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The crystal chemistry of the milarite-group minerals

Hawthorne, Frank C. and Kimata, Mitsuyoshi and Černý, Petr and Ball, Neil and Rossman, George R. and Grice, Joel D. (1991) The crystal chemistry of the milarite-group minerals. American Mineralogist, 76 (11-12). pp. 1836-1856. ISSN 0003-004X. https://resolver.caltech.edu/CaltechAUTHORS:20140304-114047933

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Abstract

The crystal sructures of six natural milarites (A_2B_2C[T2_3Tl_(12)O_(30)] (H_2O)_(x,)) P6/mcc, ɑ ~ 10.40, c ~ 13.80 Å, Z=2) covering the range of known compositions have been refined to R indices of ~ 3% using MoKɑ X-ray intensity data. Previously proposed split-atom models for the A and B sites are confirmed. Polarized spectroscopy in the infrared region shows H_2O to be the only significant H-bearing group. Band polarization characteristics show the H-H vector to be ⊥c and the H_2O plane to be ‖c; this corresponds to the type II H_2O found in the related structures of beryl and cordierite. There is a quantitative correlation between band intensity and H_2O 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 l5 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, Fe^(2+), Fe^(3+), Mn^(2+), and Zn) are strongly to completely ordered into the ring-linking tetrahedra. The important substitution in normal milarite is Na^B + Be^(T2) = □^B + Al^(T2) 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 + Be^(T2) = Ca^A + Al^(T2) 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 Tl 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 parageneseiss given. Milarite is a low-temperature hydrothermal mineral crystallizing in the range of 200-250 ºC at low pressures. Associated minerals suggest hat 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.


Item Type:Article
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http://www.minsocam.org/msa/collectors_corner/amtoc/toc1991.htmPublisherIssue table of contents
ORCID:
AuthorORCID
Rossman, George R.0000-0002-4571-6884
Additional Information:© 1991 Mineralogical Society of America. Manuscript received October 1, 1990. Manuscript accepted July 13, 1991. We would like to thank J.L. Jambor (CANMET, Ottawa) for the Strange Lake sample, R .V. Gaines for the Jaguaraçú samples, and John S. White for the samples from the NMNH-Smithsonian Institution (Washington). W.W. Gould assisted with the early stages of the spectroscopic work. Financial assistance was provided by the Natural Sciences and Engineering Research Council of Canada (operating grants to F.C.H. and P.C.; a major equipment grant, a University Research Fellowship, and an Infrastructure grant to F.C.H.; and a major installation grant to P.C.) and the NSF grant EAR-8618200 to G.R.R.
Funders:
Funding AgencyGrant Number
Natural Sciences and Engineering Research Council of Canada (NSERC)UNSPECIFIED
NSFEAR-8618200
Issue or Number:11-12
Record Number:CaltechAUTHORS:20140304-114047933
Persistent URL:https://resolver.caltech.edu/CaltechAUTHORS:20140304-114047933
Official Citation:The crystal chemistry of the milarite-group minerals FRANK C. HAWTHORNE, MITSUYOSHI KIMATA, PETR CERNY, NEIL BALL, GEORGE R. ROSSMAN, JOEL D. GRICE 1836-1856
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:44125
Collection:CaltechAUTHORS
Deposited By: Ruth Sustaita
Deposited On:04 Mar 2014 22:04
Last Modified:03 Oct 2019 06:14

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