The phrase "magnesium ammonium" refers to a compound that includes both magnesium and ammonium elements. It could be a combination of magnesium and ammonium ions, or a compound formed by magnesium reacting with ammonium compounds. Full definition
The crystals can be made of magnesium ammonium phosphate (known as struvite) or they can be calcium oxalate. (catbehaviorassociates.com)
Remember that struvite stones are made of magnesium ammonium phosphate. (cat-health-guide.org)
After separation of the solid fraction, the released phosphate dissolved in the liquid fraction can be precipitated as magnesium ammonium phosphate and calcium phosphate, which in turn are directly usable as high value fertilising salts. (sciencedaily.com)