Table 1.

XPS characterizations of the element contents near the surface at each step of the nanopurification process

Material (wt%)	Si	Fe	Al	Ca	Mn	Mg
Bulk	81.1	9.7	5.4	3.5	0.2	0.1
After ball milling	61.7	12.3	18.2	6.3	1.3	0.2
After purification	>99.9	<0.1	<0.1	<0.1	<0.1	<0.1