Ice Ic  Ice Ic (pronounced "ice one c" or "ice icy") is a metastable cubic crystalline variant of ice. H. König was the first to identify and deduce the structure of ice Ic. The oxygen atoms in ice Ic are arranged in a diamond structure and is extremely similar to ice Ih having nearly identical densities and the same lattice constant along the hexagonal puckered-planes. It forms at temperatures between upon cooling, and can exist up to upon warming, when it transforms into ice Ih. Apart from forming from supercooled water, ice Ic has also been reported to form from amorphous ice as well as from the high pressure ices II, III and V. It can form in and is occasionally present in the upper atmosphere and is believed to be responsible for the observation of Scheiner's halo, a rare ring that occurs near 28 degrees from the Sun or the Moon. Ordinary water ice is known as ice Ih (in the Bridgman nomenclature). Different types of ice, from ice II to ice XVI, have been created in the laboratory at different temperatures and pressures. But there are doubts whether "ice Ic" really has a cubic crystal system: Some authors claim that it is merely "stacking-disordered ice I" (“ice Isd”), and it has been dubbed the ″most faceted ice phase in a literal and a more general sense.″