Idocrase: Synthesis, phase relations and crystal chemistry¹

Phase syntheses of Mg-idocrase indicate that the mineral's compositional range may be written as Ca₁₈(Ca_2-xMg_x)Mg₂(Mg_2-yAl_y)Al₈(Al_ySi_2-y)Si₁₆O₆₈(OH)₈ where 0.0<x<1.5 and 0.0<y<1.5. Mg²⁺ ions in 6-coordinated sites seem to be essential for idocrase formation, unlike the case for grossular garnet.

Pressure-temperature stability fields under pure water in the sodium-free system are wide. With water pressures of 2 kbar, idocrase is stable from 4S0°C to 720°C,

The mineral associations of idocrase (in contact metamorphism) can be ideally illustrated as follows: 2 grossular+2 diopside+wollastonite+calcite+2 water$⇌$iidocrase+2 quartz+CO₂. The presence of three mineral zones (garnet, idocrase and monticellite) at Crestmore, California, is best explained in terms of compositional changes, rather than pressure-temperature variation.

Unit cell dimensions of Mg²⁺, Ni²⁺, Co²⁺ and Cu²⁺ analogs and those of Mg-Idocrase which also contained Fe³⁺, Mn²⁺ or Ti⁴⁺ were obtained from computer-refined X-ray powder diffractometer data. Mg-idocrase with the formula Ca_18.6Mg_3.2Al_10.1Si_17.6O₆₈(OH)₈ (electron probe analysis) has unit cell dimensions:a=15.600 ± 0.001 Å; c=11.829 ± 0.002 Å.

Qualitative kinetics studies showed that the rate of growth and nucleation of idocrase are strongly dependent on temperature and pressure. The use of different gel compositions, and of various starting materials, demonstrates the diversity of kinetic processes in idocrase crystallization.