The powder in this capsule becomes black. What reaction is this?

Calcium Carbonate 250mg Calcium Bisglycinate 17.9% 139, 665mg Chromium Piccolinat 12% 0, 75mg Potassium Iodide 0, 065mg Tri Potassium citrate 138, 89mg Copper BISGLYCINATE 18.4% Soluble 2, 04mg Manganese Bisglycinate (Mn 17.2%) 14, 55mg Sodium Molybdate 0.2% 12, 5mg Selenomethionine 5000MCG/G 10mg Zinc Mono L-Methionine 56, 25mg Vitamin K2 MK-7 MenaQ7 0.2% 5mg Vitamin C, Ascorbic Acid 90mg Silicon Dioxide 50mg Magnesium Stearate Frutarom 15mg