soft, dark brownish-black masses with a confused fibrous structure. Associated with chalcophanite ) and secondary calcite () crystals in franklinite () - willemite () ore. At Saipan in the Mariana Islands, in the Pacific Ocean, todorokite has been found with the same mode of aggregation and color as that from Charco Redondo in Cuba. It is relatively brittle and hard because it is intimately admixed with finely divided silica, amounting in some cases to as much as 50% of the sample. The content of magnesium is greater than that of barium or calcium. See also Manganese oxide References Magnesium minerals Calcium minerals Barium minerals Potassium minerals Sodium minerals Manganese minerals Oxide minerals Monoclinic minerals Minerals in space group 11