Apatite or calcium phosphorous stones are the least common type of stone I see in practice. These rare stones form from the same risk factors responsible for calcium oxalate stones. These include increased amounts of calcium and/or phosphate in the urine. The one big difference is pH. While calcium oxalate stones precipitate out of acidic urine, calcium phosphate stones form in urine with an increased or basic pH. This increased pH may be caused by urease producing bacterial infections.
The same breeds that show an increased incidence of calcium oxalate are also prone to calcium phosphate stones. In addition to miniature schnauzers, Yorkshire terriers, shih tzu and miniature poodles, cocker spaniels are also on this list. Since females are more prone to urinary tract infections, they tend to have more calcium phosphate stones.
Treating calcium phosphate stones is tough because they often form in the kidney. Since surgical extraction will damage the kidney, I usually recommend lithotripsy for this condition. More information on lithotripsy may be found at my blog post; Removing Urinary Stones With Lithotripsy in Dogs. Once the stones are gone, prevention is based on keeping the urine dilute to flush the crystals out before they can form stones. Giving urinary acidifiers is not recommended as it might cause the formation of calcium oxalate stones.
-Shell, Linda. Urolithiasis, Calcium Phosphate. Canine Partner, VIN 3/1/2010.