barrington Tops , New South Wales , Australia ."
OK so it's a bit of conjecture but it seems likely that it is the dihydrate they are using to make their chalk. The calcium carbonate reference seems to be a red herring/irrelevance.
I was highly sceptical about the idea of this being better than 'normal' chalk when I read the article and, while I wouldn't be convinced without trying it, there seems to be a rational explanation for there being a difference in performance characteristics well as a potential justification for the cost.
The obvious question is why not just use 100% MgCO3 as it is readily available and water free. Perhaps it is too aggressive or it doesn't 'wet' readily.