Introduction

Diamond anvil cells (DAC) have been used in a wide variety of high-pressure studies both at room and high temperature. Under external heating, oxidation of diamonds and the metal heaters require protection of the inner cell assembly by an inert or reducing gas atmosphere. Graphitization of diamond sets in above 900 - 1000 C with increasing intensity. Furthermore, the strength of necessary supporting material (e.g. tungsten carbide) is considerably reduced at temperatures above 800 C, thus limiting the reachable pressure in an externally heated DAC at very high temperatures to at least 2.5 GPa [1]. Instead of diamond, we used cubic garnet single crystals (YAG) as transparent anvils in a high-pressure cell. Among the various materials of the garnet structure, Y₃Al₅O₁₂ (YAG) has the highest melting point (T_m = 2273 K) and hence is likely to be the most resistant to plastic deformation at high temperatures. Garnets in general appear to have a creep strength higher by a factor of 3-10 than other oxides, compared at the same homologous temperatures T/T_m  [2]. From these findings, we expect that a garnet anvil cell (GAC) can operate even up to 1500 C under external heating, i.e. under nearly hydrothermal conditions compared to laser-heated cells. We have performed experiments with a garnet anvil cell up to 800 C without any special precautions, and up to 1200 C under protection gas. The reachable pressure depends on the size and shape of the active anvil face, with coned (diamond-like) garnet cylinders with 5 mm outer diameter and a 500 mm anvil face we were able to get 6.0 GPa without any problems. With a gasket-free preparation, we used a special cell with shear mechanism to study the brittle-ductile transition in calcite by deforming small calcite cylinders (2 mm diameter and 30 mm thickness) in the GAC under visual control. [3]