Calcium Phosphate Kidney Stone
Calcium Phosphate crystals form when calcium atoms combine with phosphoric acid instead of oxalic acid.
Phosphoric Acid is simply a phosphorus atom with 4 oxygen atoms bound to it. The bond between one of the oxygen atoms cannot provide any charge with which to bond to calcium atoms in order to form a crystal. The other three oxygen atoms have ordinary bonds.
One of the three negatively charged oxygens never has a hydrogen on it in urine but only in exceedingly acidic solutions. A second charged oxygen is always occupied by a hydrogen atom in urine. This makes the third oxygen, variably occupied by a hydrogen in urine, a tie breaker.
In a urine of average normal acidity (pH around 6), most of the tie breaker oxygens have their hydrogen leaving the phosphate ion only one negative charge. Not enough to make a crystal. But, when the urine is abnormally alkaline (pH above 6.3 or 6.5), the variable oxygen becomes charged so the ion has two negative charges that can combine with calcium to make crystals. For this reason the calcium phosphate kidney stone tends to occur in people who produce a more alkaline urine than those who produce calcium oxalate kidney stones.