Study On Oriental Relationship Between Rutile Needle And Corundum Matrix

Rutile needles are usually encountered in corundum, parallel to three different directions at the angle of 60°/120°. Large amount of this needles will give rise to asterism in cabochon corundum. In this study, the oriental relationship between rutile and corundum and formation mechanism of rutile exsolution were explored and discussed by various analysis methods, including microscopic observations under polarized microscope, Infrared spectra, Raman spectra and digital modeling of rutile and corundum.Observations under microscope reveal various shapes of rutile, including dot-like, arrow-like, short prismatic etc. The presence and various shapes of rutile strongly indicate the distribution of Ti cations in corundum, since Ti cations are very unlikely to move at a large distance due to the strong viscosity in solid, compared to liquid. The value of indent angle（111°） seen in arrow-like and short prismatic rutiles is very close to the theoretical value（114°） of twinning angles in rutiles twinning on {101}. And the difference in extinction under reflective light also suggest the existence of twinning and that rutile needles elongate along the twinning plane. A broad band centered about 490-500cm-1 of Infrared spectra and two prominent peaks at 610cm-1 and 448cm-1 strongly confirm the needles as rutile.Proximity between crystal radii of Al and Ti enables Ti cation to enter corundum crystalline structure without apparent distortion. Stability of corundum and rutile at high temperature contributes to conserve the Ti2O3-Al2O3 solid solution. Ti may concentrate along certain crystalline plane of corundum, such as hexagonal bipyrimaid {112₂}, mainly depending on the external factors such as growing environment and the source of Ti. Structurally, similarities between {101} plane of rutile and {112₀} plane of corundum facilitate the replacement, making the replacement of Al by Ti possible in the following order: "Ti4+- V- Ti4+- V- Ti4+- V" replacing "Al3+- Al3+- V- Al3+- Al3+-V"When temperature becomes favorable, Ti cations leave corundum matrix and precipitate as rutile needles. Hence, based on the above evidence, the rutile-corundum orientation relations can be summed as follows: {101}r // {112_<sub>0}c, {010}r //{0001}cMoreover, the existence of rutile inclusion interferes the continuation of corundum. Owing to the arrangement of these needles, the force between basal plane {0001} is decreased and the force between {101-1} is increased. Generally, the presence of rutile enhance the durability of corundum.