Manganiakasakaite-(La) and Ferriakasakaite-(Ce), Two New Epidote Supergroup Minerals from Piedmont, Italy

Abstract

Two new monoclinic (P2 1 /m) epidote supergroup minerals manganiakasakaite-(La) and ferriakasakaite-(Ce) were found in the small Mn ore deposit of Monte Maniglia, Bellino, Varaita Valley, Cuneo Province, Piedmont, Italy. Manganiakasakaite-(La) occurs as subhedral grains embedded in pyroxmangite. Its empirical formula is A(1) (Ca 0.62 Mn 2+ 0.38 ) A(2) (La 0.52 Nd 0.08 Pr 0.07 Ce 0.07 Y 0.01 Ca 0.25 ) M(1) (Mn 3+ 0.52 Fe 3+ 0.28 Al 0.18 V 3+ 0.01 ) M(2) Al 1.00 M(3) (Mn 2+ 0.60 Mn 3+ 0.27 Mg 0.13 ) T(1−3) (Si 2.99 Al 0.01 ) O 12 (OH), corresponding to the end-member formula CaLaMn 3+ AlMn 2+ (Si 2 O 7 )(SiO 4 )O(OH). Unit-cell parameters are a = 8.9057(10), b = 5.7294(6), c = 10.1134(11) Å, β = 113.713(5) • , V = 472.46(9) Å 3 , Z = 2. The crystal structure of manganiakasakaite-(La) was refined to a final R 1 = 0.0262 for 2119 reflections with F o > 4σ(F o ) and 125 refined parameters. Ferriakasakaite-(Ce) occurs as small homogeneous domains within strongly inhomogeneous prismatic crystals, where other epidote supergroup minerals coexist [manganiandrosite-(Ce), "androsite-(Ce)", and epidote]. Associated minerals are calcite and hematite. Its empirical formula is A(1) (Ca 0.64 Mn 2+ 0.36 ) A(2) (Ce 0.37 La 0.17 Nd 0.06 Pr 0.03 Ca 0.35 0.02 ) M(1) (Fe 3+ 0.61 Al 0.39 ) M(2) Al 1.00 M(3) (Mn 2+ 0.64 Mn 3+ 0.33 Fe 3+ 0.02 Mg 0.01 ) T(1−3) Si 3.01 O 12 (OH), the end-member formula being CaCeFe 3+ AlMn 2+ (Si 2 O 7 )(SiO 4 )O(OH). Unit-cell parameters are a= 8.9033(3), b = 5.7066(2), c = 10.1363(3) Å, β = 114.222(2) • , V = 469.66(3) Å 3 , Z = 2. The crystal structure of ferriakasakaite-(Ce) was refined to a final R 1 = 0.0196 for 1960 unique reflections with F o > 4σ(F o ) and 124 refined parameters. Minerals 2019, 9, 353 2 of 15and 2 to the dollaseite group. Two additional minerals stand alone: zoisite, the unique example of epidote with orthorhombic symmetry, and åskagenite-(Nd), the unique example of epidote having a divalent anion dominant at the O(10) site. We herewith describe manganiakasakaite-(La) and ferriakasakaite-(Ce), two new minerals belonging to the allanite group. In this group, a trivalent cation, typically a REE, is dominant at the A(2) site, and a divalent cation is dominant at the M(3) site. According to the general guidelines for the nomenclature of epidotes [1], each distinct combination of dominant cations at the A(1) and M(3) sites deserves a proper root-name. In particular, the root-name "akasakaite" (after Masahide Akasaka, professor of mineralogy at the Shimane University) corresponds to the pair A(1) = Ca and M(3) = Mn 2+ [2]. Then, an adjectival prefix is appended to the root-name for the dominant cation at the M(1) (other than Al), and a Levinson suffix [3] is appended to the root-name for the dominant REE at the A(2) site. The names of the two new minerals are in keeping with the above guidelines, as they represent chemical variants of the mineral ferriakasakaite-(La), which was first described from Japan [2]. Both manganiakasakaite-(La) and ferriakasakaite-(Ce) were found in the Mn ore d...

Ferriandrosite-(Ce), a new member of the epidote supergroup from Betliar, Slovakia

“…The partial replacement of Mn 2+ by Ca 2+ at the A 1 site in androsite-series minerals is in keeping with the possible solid solution between members of this series and those of the akasakaite series, as suggested, for instance, by Biagioni et al (2019), who described strongly zoned grains containing ferriakasakaite-(Ce), manganiandrosite-(Ce), and the potential ‘androsite-(Ce)’ end-member in samples from the Mn mineralisation of Monte Maniglia, Piedmont, Italy.…”

“…137.4(6) and 137.4(8)°, respectively, and slightly smaller than that given by Nagashima et al (2015) for ferriandrosite-(La), i.e. 139.4(2)°; however, it is smaller than those reported by Biagioni et al (2019) in manganiakasakaite-(La) and ferriakasakaite-(Ce), i.e. 140.88(19) and 144.47( 14)°, respectively.…”

“…If one defines the difference between the bond distances A1-O6 and A1-O5 (with O6 and O5 being the seventh and sixth ligands of A1, respectively) as δ 7-6 , a direct relation between the Mn 2+ content at A1 and the δ 7-6 value can be observed (e.g. Bonazzi et al, 1996;Nagashima et al, 2010Nagashima et al, , 2013Nagashima et al, , 2015Biagioni et al, 2019). In ferriandrosite-(Ce), this difference is 0.508 Å, to be compared with the theoretical value of 0.502 Å calculated on the basis of the regression line given by Bonazzi et al (1996).…”

“…The crystal structure of ferriandrosite-(Ce) was refined using Shelxl-2018 (Sheldrick, 2015) starting from the atomic coordinates of ferriakasakaite-(Ce) (Biagioni et al , 2019). The position of the H atoms was located through the difference-Fourier map.…”

“…Data were corrected for Lorentz-polarisation, absorption, and background using the package of software Apex4 (Bruker AXS, 2022). Unit-cell parameters were refined on the basis of the XYZ centroids of 9895 reflections above 20 σ I with 7.898°< 2θ < 66.315°.Ferriandrosite-(Ce) is monoclinic, space group P2 1 /m (#11), with the following unit-cell parameters: a = 8.8483(4) Å, b = 5.7307(3) Å c = 10.0314(5) Å, β = 113.3659(15)°, V = 10.0314(5) Å 3 and Z = 2.The crystal structure of ferriandrosite-(Ce) was refined using Shelxl-2018(Sheldrick, 2015) starting from the atomic coordinates of ferriakasakaite-(Ce)(Biagioni et al, 2019). The position of the H atoms was located through the difference-Fourier map.…”