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Linear Structural Trends and Multi-Phase Intergrowths in Helvine-Group Minerals, (Zn,Fe,Mn)8[Be6si6o24]S2

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Linear Structural Trends and Multi-Phase Intergrowths in Helvine-Group Minerals, (Zn,Fe,Mn)8[Be6si6o24]S2 minerals Article Linear Structural Trends and Multi-Phase Intergrowths in Helvine-Group Minerals, (Zn,Fe,Mn)8[Be6Si6O24]S2 Sytle M. Antao Department of Geoscience, University of Calgary, Calgary, AB T2N 1N4, Canada; [email protected] Abstract: Synchrotron high-resolution powder X-ray diffraction (HRPXRD) and Rietveld structure refinements were used to examine the crystal structure of single phases and intergrowths (either two or three phases) in 13 samples of the helvine-group minerals, (Zn,Fe,Mn)8[Be6Si6O24]S2. The helvine structure was refined in the cubic space group P43n. For the intergrowths, simultaneous refinements were carried out for each phase. The structural parameters for each phase in an intergrowth are only slightly different from each other. Each phase in an intergrowth has well-defined unit-cell and structural parameters that are significantly different from the three endmembers and these do not represent exsolution or immiscibility gaps in the ternary solid-solution series. The reason for the intergrowths in the helvine-group minerals is not clear considering the similar radii, identical charge, and diffusion among the interstitial M cations (Zn2+, Fe2+, and Mn2+) that are characteristic of elongated tetrahedral coordination. The difference between the radii of Zn2+ and Mn2+ cations is 10%. Depending on the availability of the M cations, intergrowths may occur as the temperature, pressure, fugacity f S2, and fluid composition change on crystallization. The Be–Si atoms are fully ordered. The Be–O and Si–O distances are nearly constant. Several structural parameters (Be–O–Si bridging angle, M–O, M–S, average <M–O/S>[4] distances, and TO4 rotational angles) vary linearly with the a unit-cell parameter across the series because of the size of the M cation. Citation: Antao, S.M. Linear Structural Trends and Multi-Phase Intergrowths in Helvine-Group Keywords: helvine-group minerals; crystal chemistry; ternary solid solution; zoning; intergrowths; Minerals, (Zn,Fe,Mn)8[Be6Si6O24]S2. HRPXRD; linear structural variations Minerals 2021, 11, 325. https:// doi.org/10.3390/min11030325 Academic Editor: Sergey V. 1. Introduction Krivovichev The helvine-group minerals with the general formula (Mn,Fe,Zn)8[Be6Si6O24]S2 occur in granites, pegmatites, contact metamorphic rocks, and skarns. The endmembers have Received: 1 March 2021 interstitial M cations where M = Zn2+ (genthelvite), Fe2+ (danalite), and Mn2+ (helvine). Accepted: 18 March 2021 The latter was formerly called helvite. Helvine is the most common, follow by danalite, and Published: 20 March 2021 genthelvite is by far the rarest. Helvine-group minerals form an isomorphous series that is isostructural with sodalite, Na8[Al6Si6O24]Cl2 [1,2]. The Al atom in sodalite corresponds Publisher’s Note: MDPI stays neutral to the Be atom in helvine-group minerals, the Na to the M atom, and the Cl to the S with regard to jurisdictional claims in atom. The crystal structure of the helvine-group minerals is known [1–6]. Reports on published maps and institutional affil- the high-temperature behavior of some sodalite-group minerals are available [7–11]. The iations. framework structure of the helvine-group minerals consists of an ordered distribution of BeO4 and SiO4 tetrahedra that are linked to form four- and six-membered rings that create the sodalite or zeolite β cage, which contains tetrahedral SM4 clusters (Figure1). The M cation is coordinated by three framework O atoms and one S atom in an elongated Copyright: © 2021 by the author. tetrahedral configuration. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Minerals 2021, 11, 325. https://doi.org/10.3390/min11030325 https://www.mdpi.com/journal/minerals Minerals 2021, 11, x FOR PEER REVIEW 2 of 22 Minerals 2021, 11, 325 2 of 22 ØSi ØB FigureFigure 1. 1.Projection Projection of of the the structure structure of of helvine-group helvine-group minerals minerals down down [001] [001] showing showing the the lower lower half half of theof the unit unit cell. cell. The The BeO BeO4 4 4 4 (yellow)(yellow) and and SiO SiO4 (green) (green) tetrahedra tetrahedra alternate alternate in in the the four- four- and and six-membered six-membered rings. rings. The The angles angles of of rotation rotation of of these these TO TO4 tetrahedratetrahedra from from the the fully fully expanded expanded to theto the partially partially collapsed collapsed structure structure are are indicated indicated by ØBeby Ø andBe and ØSi. ØSi The. The S atomS atom is shownis shown with large circles (grey), and the M (= Mn, Fe, Zn) cations (violet) are on the body diagonals. The unit-cell edges are shown with large circles (grey), and the M (= Mn, Fe, Zn) cations (violet) are on the body diagonals. The unit-cell edges are shown (black lines). (black lines). OscillatoryOscillatory zoning zoning is is common common in in garnet garnet and and plagioclase plagioclase and and is is based based on on repetitious repetitious compositionalcompositional variations variations that that result result from from cyclical cyclical changes changes in in the the chemical chemical environment environment duringduring crystal crystal growth. growth. A A zoned zoned genthelvite genthelvite sample sample from from the the Cairngorm Cairngorm Mountains, Mountains, Scot- Scot- land,land, has has been been reported reported [ 12[12].]. MarkedMarked lineline splittingsplitting in in a a large large diameter diameter (11.46 (11.46 cm) cm) was was ob- observedserved with with an X-ray camera, notnot detectabledetectable onon filmsfilms taken taken with with a a small small diameter diameter (6 (6 cm) cm) camera,camera, indicating indicating the the presence presence of of more more than than one one helvine-group helvine-group phase phase in in the the powder. powder. The The 79 16 5 oscillatoryoscillatory zoning zoning varies varies in in composition composition from from a central a central region region of Gof79 GD16DH5H(G (G = genthelvite,= genthelvite, DD = danalite,= danalite, and and H H = helvine)= helvine) to ato crustal a crustal coating coating around around G29 GD2938DH3833H.33 Zoned. Zoned genthelvite genthelvite samplessamples from from the the Aïr Aïr Mountains, Mountains, Niger, Niger, have have also also reported reported [13 [13].]. Oscillatory Oscillatory zoning zoning in in helvine-grouphelvine-group minerals minerals was was also also reported reported by by other other researchers researchers [14 [14–19].–19]. For For excellent excellent im- im- agesages of of oscillatory oscillatory zoning zoning in in helvine, helvine, see see Figure Figure 3 in3 in Raade Raade [19 [19].]. Powder Powder X-ray X-ray diffraction diffraction hashas detected detected intergrowths intergrowths in in several several other other minerals minerals [20 [20–22].–22]. AA geometrical geometrical model model for for a structurea structure based based on on the the sodalite sodalite framework framework topology topology is is availableavailable [23 [23],], with with the the assumption assumption that that the the interframework interframework ions ions (Na, (Na, K, K, Ca, Ca, Cl, Cl, OH, OH, H 2HO,2O, SOSO4,4 etc.), etc.) have have nono effecteffect on th thee Si–O Si–O and and Al–O Al–O distances. distances. The The helvine-group helvine-group minerals minerals pro- providevide an an ideal ideal opportunity opportunity to examine to examine the theisolated isolated effect effect of only of onlyinterframework interframework M cations M cationson analogous on analogous Be–O Be–O and andSi–O Si–O distances, distances, together together with with the the use use of ofthe the sodalite sodalite model model to toexplain explain variations variations in in structural structural parameters. parameters. CompleteComplete miscibility miscibility occurs occurs between between the the Fe 2+Feand2+ and Mn Mn members members and and between between the the Fe 2+Fe2+ andand Zn Zn members, members, but but an an immiscibility immiscibility gap gap exists exists between between the the Mn Mn and and Zn Zn members members [14 [14].]. Minerals 2021, 11, 325 3 of 22 The pure endmembers helvine and genthelvite occur naturally, but danalite is about 86% of the Fe2+ endmember. Danalite is the only endmember that could not be synthesized in an early study [24], but was later synthesized [25]. Based on the crystal structure, the radii of the M atoms, and the sodalite model, complete miscibility should exist among the three endmembers [1,23]. The lack of pure danalite in nature may indicate that another phase may be more stable relative to danalite. Pure danalite (a = 8.203(1) Å) has been synthesized, as well as the mixed M cation varieties, (Mn4Fe4)[Be6Si6O24]S2 with a = 8.223(1) Å, and (Fe4Zn4)[Be6Si6O24]S2 with a = 8.097(1) Å [25]. Mössbauer studies of these samples showed quadrupole splitting, especially for the mixed M cation varieties. A zero-quadrupole split- ting is expected if the anions surrounding the Fe2+ ion are identical. However, quadrupole splitting may also be explained by zoned intergrowths with slightly different compositions. The sodalite structure has also been modeled by several researchers [26–30]. These models have been used to analyze the structural and thermal-expansion behavior of alumi- nosilicate sodalite [23]. The sodalite framework topology is in a partially-collapsed state because of the relatively small non-framework interstitial ions [6]. Heating (or substitu- tion) causes the framework tetrahedra to rotate in a cooperative manner. This rotational mechanism is described in terms of the angles ØSi and ØAl through which the distinct TO4 tetrahedra are rotated relative to their position in the fully expanded state (see Figure 2 in Hassan and Grundy [23]). Linear trends in structural parameters are expected for the helvine-group minerals, but only general trends were observed in a previous single-crystal study [1].
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