Grabbing all the clays and kaolins I can find to push through testing for the soda wash recipe.
This one is Red Horse Clay.

Looks like a heavy layer of iron oxide, a nice metallic sheen.
I'd never heard of it before, but I like what it does.

Red Horse Clay Chemistry % CaO 0.45 MgO 0.62 K2O 1.20 Na2O 0.30 Al2O3 22.03 SiO2 67.59 Fe2O3 6.62 Volatiles % LOI 1.18 Another analysis show 50% SiO2, 28% Al2O3 and 10% LOI.