http://phys.org/news/2013-04-mineral...r-deposit.html

Magnesium pyroxene (MgSiO3), calcium pyroxene (CASiO3), iron pyroxene
(FeSiO3)
are water-insoluble salts; derived from water soluble metasilicic acid
(H2SiO3) which
combines with water soluble cations (Mg,Ca,Fe) and precipitates to the
seafloor.
John Curtis