Hydrothermal investigation of the bulk compositions Ca 19 Mg 4 Al 10 Si 17 O 68 (OH) 8 and Ca 19 Mg 3 Al 10 Si 18 O 68 (OH) 10 (+ or -4SiO 2 + H 2 O) has been conducted by using conventional cold seal and internally heated pressure apparatus at 320 degrees to 900 degrees C and 1 to 2 kb P fluid . Magnesium idocrase was readily crystallized from oxide mixes at 600 degrees C. The synthetic material is extremely fine grained and is nearly isotropic with an average refractive index of 1.698. The stability fields of idocrase and idocrase + quartz were reversibly determined using the synthetic phases as starting materials with run durations of between 6 and 45 days. The hightemperature decomposition of Mg idocrase is experimentally determined to be a continuous reaction in which the coexistence of idocrase and melilite + monticellite occurs over the temperature range of approximately 722 degrees C to 825 degrees C + or - 12 degrees C at 1 kb and 722 degrees C to 862 degrees C + or - 12 degrees C at 2 kb P fluid . Cell dimensions of the idocrase grown at 600 degrees C and 1 kb P fluid are a = 15.559(4)Aa and c = 11.805(4)Aa, and those of the idocrase grown at 800 degrees C and 1 kb P fluid are a = 15.546(4)Aa and c = 11.802(4)Aa. This small but statistically significant difference in a may indicate a minor difference in composition of the high- and low-temperature idocrases according to the above reaction. Above this temperature range, idocrase becomes unstable according to the reaction: idocrase = wollastonite + melilite + monticellite + H 2 O.The thermal stability of idocrase is greatly shifted toward lower temperatures in the presence of quartz with the dehydration reaction defined by: idocrase + 4 quartz = 5 grossular + 3 diopside + wollastonite + 4 H 2 O. This reaction was experimentally determined to occur at 393 degrees C + or - 10 degrees C (1 kb P fluid ) and 405 degrees C + 5 degrees C (2 kb P fluid . Using the present and previous experimentally determined results and ehemographie relationships in the system CaAl 2 Si 2 O 8 -CaMgSi 2 O 6 -CaSiO 3 -SiO 2 -H 2 O-CO 2 , isobaric T-X (sub CO 2 ) diagrams were constructed which indicate that idoerase in the presence of quartz should be stable only at temperatures lower than 400 degrees C and at X (sub CO 2 ) less than 0.015 at 2 kb P fluid . The experimentally determined P-T conditions for the stabilities of idoerase and idoerase + quartz are consistent with those deduced from natural parageneses of idoerase in skarns and regionally metamorphosed rocks. It is suggested that the growth and stability of idoerase implies a fluid phase with exceedingly low X (sub CO 2 ) .