MgO.SiO2

from The Collaborative International Dictionary of English v.0.48

Unisilicate \U`ni*sil"i*cate\, n. [Uni- + silicate.] (Min.)
   A salt of orthosilicic acid, {H4SiO4}; -- so called because
   the ratio of the oxygen atoms united to the basic metals and
   silicon respectively is 1:1; for example, {Mg2SiO4} or
   2{MgO.SiO2}.
   [1913 Webster]