Staghorn calculi are upper urinary tract kidney stones which involve the renal pelvis and involve at least two of the calyces. They are formed in the setting of an alkaline pH and urease-producing micro-organisms, which leads to the formation of "triple phosphate" or "struvite" stones comprised of magnesium ammonium phosphate crystals (MgNH4PO4•6H2O) mixed with carbonate apatite (Ca10 (PO4) 6•CO3) crystals. Urea-producing bacteria include Proteus, Ureaplasma urealyticum, Klebsiella, Staph species, and Pseudomonas; E. coli does not produce struvite stones.
A few interesting historical tidbits: the name "struvite" was named after the Russian naturalist and minerologist Baron von Struve. Also, urease was the first-ever enzyme which was purified; it earned chemist James Sumner the 1946 Nobel Prize in Chemistry.