Trivalent iron ions substituting for Al3+ are present in many beryls and are abundant in dark red and dark blue beryl. These ions do not contribute any color to the crystals. Dark blue beryl also contains octahedral Fe2+ ions, which are involved in giving the crystal the blue color. The colors of dark red and dark blue beryls are very stable. Tetrahedral iron ions are practically absent in these beryls, but are present in other beryls which change their color upon irradiation and heating. Their colors depend more on the radiation and temperature history of the crystals than on chemistry.
Trivalent iron ions substituting for Si4+ also do not contribute color. Rare observations by EPR (electron paramagnetic resonance) indicate that the charge difference can be compensated either by protons or by alkali ions in the beryl channels. It is shown that about 1% of the Fe3+ ions in dark red beryl substitute for Si4+.
The changing colors of iron-containing beryls have been interpreted in many different ways, but it is likely that they are due to ions substituting for Be2+. The present study suggests that the iron ions do not substitute isomorphically for Be, but that they enter a distorted tetrahedron which consists of one O(1) oxygen and three O(2) oxygens. The center of this T(3) tetrahedron is at (0.432, 0.344, 0.167) in the beryl structure and the distance to the four oxygen ions is 1.84 Å, compared to 1.65 Å in the Be tetrahedron, thus providing more space for the much larger iron ions. This site is so close to the Be site that both sites cannot be simultaneously occupied. Beryl crystals with Fe2+ ions in T(3) are colorless. Irradiation oxidizes these Fe2+ ions to Fe3+, which gives the beryl a yellow color. The aquamarine color is caused by pairs of Fe2+ and Fe3+ ions in neighboring T(3) tetrahedra. When both configurations are present, the crystal assumes a shade of green, depending on their relative proportion. The electrons released by the irradiation are trapped at sites which are probably associated with water molecules in the beryl channels. These electrons leave the traps upon heating and reduce the Fe3+ ions in the T(3) tetrahedra. The yellow color vanishes and the beryl becomes aquamarine blue. More electrons are released at higher temperatures and reduce both ions of the pairs, leaving the beryl colorless. The oxidation of the iron ions at even higher temperatures is connected with the release of water from the beryl crystal.
A number of small EPR signals in addition to the large signal from octahedral Fe3+ ions in iron-containing beryl crystals are investigated. Many arise from exchanged-coupled Fe3+ ion pairs, and one is related to the yellow color.