(1) The outermost layer of the calcium atom loses two electrons to form ions with two positive charges, and the calcium ions are expressed as: Ca2+;

(2) According to the label, the pH value of mineral water is 7.5 ~ 8.5, so mineral water is alkaline;

(3) The mass fraction of silicon in metasilicic acid = 2878× 100% ≈ 35.9%.

So the answer is: (1) Ca2+; (2) alkaline; (3)35.9%.