A 75-year-old woman presents to the clinic with sudden onset of severe right knee pain and swelling for the past 24 hours. She denies trauma but reports a low-grade fever. On examination, the knee is warm, erythematous, and holds approximately 30 mL of turbid fluid on aspiration. Plain radiographs demonstrate meniscal chondrocalcinosis. Under polarized light microscopy, the synovial fluid reveals weakly positively birefringent, rhomboid-shaped crystals. Which of the following best describes the composition of these crystals?