The low-Fe clay was a purified china clay (Imerys PolwhiteE). This is a commercial kaolin product from the well-known kaolinized granite deposit in Cornwall, U. , in which muscovite is commonly found alongside remaining K-feldspar and quartz as associated minerals . The quantities of each mineral phase were calculated independently by quantitative XRD analysis using the Rietveld refinement method as 73.5 wt.% kaolinite, 17.9 wt.% muscovite, 5.6 wt.% K-feldspar and 3.0 wt.% quartz.