Table 2.

Chemical composition of the surface layers.

Element	Inner layer		Outer layer
	wt.%	at.%	wt.%	at.%
O	32.6	55.7	52.4	66.4
Na	2.2	2.6	1.9	2.2
Al	3.7	3.8	–	–
Si	7.1	6.9	41.5	29.7
Ti	52.4	29.9	3.9	1.5
V	1.8	0.9	0.2	0.1