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

: Two new monoclinic (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 > 4 σ ( F o ) and 124 reﬁned parameters.


Introduction
Epidotes are mixed-anion silicates with general formula A 2 M 3 [T 2 O 7 ][TO 4 ](O,F)(OH,O) [1] which typically occur as rock-forming minerals in low-to medium-grade metamorphic rocks, in hydrothermal settings, or as accessory phases in magmatic rocks. As of today, the epidote supergroup includes 30 monoclinic mineral species; of them, 17 belong to the allanite group, 10 to the clinozoisite group,

Chemical Data
The examined crystals of the epidote supergroup minerals from Monte Maniglia are usually chemically zoned. Whereas the studied grain of manganiakasakaite-(La) was quite homogeneous, the grains sampled from specimen #19903, where ferriakasakaite-(Ce) was found, are inhomogeneous.
Quantitative chemical analyses for both minerals were carried out using a Superprobe JEOL JXA 8200 electron microprobe (WDS mode, 20 kV, 10 nA, 1 μm beam diameter) at the Eugen F. Stumpfl laboratory, Leoben University, Austria. Counting times were 20 s on the peak and 10 s on the left and right backgrounds. The position of the measurement of backgrounds was carefully selected to avoid overlapping among the analyzed elements. The ZAF correction method was used. Table 1 gives the results of the chemical analyses and the used standards. Fluorine was sought but not detected in manganiakasakaite-(La), whereas Y, Ti, V, and F were sought but not detected in ferriakasakaite-(Ce). No direct H2O determination was performed, owing to the scarcity of available material. Similarly, Mössbauer spectra could not be collected due to the limited amount of available samples. Thus, the Fe 2+ /Fe 3+ and Mn 2+ /Mn 3+ values were calculated in agreement with the recommendations of Armbruster et al. [1], oxidizing first Fe 2+ , then Mn 2+ , in order to account for their different redox potentials. For both manganiakasakaite-(La) and ferriakasakaite-(Ce) all Fe is in its trivalent state, in keeping with the oxidized nature of the mineral assemblages, characterized by the widespread occurrence of hematite.  Holotype material of manganiakasakaite-(La) (a) and ferriakasakaite-(Ce). In (a), manganiakasakaite-(La) occurs as a subhedral grain hosted in pyroxmangite. Field of view: 1.5 × 2 mm. Collection Museo di Storia Naturale, University of Pisa (catalogue number 19907). In (b), ferriakasakaite-(Ce), as prismatic crystals in calcite, up to 1 mm long. Collection Museo di Storia Naturale, University of Pisa (catalogue number 19903).

Chemical Data
The examined crystals of the epidote supergroup minerals from Monte Maniglia are usually chemically zoned. Whereas the studied grain of manganiakasakaite-(La) was quite homogeneous, the grains sampled from specimen #19903, where ferriakasakaite-(Ce) was found, are inhomogeneous.
Quantitative chemical analyses for both minerals were carried out using a Superprobe JEOL JXA 8200 electron microprobe (WDS mode, 20 kV, 10 nA, 1 µm beam diameter) at the Eugen F. Stumpfl laboratory, Leoben University, Austria. Counting times were 20 s on the peak and 10 s on the left and right backgrounds. The position of the measurement of backgrounds was carefully selected to avoid overlapping among the analyzed elements. The ZAF correction method was used. Table 1 gives the results of the chemical analyses and the used standards. Fluorine was sought but not detected in manganiakasakaite-(La), whereas Y, Ti, V, and F were sought but not detected in ferriakasakaite-(Ce). No direct H 2 O determination was performed, owing to the scarcity of available material. Similarly, Mössbauer spectra could not be collected due to the limited amount of available samples. Thus, the Fe 2+ /Fe 3+ and Mn 2+ /Mn 3+ values were calculated in agreement with the recommendations of Armbruster et al. [1], oxidizing first Fe 2+ , then Mn 2+ , in order to account for their different redox potentials. For both manganiakasakaite-(La) and ferriakasakaite-(Ce) all Fe is in its trivalent state, in keeping with the oxidized nature of the mineral assemblages, characterized by the widespread occurrence of hematite. As reported above, ferriakasakaite-(Ce) occurs in specimen #19903, whose grains are strongly inhomogeneous. Back-scattered electron (BSE) images and X-ray maps allowed to describe such an inhomogeneity (Figures 2 and 3). Sample #19903-1 is composed by two grains (Figure 2). The larger one is formed by two domains, whereas the smaller one is homogeneous and was used for the collection of chemical data and single-crystal X-ray diffraction study of ferriakasakaite-(Ce). The main chemical inhomogeneity is related to the Ca and La contents; indeed, Ca is depleted and La is enriched in one of the domains occurring in the larger grain and characterized by a light grey color in BSE image.  As reported above, ferriakasakaite-(Ce) occurs in specimen #19903, whose grains are strongly inhomogeneous. Back-scattered electron (BSE) images and X-ray maps allowed to describe such an inhomogeneity (Figures 2 and 3). Sample #19903-1 is composed by two grains (Figure 2). The larger one is formed by two domains, whereas the smaller one is homogeneous and was used for the collection of chemical data and single-crystal X-ray diffraction study of ferriakasakaite-(Ce). The main chemical inhomogeneity is related to the Ca and La contents; indeed, Ca is depleted and La is enriched in one of the domains occurring in the larger grain and characterized by a light grey color in BSE image.   Table 2 gives average chemical analyses of the different domains occurring in the two studied samples, with the exception of data of the grain of ferriakasakaite-(Ce) used for single-crystal X-ray diffraction, given in Table 1. Note: * Calculated in order to yield 8 (A + M + T) cations and 25 positive charges per formula unit; ** Calculated in order to yield 1 OH group per formula unit. Only average chemical data and estimated standard deviations (within brackets) are given.  Table 2 gives average chemical analyses of the different domains occurring in the two studied samples, with the exception of data of the grain of ferriakasakaite-(Ce) used for single-crystal X-ray diffraction, given in Table 1. Table 2. Chemical composition (in wt %) of ferriakasakaite-(Ce) and coexisting epidote group minerals in samples #19903-1 and 19903-2.   4 )O(OH), i.e., "androsite-(Ce)". This phase has not been approved yet as a valid mineral species. Both analyses collected on the larger grain of sample #19903-1 have Si > 3 atoms per formula unit (apfu). According to Armbruster et al. [1], if Si becomes > 3.05 apfu, the formula may be renormalized on Si = 3 apfu. However, although the analysis of the light grey domain has Si = 3.07 apfu, we did prefer to maintain the same kind of normalization used for the other domains.
The core (light grey in Figure 3)  Likely, the quality of this set of analyses is not satisfying, as proved by the low analytical total (Table 2). However, following [1], we could consider this as a border-line case where REE < 0.5, close to 0.5 apfu (i.e., 0.44 apfu). In this case, the mineral should be assigned to the clinozoisite group (REE < 0.5 apfu) and the species name should be determined by the dominant M 3+ at M(3), i.e., Fe 3+ . Consequently, this domain could be classified as epidote.

X-ray Crystallography
Powder X-ray diffraction data of manganiakasakaite-(La) and ferriakasakaite-(Ce), collected using a 114.6 mm Gandolfi camera and CuKα radiation, are listed in Table 3.
Single-crystal X-ray diffraction data were collected using a Bruker Smart Breeze single-crystal diffractometer with an air-cooled CCD area detector, and graphite-monochromatized MoKα radiation. The detector-to-crystal distance was set at 50 mm. Intensity data were collected using ω and ϕ scan modes, in 0.5 • slices, with an exposure time of 10 s per frame. The data were corrected for Lorentz and polarization factors and absorption using the software package Apex2 [12] for manganiakasakaite-(La) and Apex3 [13] for ferriakasakaite-(Ce).
The statistical tests on the distribution of |E| values and the systematic absences confirmed the space group symmetry P2 1 /m. The crystal structures of both minerals were refined using Shelxl-2018 [14] starting from the atomic coordinates of ferriakasakaite-(La) given by Nagashima et al. [2]. The position of the H atom was located through difference-Fourier syntheses. The following scattering curves for neutral atoms, taken from the International Tables for Crystallography [15], were used:  Table 3. Observed and calculated X-ray powder diffraction data for manganiakasakaite-(La) and ferriakasakaite-(Ce).

Manganiakasakaite-(La)
Ferriakasakaite-(Ce)  [11] on the basis of the structural models given in Table 5. Only the reflections with I calc > 10 are given, if not observed. The eight strongest reflections are highlighted in bold. Intensities were visually estimated: vs = very strong; s = strong; ms = medium-strong; m = medium; mw = medium-weak; w = weak; vw = very weak.
After several cycles of refinement, an anisotropic structural model (only H atoms were refined isotropically) converged to 0.0262 for 2119 reflections with F o > 4σ(F o ) for manganiakasakaite-(La), and to 0.0196 for 1960 reflections with F o > 4σ(F o ) for ferriakasakaite-(Ce). Details of data collection and refinement are given in Table 4.  Fractional atomic coordinates, site occupation factors, and displacement parameters are given in Table 5.   Table 6 reports selected bond distances, whereas Table 7 shows the comparison between observed site scattering and that calculated on the basis of the proposed site population.   Table 7. Refined site scattering vs. calculated site scattering (in electrons) and site population at the A and M sites in manganiakasakaite-(La) and ferriakasakaite-(Ce).

A(1) A(2) M(1) M(2) M(3)
Si ( Note: Σ c v and Σ a v are the bond-valence sums over cations and anions, respectively. Left and right superscripts indicate the number of equivalent bonds for each anion and cation, respectively. For sites with mixed occupancy, the bond valences have been weighted. The contribution of H-bond has been evaluated following Ferraris and Ivaldi [17]. a Includes +0.14 v.u. for the hydrogen bond. b Includes −0.14 v.u. for the hydrogen bond. The CIF of manganiakasakaite-(La) and ferriakasakaite-(Ce) are available as Supplementary Material linked to this article.

Crystal Structure Description
Manganiakasakaite-(La) and ferriakasakaite-(Ce) are isotypic with the other members of the epidote supergroup, e.g., [18]. Their crystal structure (Figure 4) consists of single chains of edge-sharing M(2)-centered octahedra and zig-zag chains of M(1)-centered octahedra with M(3)-centered octahedra attached on alternate sides along b. Si2O7 and SiO4 groups are linked to the octahedral chains. In this way, a framework hosting two types of structural cavities arises. The smaller cavity hosts the A(1) site, whereas the larger one hosts the A(2) site.
The decrease of coordination number is also related to the shift of O(9) away from the cation at the A(1) site. This shift is associated with a reduction of the Si(1)-O(9)-Si(2) angle, related not only to the site occupancy at A(1) but also to the mean bond length at M(3) [10]. In manganiakasakaite-(La), O(9) is at 3.14 and 3.27 Å from A(1), and the Si(1)-O(9)-Si(2) angle is 140.88 (19) • ; in ferriakasakaite-(Ce), the same values are 3.09 and 3.26 Å, and the angular value is 144.47 (14) • .
The A(2) site is a mixed (REE,Ca) site. Lanthanum is the dominant REE in manganiakasakaite-(La), whereas Ce is dominant in ferriakasakaite-(Ce). Bonazzi et al. [19] discussed the relation between the ratio of c to the unit-cell volume and the REE content in minerals of the piemontite-androsite series. Moreover, they observed a linear relation between the REE content and the β angle. In particular, the latter decreases systematically with increasing REE and the values observed in manganiakasakaite-(La) (β = 113. Whereas the assignment of Mn 2+ is straightforward, the ordering of Mn 3+ and Fe 3+ between M(1) and M(3) is more questionable. Bonazzi and Menchetti [26] reviewed the studies about the ordering of these two transition elements between M(1) and M(3) in natural and synthetic piemontite and pointed out that Mössbauer spectroscopy [27] and neutron diffraction [28] data indicate that not all Mn 3+ is ordered at M(3) at the expense of Fe 3+ . As Langer et al. [29] suggested, although the compression of the M(3) octahedron along the O(4)-M(3)-O(8) axis is directly related to the Mn 3+ content at this site, there is evidence that the distortion of M(3) in Mn 3+ -bearing epidote supergroup minerals is not related to the Jahn-Teller effect only, and that the distortion of the M sites is only partially affected by their composition. Moreover, a significant distortion of the M(1) octahedron along the O(4)-M(1)-O(4) axis seems to be also related with the increasing total Mn 3+ content in piemontites [29], thus indicating that part of Mn 3+ can be hosted at M(1). Nonetheless, because the derivation of a mineral name from chemical data should be possible also in the lack of non-conventional structural data (e.g., neutron diffraction), the octahedral cations are assigned to M(3) and M(1) following the procedures suggested by Armbruster et al. [1], i.e., according to a sequence based on decreasing ionic radii, filling first M(3) and then M(1), and regarding Mn 3+ as larger than Fe 3+ owing to the Jahn-Teller distortion. Therefore, Mn 3+ rather than Fe 3+ is assumed to fill first M(3) and then M(1), resulting in the following cation distributions: M (Table 7) and the related bond-valence sums are in accord with the theoretical value (Table 8).

Relationships with other Allanite Group Minerals
Manganiakasakaite-(La) and ferriakasakaite-(Ce) are two new additions to the allanite group (Table 9), which is currently formed by 17 mineral species. Within the allanite group, species having Ca 2+ or Mn 2+ as dominant A(1) constituents occur. The presence of about 0.4 A(1) Mn 2+ in the two new