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Re-definition, nomenclature and crystal-chemistry of the hellandite group. (Articolo in rivista)

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Re-definition, nomenclature and crystal-chemistry of the hellandite group. (Articolo in rivista) (literal)

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OBERTI R.; DELLA VENTURA G.; OTTOLINI L.; HAWTHORNE F.C.; BONAZZI P. (2002)
Re-definition, nomenclature and crystal-chemistry of the hellandite group.
in The American mineralogist
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This work benefited from the simultaneous development of new analytical procedures for SIMS analysis of complex mineral matrices, which has been done at the CNR-IGG, sezione di Pavia. Minerals with complex structures, where elements with atomic numbers from 1 to 92 can occur and distribute into multiple sites can now be characterised with a high level of accuracy. As a consequence, a redefinition of the hellandite group and the new nomenclature rules have been proposed and approved by the IMA Commission on New Minerals and Mineral names. Interestingly, the understanding of hellandite crystal-chemistry allowed the authors to propose the use of this structure (with properly designed synthetic compositions) for radioactive waste disposal. The American Mineralogist is the journal with the highest impact factor in mineralogy. (literal)

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Detailed X-ray single-crystal refinements and complete (SIMS + EMPA) microprobe chemical analyses of a series of non-metamict samples of hellandite recently found in Latium province (Italy), together with a critical re-evaluation of the existing data on hellandite, allow a better understanding of the crystal-chemistry of the hellandite group. Relative to the previously determined crystal-structure arrangement, a new tetrahedrally coordinated site has been detected; this may be fully, or in part, occupied by Li and Be. These cations occur at the center of the tetrahedral cavity where the H atom, which is bonded to the O5 oxygen, protrudes; thus the H content in hellandite is constrained to values equal to or lower than 2 - (Li + Be + F). A new general formula for hellandite-group minerals could therefore be proposed: X4 Y2 Z T2 [B4 Si4 O22] W2, where X = Na, Ca, Y, LREE3+ at the [8]-coordinated M3 and M4 sites; Y = Ca, Y, HREE3+, Th4+, U4+ at the [8]-coordinated M2 site; Z = Al, Mn3+, Fe3+, Ti4+ at the octahedral M1 site; T = Vacancy, Li, Be at the new tetrahedrally coordinated site; W = OH, F, O2- at the O5 site. Eight root end-member compositions are identified: four of these correspond to known compositions: hellandite-(REE), tadzhikite-(REE), and two correspond to new minerals, mottanaite-(Ce) and ciprianiite. A sequence of incorporation based on crystal-chemical arguments is provided and allows evaluation of the site populations for nomenclature purposes. (literal)

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R. Oberti a), G. Della Ventura b), L. Ottolini a), F.C. Hawthorne c), P. Bonazzi d). a) CNR-IGG - Sezione di Pavia, Via Ferrata 1, I-27100 Pavia, b) Dipartimento di Scienze della Terra, Università della Calabria, I-87030 Arcavacata di Rende (CS), c) Department of Geological Sciences, University of Manitoba, Winnipeg, MB, Canada, R3T 2N2, d) Dipartimento di Scienze della Terra, Università di Firenze, I-50121 Firenze (literal)

Titolo

Re-definition, nomenclature and crystal-chemistry of the hellandite group. (literal)

Abstract

Detailed X-ray single-crystal refinements and complete (SIMS + EMPA) microprobe chemical analyses of a series of non-metamict samples of hellandite recently found in Latium province (Italy), together with a critical re-evaluation of the existing data on hellandite, allow a better understanding of the crystal-chemistry of the hellandite group. Relative to the previously determined crystal-structure arrangement, a new tetrahedrally coordinated site has been detected; this may be fully, or in part, occupied by Li and Be. These cations occur at the center of the tetrahedral cavity where the H atom, which is bonded to the O5 oxygen, protrudes; thus the H content in hellandite is constrained to values equal to or lower than 2 - (Li + Be + F). A new general formula for hellandite-group minerals could therefore be proposed: X4 Y2 Z T2 [B4 Si4 O22] W2, where X = Na, Ca, Y, LREE3+ at the [8]-coordinated M3 and M4 sites; Y = Ca, Y, HREE3+, Th4+, U4+ at the [8]-coordinated M2 site; Z = Al, Mn3+, Fe3+, Ti4+ at the octahedral M1 site; T = Vacancy, Li, Be at the new tetrahedrally coordinated site; W = OH, F, O2- at the O5 site. Eight root end-member compositions are identified: four of these correspond to known compositions: hellandite-(REE), tadzhikite-(REE), and two correspond to new minerals, mottanaite-(Ce) and ciprianiite. A sequence of incorporation based on crystal-chemical arguments is provided and allows evaluation of the site populations for nomenclature purposes. (literal)

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