FRANK C. HAWTHORNE, MITSUYOSHI KIMATA, PETR CERNY, NEIL BALL
Department of Geological Sciences. University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada
GEORGE R. ROSSMAN
Division of Geological and Planetary Sciences, California Institute of Technology
Pasadena, California 91125-2500, U.S.A.
JOEL D. GRICE
Mineral Sciences Division, Canadian Museum of Nature
Ottawa, Ontario K1P 6P4, Canada
Milarite has a beryllo-alumino-silicate framework structure. The framework is a four-connected three-dimensional net, one of a series of seven simple nets with prominent double rings of tetrahedra previously derived by Hawthorne and Smith (1986). There are 15 minerals with this net as the basic tetrahedral arrangement; in general, Si is strongly to completely ordered into the ring tetrahedra. and the other tetrahedral cations (Li, Be, B, Al. Mg, Fe2+, Fe3+, Mn2+, and Zn) are strongly to completely ordered into the ring-linking tetrahedra. The important substitution in normal milarite is NaB + BeT2 = B + AlT2 with the amount of Be varying between approximately 1.5 and 2.5 apfu: in (Y,REE) bearing milarite, the important substitution is (Y,REE)A + BeT2 = CaA + AlT2 with the amount of Be varying up to 3.0 atoms pfu. The mean bond lengths of the polyhedra involved in these substitutions vary accordingly, and linear models of this behavior are developed. In addition, there is chemical evidence of a small amount of Al = Si substitution at the T1 site. Similar linear models are developed for the whole group of milarite type minerals.
Milarite is found in a variety of environments, from vugs in plutonic rocks through diverse pegmatites to hydrothermal ore deposits and alpine veins: a review of specific parageneses is given. Milarite is a low-temperature hydrothermal mineral crystallizing in the range of 200- 250°C at low pressures. Associated minerals suggest that milarite crystallizes from alkaline fluids with Be being transported as fluor-complexes and carbonate complexes. Other members of the milarite group occur in peralkaline rocks. meteorites, peraluminous volcanic rocks, and high-grade metamorphic rocks, as well as other geochemically more unusual rocks, and form under a wide variety of P-T conditions.