The crystal-chemistry of holmquistites: Ferroholmquistite from Greenbushes (Western Australia) and hints for compositional constraints in BLi amphiboles
-
Fernando Cámara
and
Roberta Oberti
Abstract
A systematic crystal-chemical investigation of orthorhombic holmquistites has been done to determine the reasons for their- limited compositional variations. Structural constraints to the relative stability of BLi amphiboles are also suggested by the occurrence of ferro- and feni-ferroclinoholmquistites, and the lack of clinohohnquistite. Detailed crystal-chemical analysis shows: (1) a remarkable constancy in composition, both in terms of charge arrangement and of limited homovalent M1,3(Mg-1Fe2+), M2(Al-1Fe3+), and 03(OH-1F) exchanges, (2) a remarkable constancy in the unit-cell dimensions, with the Fe3+ content at the M2 site being the only factor affecting the b edge; (3) complete ordering of Li at the M4 site, in contrast with the common partitioning between the M4 and M3 sites in clinoamphiboles, which however couples with partial A-site occupancy; (4) complete ordering of bivalent cations at the M2 site; (5) an inverse relationship between the Fe2+ and the Fe3+ contents, which is interpreted as a way to keep the size of the octahedral strip constant; (6) a strong distortion of the octahedral sites, both in terms of angular variance and quadratic elongation.
A⃞BLi2c(Mg3Al2)TSi8O22(OH)2 is the amphibole composition composed of the smallest possible structural moduli. Crystallization in Puma symmetry, where the two double-chains of tetrahedra can assume different conformations, is probably required by the need to obtain a more suitable [5 + 1]- coordination for BLi, and to shrink the cation-cation distances. This arrangement does not allow for extensive incorporation of larger homovalent substituents, which are hosted via mechanisms implying distortion of the octahedral sites.
During this work, a sample with Fe2+ slightly but significantly higher than Mg was char acterized, and then recognized as a mineral species by the IMA-CNMMN (2004-030). Holotype ferroholmquistite has a = 18.287 (1), b = 17.680 (1), and c = 5.278 (1) Å, and V= 1706.6(1) A3. Its crystal-chemical formula isAK0.01Na0.01B(Li1.88Mg0.08Na0.03Fe2+0.01)c(Al1.89Fe2+1.07Mg1.39Mn2+0.02)TSi8.00O22(OH1.97F0.03). Ferroholmquistite occurs as elongated black to bluish-violet prismatic crystals; it is biaxial negative, with a = 1.628, (3 = 1.646, and y = 1.651 (X = 589 nm), 2VX (calc.) = 55.1°. It is weakly pleochroic, with a = colorless, (3 = pale violet-blue, and y = blue to deep violet; the calculated density is 3.145 g/cm3. The holotype specimen belongs to the mineral collection of Renato and Adriana Pagano (Italy), and comes from the Greenbushes pegmatite (Western Australia). The analyzed sample has been deposited at the Museum of the Dipartimento di Scienze della Terra, Università di Pavia (Italy) under the code 2004-01.
© 2015 by Walter de Gruyter Berlin/Boston
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Articles in the same Issue
- A synthesis of systematic mineralogy
- Oxidation/alteration of pentlandite and pyrrhotite surfaces at pH 9.3: Part 1. Assignment of XPS spectra and chemical trends
- Oxidation of pentlandite and pyrrhotite surfaces at pH 9.3: Part 2. Effect of xanthates and dissolved oxygen
- X-ray Rietveld refinement and FTIR spectra of synthetic (Si,Ge)-richterites
- Influence of crystal chemistry on ideal plastic shear anisotropy in forsterite: First principle calculations
- High-temperature Mössbauer study of Fe-substituted mullite
- Pressure and temperature-dependence of water solubility in Fe-free wadsleyite
- Quartz exsolution in clinopyroxene is not proof of ultrahigh pressures: Evidence from eclogites from the Eastern Blue Ridge, Southern Appalachians, U.S.A.
- The visible and infrared spectral properties of jarosite and alunite
- Wakabayashilite, [(As,Sb)6S9][As4S5]: Crystal structure, pseudosymmetry, twinning, and revised chemical formula
- Compression of albite, NaAlSi3O8
- Effect of polymorphic transition on oxygen isotope fractionation between aragonite, calcite, and water: A low-temperature experimental study
- Effect of current density on the electron microprobe analysis of alkali aluminosilicate glasses
- A high-pressure polytypic transformation in type-I chlorite
- Experimental determination of crystal growth rates in highly supercooled aluminosilicate liquids: Implications for rate-controlling processes
- P-V equation of State, thermal expansion, and P-T stability of synthetic zincochromite (ZnCr2O4 spinel)
- Niksergievite, [Ba1.33Ca0.67Al(CO3)(OH)4][Al2(AlSi3O10)(OH)2]·nH2O, a new phyllosilicate related to the surite-ferrisurite series
- The crystal-chemistry of holmquistites: Ferroholmquistite from Greenbushes (Western Australia) and hints for compositional constraints in BLi amphiboles
- Dissakisite-(La) from the Ulten zone peridotite (Italian Eastern Alps): A new end-member of the epidote group
- Mazzite-Na, a new zeolite from Boron, California: Its description and crystal structure
- Low-temperature illitization of smectite in the late eocene and early oligocene of the Isle of Wight (Hampshire basin), U.K.
- First reported sedimentary occurrence of berlinite (AlPO4) in phosphate-bearing sediments from Cioclovina Cave, RomaniaComment
- Optical and near infrared spectra of ringwoodite to 21.5 GPa: Implications for radiative heat transport in the mantle
- Letter. Observation of nano-clustered calcite growth via a transient phase mediated by organic polyanions: A close match for biomineralization
- Letter. Aluminum coordination and the densification of high-pressure aluminosilicate glasses
- Letter. Crystal chemistry of a high-pressure C2/c clinopyroxene with six-coordinated silicon
