Crystal chemistry of dorrite from the Eifel volcanic region, Germany, and chemical variations in the khesinite-dorrite-rhönite-kuratite solid-solution system

Abstract

Dorrite, khesinite and rhönite from metamorphosed calcic xenoliths of the Bellerberg paleovolcano (Eifel, Germany) were studied, including first determination of the crystal structure of natural dorrite (R = 0.0636). Dorrite is triclinic, P-1, unit-cell parameters are: a = 10.4316(7), b = 10.8236(9), c = 8.9488(7) Å, α = 105.972(6), β = 96.003(9), γ = 124.67(10)° and V = 754.10(11) ų. Its crystal-chemical formula is (Z = 1): ^M8Ca₂^M9Ca₂^M1Fe^3 + M2Fe^3 + M3(Fe³⁺_0.8Mg_0.2)₂^M4(Fe³⁺_0.8Mg_0.2)₂^M5(Mg_0.9Fe³⁺_0.1)₂^M6(Mg_0.5Fe³⁺_0.5)₂^M7(Fe³⁺_0.9Al_0.1)₂[^T1(Al_0.75Si_0.20Fe³⁺_0.05)₂^T2(Al_0.77Si_0.20Fe³⁺_0.03)₂^T3(Al_0.9Fe³⁺_0.1)₂^T4Si₂^T5(Fe³⁺_0.6Al_0.4)₂^T6(Fe³⁺_0.6Al_0.4)₂]O₄₀. New and earlier published data show that khesinite, dorrite, rhönite and kuratite form a solid-solution system without significant gaps. The chemical variation and isomorphous substitutions in this system are discussed and the following simplified formulae are suggested: dorrite, Ca₂(Fe³⁺,Mg)₅Mg[(Al,Fe³⁺,Si)₅SiO₂₀], khesinite, Ca₂(Fe³⁺,Mg)₅Mg[(Fe³⁺,Al,Si)₅SiO₂₀], and rhönite, Ca₂(Mg,Fe³⁺)₅Ti[(Si,Al)₆O₂₀].

Introduction

The rhönite group, a subdivision of the sapphirine supergroup (Kunzmann 1999; Grew et al. 2008; Mills et al. 2009), unites minerals with the general formula M₁₆T₁₂O₄₀ (Z = 1) in which species-defining components are M = Ca, Mg, Fe²⁺, Fe³⁺, Al, Ti⁴⁺, Sb⁵⁺ [distributed between nine positions with coordination numbers 6 (M1–7) and 7, 8 (M8–9)] and T = Si, Fe³⁺, Al, B, Be (distributed between six tetrahedrally coordinated positions). Different wide-range substitutions in the positions M1–7 and T are typical for this group. Recent data, published and freshly obtained by us, demonstrate interesting isomorphism for rhönite-group members with the formation of a continuous solid-solution system between silicates and silico-oxides, i.e., minerals with ^T(Al + Fe) ≤ Si and ^T(Al + Fe) > Si, respectively.

Rhönite-group silico-oxides are dorrite, firstly described by Cosca et al. (1988) with the idealized formula Ca₂Mg₂Fe³⁺₄(Al₄Si₂)O₂₀ (Z = 2), and khesinite for which the idealized formula Ca₂(MgFe³⁺₅)(Fe³⁺₅Si)O₂₀ was given (Galuskina et al. 2017). Both minerals are known only in the pyrometamorphic rocks. The overview of the published data on dorrite is given in our paper on the crystal structure of its anthropogene counterpart from a burnt dump of a coal mine in Kopeisk, South Urals, Russia (Shchipalkina et al. 2016). The holotype khesinite originates from veins of paralava in gehlenite rocks of the Hatrurim Complex, Negev Desert, Israel (Galuskina et al. 2017). Earlier its anthropogene analogue from a burnt dump in Kopeisk was described as “malakhovite” (Chesnokov et al. 1993b).

The existence of the dorrite–khesinite solid-solution series was reported by Galuskina et al. (2017). The end-member formulae for dorrite and khesinite were suggested in the cited paper as Ca₄Mg₂Fe³⁺₁₀(Al₁₀Si₂)O₃₆O₄₀ and Ca₄Mg₂Fe³⁺₁₀(Fe³⁺₁₀Si₂)O₃₆O₄₀, respectively. However, the chemical and crystal chemical data presented by the same authors and by Shchipalkina et al. (2016) demonstrate some discrepancy with the idealized formulae suggested by Cosca et al. (1988) and Galuskina et al. (2017) because of significant variations in the Al: Fe³⁺: Si ratios in tetrahedral sites and the Mg: Fe³⁺ ratio in octahedral sites. In fact, the situation is even more complicated due to the diversity of M- and T-ordering schemes in minerals of the dorrite-khesinite series.

The relationship between dorrite and an aluminosilicate rhönite was slightly noted by Cosca et al. (1988, and references therein) and Jensen (1996). The idealized formula of rhönite was written as Ca₂(Mg,Fe²⁺,Fe³⁺,Ti)₆(Si,Al)₆O₂₀ by Bonaccorsi et al. (1990) and Ca₄(Mg₈Fe³⁺₂Ti₂)O₄[Si₆Al₆O₃₆] by Grew et al. (2008). The Fe²⁺- and Si-enriched analogue of rhönite, a new mineral kuratite with the idealized formula Ca₄(Fe²⁺₁₀Ti⁴⁺₂)(Si₈Al₄)O₄₀ was recently discovered in the D’Orbigny angrite meteorite (Hwang et al. 2016). We also note so-called “rhönite from the Allende meteorite” studied in several works; its chemical composition significantly differs from that of rhönite and, according to the majority of published analyses (see review by Bonaccorsi et al. 1990), corresponds to the simplified formula Ca₂(Mg₃AlTi³⁺Ti⁴⁺)(Al₄Si₂)O₂₀. Thus, it could be considered as a potentially new species.

In this paper we report new chemical and crystal chemical data for dorrite (including first structure determination for its natural sample), khesinite and rhönite formed in metamorphosed xenoliths in basalt lava from the Bellerberg paleovolcano in Eifel, Germany, and discuss the khesinite–dorrite–rhönite–kuratite solid-solution system.

Experimental

Sample description

The Bellerberg paleovolcano located 2 km north of the city of Mayen (Rheinland-Pfalz, Germany) is the largest volcano in the East Eifel Volcanic Field. Several lava flows of the Bellerberg volcano incorporated different xenoliths of neighboring rocks: limestone, clays, marls, schists and subvolcanic igneous rocks. These thermally and metasomatically altered xenoliths host wide variety of newly formed minerals (Hentschel 1987; Schüller 2013).

Dorrite, khesinite and rhönite were found by us at Bellerberg in metamorphosed calcic xenoliths. The associated minerals are cuspidine, spinel, diopside, gehlenite, calcite, fluorite, and quartz. Dorrite forms brownish-red short-prismatic crystals up to 0.3 mm long, with wedge section. Some dorrite crystals contain khesinite core (Fig. 1). Rhönite occurs as dark brown to brownish red prismatic crystals up to 0.5 mm long sometimes twinned on {010}.

Fig. 1

Backscattered-electron image of zoned crystal from Bellerberg mainly consisting of dorrite (Dor) with khesinite (Khes) core and ingrowths of hematite (Hem) embedded in diopside (Di) aggregate. FOV width is 0.2 mm

Chemical data

Electron microprobe analyses were obtained on polished grains using a JEOL JXA-8230 microprobe instrument (EDS mode) at the Laboratory of Analytical Techniques of High Spatial Resolution, Dept. of Petrology, Moscow State University. Standard operating conditions included an accelerating voltage of 20 kV and beam current of 10 nA. The following standards were used for quantitative analysis: wollastonite (Ca), diopside (Mg), Mn (Mn), Zn (Zn), hedenbergite (Fe), hornblende (Al), hyperstene (Si), and rutile (Ti). The analytical results and empirical formulae are presented in Tables 1 and 2. Iron and manganese were assumed as Fe³⁺ and Mn³⁺ because of (1) strongly oxidizing conditions of formation of the minerals (Chesnokov and Shcherbakova 1991; Chesnokov et al. 1993a, b; Chukanov et al. 2008, 2011), (2) charge balance in the formulae, and (3) the presence of only trivalent Fe and Mn in all known structurally studied samples of these and related compounds (see below). All formulae are calculated for Z = 2, which corresponds to the basis of 20 O atoms per formula unit (apfu). The distribution of Al and Fe³⁺ between tetrahedral and octahedral sites for samples with unknown crystal structures was performed using the following scheme substantiated by the structural data (Shchipalkina et al. 2016; Galuskina et al. 2017): (1) all Si is assigned to the tetrahedral sites, (2) the tetrahedral sites are filled to 6 apfu by Al and, if all Al is lacked, the rest is filled by Fe³⁺, (3) the octahedral sites are occupied by Mg, Mn, Ti and rest of Fe³⁺ and Al.

Table 1 Chemical composition of dorrite–khesinite series minerals from Bellerberg, Eifel, Germany (1–11), and their anthropogene counterparts from Kopeisk, South Urals, Russia (12–16)

Table 2 Chemical composition of rhönite from Bellerberg (Eifel, Germany)

Chemical composition of the sample of dorrite from Bellerberg used for the crystal structure determination corresponds to #3 in Table 1. Its empirical formula is: Сa_2.08(Fe³⁺_4.03Mg_1.56Mn³⁺_0.16Ti_0.17)_∑5.92[(Al_3.32Si_1.47Fe³⁺_1.21)_∑6.00O₂₀].

Single-crystal X-ray diffraction data

A full sphere of three-dimensional X-ray diffraction (XRD) data was collected using MoKα radiation (λ = 0.71073 Å) at room temperature on an Xcalibur S CCD diffractometer. Crystal data, data collection information and structure refinement details are given in Table 3. Data reduction was performed using CrysAlisPro Version 1.171.37.35 (Agilent Technologies 2014). The data were corrected for Lorentz, background, polarization and absorption effects. The mineral is triclinic, space group P-1 and a = 10.4316(7), b = 10.8236(9), c = 8.9488(7) Å, α = 105.972(6), β = 96.003(9), γ = 124.67(10)° and V = 754.10(11) ų. Typical for the minerals of the sapphirine group twinning by non-merohedry (Merlino 1972; Bonaccorsi et al. 1990, and references therein) was observed with a two-fold rotation about the b direction of the pseudo-monoclinic unit cell with the dimensions a = 10.4124(5), b = 29.8339(13), c = 10.2125(6) Å and β = 109.086(6)°. This cell can be obtained from the triclinic one through the matrix [1 1 1 / 1 2–2 / -1 0 0]. The refined twin ratio was 0.704(2): 0.296(2). The structure of dorrite from Kopeisk (Shchipalkina et al. 2016) was used as a starting model for the refinement. The structure of dorrite from Bellerberg was refined on F to the final R = 0.0636% for 2574 reflections with I > 2σ(I) using a JANA2006 program package (Petříček et al. 2006). Atom coordinates and displacement parameters are given in Table 4 and selected interatomic distances in Table 5. For the M8 and M9 sites, the two farthest oxygen atoms (O20 and O9 for M8, O10 and O19 for M9) were included in the coordination sphere as done for rhönite (Bonaccorsi et al. 1990). The distribution of cations among tetrahedral and octahedral sites was obtained on the basis of the scheme reported for the refinement of dorrite-like synthetic phases SFCAM (Sugiyama et al. 2005). For the structural model of dorrite particular octahedral sites were refined as being occupied by disordered Fe and Mg, tetrahedral sites – by disordered Al and Fe. Other components were added to these positions in agreement with the electron microprobe data, taking into account that components of pairs Mg-Al and Al-Si could not be clearly distinguished in the routine XRD analysis due to similar scattering amplitudes.

Table 3 Crystal data, data collection and structure refinement details for dorrite from Bellerberg

Table 4 Atomic coordinates, refined site occupancies, equivalent displacement parameters of atoms (U_eq, Ų) and site multiplicities (Q) in the structure of dorrite from Bellerberg

Table 5 Interatomic distances (in Å) for cation sites in crystal structure of dorrite from Bellerberg

Results and discussion

Chemical variations in the khesinite–dorrite–rhönite–kuratite solid-solution system

The earlier published (Cosca et al. 1988; Shchipalkina et al. 2016; Galuskina et al. 2017) and new data for dorrite and khesinite show that chemical variations are mostly observed in the tetrahedral sites. In the majority of samples the amounts of tetrahedrally coordinated components follow the condition Al > Si > Fe³⁺ (in atom proportions), whereas for khesinite the scheme Fe³⁺ > Al > Si is the most typical (Fig. 2).

Fig. 2

Ratios of major tetrahedrally coordinated constituents in dorrite: ♦ – this study, ● - Cosca et al. (1988); khesinite: ◇ - this study, ○ - Cosca et al. (1988), □ - Galuskina et al. (2017), ▲ – Chesnokov et al. (2008); rhönite: + − Johnston (1985), × − Bonaccorsi et al. (1990), - Grapes et al. (2003), - Grapes and Keller (2010) ★ – this study; ☆ – “rhönite from the Allende meteorite” (Bonaccorsi et al. 1990); kuratite: ■ – Hwang et al. (2016); ∆ - Peretyazhko et al. (2017)

The dorrite analyses could be divided to two groups. For analyses reported by Cosca et al. (1988) the major substitution scheme describing the transition from dorrite to khesinite, is simple: ^IVAl → ^IVFe³⁺ (Roman numerals here and below mean coordination numbers). The ^IVFe³⁺-enriched and ^IVAl-depleted variety of dorrite forms a continuous solid-solution series with khesinite (Fig. 1). However, in dorrite and khesinite from Bellerberg both tetrahedrally and octahedrally coordinated components are involved in coupled substitutions (Table 1; Figs. 2 and 3) with the major scheme: ^VIMg²⁺ + ^IVSi⁴⁺ → ^VIFe³⁺ + ^IVFe³⁺.

Fig. 3

Contents of major осtahedrally coordinated cations Mg and Fe (apfu, Z = 2) in dorrite. Rhombs: this study, circles: data by Cosca et al. (1988)

Change of composition from dorrite to khesinite can occur in one crystal. A typical zoned crystal from Bellerberg (Fig. 1) contains small khesinite core whereas its main volume is composed by dorrite.

Dorrite and rhönite form a continuous solid-solution series in part of octahedrally coordinated components (Fig. 4) and a series with gap in part of T components (Fig. 2). In comparison with the dorrire-khesinite series, the situation with heterovalent substitutions here is more complicated due to the presence of species-defining tetravalent ^VITi and significant admixture of univalent ^VIIINa in rhönite. The major substitution scheme describing the transition from dorrite to rhönite, based on the data shown in Tables 1 and 2 and Figs. 2 and 4, can be presented as follows: ^VIMg²⁺ + ^IVSi⁴⁺ → ^VIFe³⁺ + ^IVAl³⁺. The isomorphism is complicated by the schemes 2^VIFe³⁺ → ^VIMg²⁺ + ^VITi⁴⁺ (Fig. 4) and (in the rhönite part of the series, Table 2) ^VIIICa²⁺ + ^VIFe³⁺ → ^VIIINa⁺ + ^VITi⁴⁺. Rhönite from Bellerberg studied by us (Table 2) is characterized by higher Fe³⁺ content in comparison with samples of this mineral from other localities (Johnston 1985; Bonaccorsi et al. 1990; Grapes et al. 2003; Grapes and Keller 2010). Its analyses fill a gap between dorrite and rhönite in the compositional diagram plotted for octahedrally coordinated components (Fig. 4). The field of Ti-rich minerals also includes points corresponding to kuratite (Hwang et al. 2016) and intermediate members of the rhönite-kuratite series from paralavas of Choir-Nyalga, Mongolia, reported by Peretyazhko et al. (2017). The major substitution schemes in the rhönite-kuratite series can be written as follows: ^VIFe²⁺ + ^IVSi⁴⁺ → ^VIFe³⁺ + ^IVAl³⁺ and ^VIFe²⁺ → ^VIMg²⁺.

Fig. 4

Ratios of major octahedrally coordinated constituents in dorrite, khesinite, rhönite, kuratite and “rhönite from the Allende meteorite”. For legend see Fig. 1

Analyses of the so-called “rhönite from the Allende meteorite” lie in Fig. 2 within the dorrite field but this phase strongly differs from dorrite in part of octahedrally coordinated cations (Fig. 4).

The presence of the solid-solution series between dorrite and rhönite was not discussed in literature. The totality of new and earlier published chemical data demonstrates that this series occurs in nature. Involving khesinite, kuratite and the “rhönite from the Allende meteorite”, we observe a multicomponent solid-solution system without gaps in part of M cations (Fig. 4) and with only insignificant gaps in part of tetrahedrally coordinated constituents (Fig. 2).

Crystal chemistry of dorrite-khesinite series minerals

Dorrite and khesinite belong to the aenigmatite structural type. This structure is characterized by nine crystallographically nonequivalent M sites centering six-, seven- or eight-fold polyhedra and six T sites centering tetrahedra (Fig. 5). The M(3–9)-centred polyhedra share edges forming walls with the width of three or four polyhedra running parallel to the a axis. Two octahedra M1O₆ and M2O₆ are located between neighboring walls sharing edges with the polyhedra of the walls (Fig. 6). The TO₄ tetrahedra form pyroxene-like tetrahedral chains T₆O₁₈ with lateral appendices running along the a axis. The T1–4 sites belong to an “axial” pyroxene-like chain whereas the T5O₄ and T6O₄ tetrahedra form its side group (Kunzmann 1999). Compositional variations in tetrahedral and octahedral sites can result in different cation-ordering schemes in members of the dorrite-khesinite series.

Fig. 5

General view of the wall composed by the M-centred polyhedra and its joint with tetrahedral chains in the crystal structure of dorrite. Small black circles are oxygen atoms. For composition of the M and T sites see Table 4

Fig. 6

Important building units of the crystal structure of dorrite: a – the slightly corrugated polyhedral layer (projection along the a axis), b – its fragment with the marked M and T sites and outlined unit cell. Small black circles are oxygen atoms

Cation ordering in dorrite from Bellerberg was expected to be slightly different from that in its anthropogene counterpart because of high content of Fe³⁺ in tetrahedral sites – 1.20 apfu. As shown in Table 6, the main difference of the Bellerberg dorrite from the sample from Kopeisk studied by Shchipalkina et al. (2016) is the predominance of Fe³⁺ in the T5 and T6 sites. The cation ordering in khesinite (Galuskina et al. 2017) is shown in Table 5 for comparison.

Table 6 Occupancies of M and T sites and average interatomic distances (in Å) in the crystal structures of dorrite from Kopeisk (Shchipalkina et al. 2016) and Bellerberg (this work) and khesinite from Hatrurim (Galuskina et al. 2017). Formula coefficients are given for Z = 2

The substitution of ^IVFe³⁺ for ^IVAl occurs mainly in the T5 and T6 sites (Table 6). These sites are characterized by the longest average interatomic distances among all tetrahedral sites: 1.78–1.87 Å. The Fe³⁺– Si substitution realizes in the T1–3 sites. The T4 site with the average T–O distance of 1.66 Å is steadily occupied by Si. The arrangement of tetrahedra with prevailing Si, Al and Fe³⁺ in structurally studied samples of dorrite and khesinite is shown in Fig. 7.

Fig. 7

Branched chains in the crystal structures of (1) dorrite from Kopeisk (Shchipalkina et al. 2016), (2) dorrite from Bellerberg (this study), and (3) khesinite from Hatrurim (Galuskina et al. 2017). Al prevails in light grey, Si in white and Fe³⁺ in dark grey tetrahedra

Simplified formulae of minerals in the khesinite–dorrite–rhönite system

Based on the above discussed chemical and crystal chemical data, the simplified formulae of minerals belonging to the khesinite–dorrite–rhönite solid-solution system can be written as follows (Z = 2):

$$ {\displaystyle \begin{array}{l}\mathrm{dorrite}:{\mathrm{Ca}}_2{\left({\mathrm{Fe}}^{3+},\mathrm{Mg}\right)}_5\mathrm{Mg}\left[{\left(\mathrm{Al},{\mathrm{Fe}}^{3+},\mathrm{Si}\right)}_5{\mathrm{SiO}}_{20}\right];\\ {}\mathrm{khesinite}:{\mathrm{Ca}}_2{\left({\mathrm{Fe}}^{3+},\mathrm{Mg}\right)}_5\mathrm{Mg}\left[{\left({\mathrm{Fe}}^{3+},\mathrm{Al},\mathrm{Si}\right)}_5{\mathrm{SiO}}_{20}\right];\\ {}\mathrm{rh}\ddot{\mathrm{o}} \mathrm{nite}:{\mathrm{Ca}}_2{\left(\mathrm{Mg},{\mathrm{Fe}}^{3+}\right)}_5\mathrm{Ti}\left[{\left(\mathrm{Si},\mathrm{Al}\right)}_6{\mathrm{O}}_{20}\right].\end{array}} $$

For dorrite and khesinite these formulae reflect their crystal chemical features including the presence of the Mg-dominant octahedral site M5 and Si-pure tetrahedral site T4, as well as the ratios of different components in other octahedral and tetrahedral sites. The suggested formula of rhönite shows the presence of the Ti-dominant octahedral site M7, the prevailing of Mg over Fe³⁺ in the totality of other octahedral sites, and the general predominance of Si over Al in the tetrahedral sites.