The crystar srructures of six natural milarites (A2B2C[T23T112O30](H2O)x, p6/mcc, a ~ 10.40, c ~ 13.80 Å, Z = 2) covering the range of known compositions have been refined to R indices of ~3% using MoKα X-ray intensity data. Previously proposed split-atom models for the A and B sites are confirmed. Polarized spectroscopy in the infrared region shows H2O to be the only significant H-bearing group. Band polarization characteristics show the H-H vector to be ⊥c and the H2O plane to be ||c; this corresponds to the type II H2O found in the related structures of beryl and cordierite. There is a quantitative correlation between band intensity and H2O content, and the observed molar absorptivities are similar to those measured in cordierite.
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 Å 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.