Ca3(PO4)2

from The Collaborative International Dictionary of English v.0.48

Superphosphate \Su`per*phos"phate\, n. (Chem.)
   An acid phosphate.
   [1913 Webster]

   {Superphosphate of lime} (Com. Chem.), a fertilizer obtained
      by trating bone dust, bone black, or phosphorite with
      sulphuric acid, whereby the insoluble neutral calcium
      phosphate, {Ca3(PO4)2}, is changed to the primary or acid
      calcium phosphate {Ca(H2PO4)2}, which is soluble and
      therefore available for the soil.
      [1913 Webster]