Original paper

News from the hellandite group: the redefinition of mottanaite and ciprianiite and the new mineral description of ferri-mottanaite-(Ce), the first Fe3+-dominant hellandite

Oberti, Roberta; Langone, Antonio; Boiocchi, Massimo; Bernabè, Ezio; Hawthorne, Frank C.

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European Journal of Mineralogy Volume 31 Number 4 (2019), p. 799 - 806

14 references

published: Dec 12, 2019
published online: May 24, 2019
manuscript accepted: Apr 7, 2019
final revised version received: Apr 3, 2019
manuscript received: Jan 9, 2019

DOI: 10.1127/ejm/2019/0031-2858

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Abstract

The ideal formulae of two root-names of the hellandite group, i.e. mottanaite and ciprianiite, were originally given as X Ca4 Y (CeCa)2 Z Al T Be2[B4Si4O22] W O2 and X Ca4 Y [(Th,U)REE]Σ2 Z Al T □2[B4Si4O22] W (OH)2. In order to conform to the later introduced dominant-valency nomenclature rule, they have been redefined as X Ca4 Y REE2 Z Al T (Be1.5□0.5)[B4Si4O22] W O2 and X Ca4 Y [(Th,U)Ca]Σ2 Z Al T (Be1.5□0.5)Σ2[B4Si4O22] W O2, respectively (IMA-CNMNC vote 18D). Also, the mineral description is provided for the first Fe3+-dominant species of the hellandite group, ferri-mottanaite-(Ce), ideally X Ca4 Y Ce2 Z Fe3+T (Be1.5□0.5)[Si4B4O22] W O2. The type specimen was found in an ejectum collected at Tre Croci (Vetralla, Vico volcanic province, Italy). The empirical formula derived from electron microprobe and LA-ICP-MS analyses, and validated by single-crystal structure refinement is: X (Ca)4 Y (Ca0.40REE0.93 ( Th , U ) 0.54 4 + 0.13)Σ2.00 Z ( Fe 0.50 3 + Al0.23 Mn 0.17 3 + Ti 0.17 4 + )Σ1.07 T (Be1.04Li0.040.92)Σ2.00[Si4.03B3.89O22] (O1.09(OH)0.38F0.53)Σ2.00. Ferri-mottanaite-(Ce) is biaxial (–), with α = 1.748(5), β = 1.762(5), γ = 1.773(5) and 2V (meas.) = 85.9(5)°, 2V (calc.) = 82.5°. The unit-cell parameters are a = 19.0548(9), b = 4.7468(2), c = 10.2560(5) Å, β = 110.906(2)°, V = 866.58(7) Å3, Z = 2, space group P2/a. The strongest reflections in the X-ray powder pattern obtained from single-crystal data [d values (in Å), I, (hkl)] are: 2.648, 100, (013, 4 ¯ 13); 2.857, 50, (411); 1.904, 48, (023, 4 ¯ 23, 6 ¯ 21); 2.919, 44, (212); 3.086, 44, ( 4 ¯ 12); 3.246, 43, ( 4 ¯ 10); 3.453, 36, ( 2 ¯ 12); 4.745, 33, (010).

Keywords

nomenclature • new mineral • ferri-mottanaite-(Ce) • crystal structure • hellandite group • mottanaite-(Ce) • ciprianiite • borosilicate • rare-earth elements • Vico volcanic province