Description, occurrence, physical properties and mineral name

Marshallsussmanite forms bladed crystals with a pink colour (Figs 1 and 2). On the holotype specimen, marshallsussmanite is intense pink with a distinct orange tinge, grading towards a peach colour. Holotype marshallsussmanite has epitaxial serandite in association (Fig. 3), as well as minor later calcite (Fig. 1). Another assemblage of marshallsussmanite is also known, associated with euhedral black prismatic aegirine, translucent white hydroxyapophyllite-(K) crystals and epitaxial pectolite overgrowths. These marshallsussmanite specimens are paler in colour and lack the orange tinge of holotype material, and possibly represent a separate find of the mineral. The lustre of marshallsussmanite is vitreous; crystals vary from transparent to translucent. The maximal crystal size observed in this study is 2.1 cm × 1.5 cm. The bladed crystals are elongated most along [010], typically with an aspect ratio of 3:2, and generally sit on matrix on their long edge.

Fig. 1.

The holotype specimen of marshallsussmanite, 9.5 cm across, showing pink bladed crystals to 1.3 cm with minor white calcite.

Fig. 2.

A fine marshallsussmanite specimen from the Wessels mine, 9.1 cm tall. Weinrich Minerals specimen. Jeff Scovil photograph.

Fig. 3.

Back-scattered electron photomicrograph of a zoned marshallsussmanite–serandite crystal, the brighter rim (right side of image) representing epitaxial serandite grown on darker marshallsussmanite.

Marshallsussmanite has a Mohs hardness of 5. The mineral shows perfect cleavage in two directions, {100} and {001}. Fracture normal to the cleavage planes is difficult to achieve, and marshallsussmanite splinters upon crushing.

Marshallsussmanite forms on dark grey-brown skarn, consisting primarily of quartz stained by iron and manganese oxide phases, in the Wessels mine at Black Rock, Northern Cape Province, South Africa. The Wessels mine lies within the economically significant Kalahari Manganese Field; collectively, the Wessels mine and the adjacent N'Chwaning mines are type locality to 25 mineral species at the time of writing (http://www.mindat.org). Marshallsussmanite probably formed via metasomatic alteration of manganese rich metasediments, within the temperature range 270–420°C and the pressure range 0.2–1.0 kbar (Gutzmer and Beukes, Reference Gutzmer and Beukes1996).

The mineral name honours Marshall I. Sussman (1944–). Mr. Sussman is a lifelong collector of minerals. Mr. Sussman periodically visits southern Africa to acquire mineral specimens. On one such visit in 2011, Mr. Sussman returned to Arizona with a finely crystallized pink-peach mineral. The initial powder X-ray diffraction study showed similarities to ferrobustamite. Noting the pink colour and occurrence in the Kalahari Manganese Field, Mr. Sussman suspected significant manganese in the mineral. He brought the material to the attention of one of the authors (MJO) who then used single-crystal methods at the University of Arizona and found a triclinic unit cell intermediate to those of pectolite and serandite (Table 1). An independent powder X-ray diffraction study at the Smithsonian also showed the material to be isomorphous with pectolite. Further independent study using scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS) at the Smithsonian Institution indicated significant Na, Ca and Mn. A careful reading of Ohashi and Finger (Reference Ohashi and Finger1978) gave an inkling that this material represents a novel ordered intermediate NaCaMnSi₃O₈OH compositionally midway between pectolite and serandite, and henceforth came this study.

Table 1.

Minerals of the pectolite group*.

* Cascandite, pectolite and serandite unit cell parameters from Mellini and Merlino (Reference Mellini and Merlino1982), Prewitt (Reference Prewitt1967) and Jacobsen et al. (Reference Jacobsen, Smyth, Swope and Sheldon2000), respectively.

Additional localities for marshallsussmanite exist. Notably, sample SCH of Ohashi and Finger (Reference Ohashi and Finger1978) corresponds to marshallsussmanite; it occurs in sodalite-syenite pegmatite at Kangerdlaursak, Julianehaab, Greenland. Carr et al. (Reference Carr, Phillips and Williams1976) report ‘manganoan pectolite’ and ‘calcian serandite’ in phonolite from Mount Goonneringerringgi, Queensland, Australia; both minerals fall into the marshallsussmanite compositional field. Marshallsussmanite also occurs in the Aris phonolite, Windhoek, Namibia (Kolitsch et al., Reference Kolitsch, Blass, Jahn, Cámara, Von Bezing, Wartha, Tremmel, Sturla, Cerea, Skebo and Ciriotti2016). Type serandite analysed in Lacroix (Reference Lacroix1931) also falls into the marshallsussmanite composition field.

Chemical analysis and chemical formula

Chemical analyses were performed on a Cameca SX-100 electron microprobe housed in the Lunar and Planetary Laboratory at the University of Arizona. Wavelength dispersive spectroscopy scanning revealed Na, Ca, Si, Mn and minor Mg, but no other elements with Z > 8; notably no significant Fe was found. Operating conditions were 20 nA and 20 kV, with an estimated spot size <1 μm. Standards included albite (Na), forsterite (Mg), orthoclase (Si), wollastonite (Ca) and rhodonite (Mn). Table 2 includes the results of 20 microprobe spot analyses. Data reduction and corrections followed the ‘PAP’ methodology (Pouchou and Pichoir, Reference Pouchou and Pichoir1984).

Table 2.

Empirical chemistry of marshallsussmanite*.

* Oxide values (Na,Mg,Ca,Mn,Si) from EPMA and Li values from ICPMS. Normalized to 3 Si atoms and assuming 1 H per formula unit, the empirical formula is Li_0.100Na_0.906Ca_0.953Mg_0.007Mn_1.071Si₃O₈(OH).

Back-scattered electron microscopy revealed chemical zonation in the material. Chemical analysis showed a clear distinction between a rather homogeneous marshallsussmanite zone with Ca:Mn ≈ 0.9:1.1 and a homogenous epitaxial serandite zone with Ca:Mn ≈ 0.1:1.9 (Fig. 3). Such zonation could represent exsolution of serandite from marshallsussmanite, or later deposition of serandite onto marshallsussmanite.

To quantify a suspected Li content, 4.2 mg of marshallsussmanite were digested in mixed concentrated hydrofluoric and nitric acids, re-precipitated by evaporation, and finally redissolved in 2% nitric acid. The sample solution was analysed with a Thermo Scientific XSeries II ICP-MS (Quadrupole Inductively Coupled Plasma Mass Spectrometer). Signal optimization employed a Thermo Fisher Scientific Multi-Element Tune A solution. A 1 ppm standard solution was made from stock solutions of 1000 ppm standards. Recursive tenfold dilutions were used to make internal standard solutions to 0.1 ppb. These diluted solutions were used to generate a calibration curve with a correlation coefficient of 0.9999, and ensured that both the pulse counting and the analogue detectors were operating synchronously. A BCR-2 standard was used during data reduction to calculate the Li concentration; estimated errors for Li are ±5%.

Reduction of the electron microprobe analytical data showed a low metal to Si ratio, i.e. ∑(Mn + Ca + Na + Mg) : Si of 2.937 : 3. In the refinement of the crystal structure of marshallsussmanite, free variation of the A-site occupancy yielded an electron density peak too low for a single Na atom, suggesting either a vacancy or presence of an element with Z < 8. Analysis using ICP-MS found significant Li in the mineral corresponding to 0.10 Li per formula unit. And crystal-structure refinement indicated the presence of 1 H apfu. The empirical formula for holotype marshallsussmanite, based on electron microprobe and ICP-MS data, calculated for 9 O atoms (and assuming 1 H apfu) is Li_0.100Na_0.906Ca_0.953Mg_0.007Mn_1.071Si₃O₈(OH).

Crystallographic study

Powder X-ray diffraction

Powder X-ray diffraction data were collected on a Bruker D8 diffractometer, generating CuKα radiation at 40 kV and 40 mA from a conventional sealed X-ray tube, filtered with Ni, and monochromated by a graphite crystal. The powder X-ray diffraction data were collected with step scans over 2θ range 5–90°, with step size 0.01° in 2θ and step count time of 2 s. Table 3 includes the powder diffraction data. Minor admixture with serandite (Fig. 3) caused minor peak broadening, precluding precise refinement of the unit-cell parameters of marshallsussmanite from the powder data.

Table 3.

Powder X-ray diffraction data for marshallsussmanite.

* broad

Single-crystal X-ray diffraction

Thermal shocking of marshallsussmanite splinters with liquid nitrogen produced a few equidimensional crystal fragments. A cuboid of 50 μm × 40 μm × 40 μm was selected and mounted for diffraction study. Single-crystal X-ray diffraction was performed using a Bruker Apex II diffractometer housed at the Department of Geosciences, University of Arizona. The X-ray source is a fine focus sealed tube with a Mo target, operated at 50 kV and 30 mA, and collimated with multilayer X-ray optics.

A hemisphere of intensity data was collected (Table 4). The unit-cell dimensions were refined from the positions of 3558 observed reflections with I > 2σ over 2θ range 5.38–68.62°. The crystal intensity data were analysed with Apex software, integrated with Bruker SAINT, and reduced with XPREP. The crystal structure analysis initially used direct methods in SHELXS (Sheldrick, Reference Sheldrick2008) and subsequent refinement cycles utilized SHELXL (Sheldrick, Reference Sheldrick2008). The cation occupancies for the A (Na, Li), M1 (Ca, Mn) and M2 (Mn, Ca) sites were allowed to vary, each constrained to full occupancy. The H atom was located within a difference-Fourier map, and its position and its isotropic displacement parameter converged without restraint. Final atom coordinates are listed in Table 5 and selected geometric details appear in Table 6. A crystallographic information file and structure factor data have been deposited with the Principal Editor of Mineralogical Magazine and are available as supplementary material (see below). The structure is illustrated in Fig. 4.

Table 4.

Marshallsussmanite crystal data and structure refinement details.

Table 5.

Fractional coordinates and displacement parameters for marshallsussmanite*.

* Refined occupancies with errors: A = Na_0.948(5)Li_0.052(5); M1 = Ca_0.793(6)Mn_0.207(6); M2 = Mn_0.937(6)Ca_0.063(6)

Table 6.

Atomic separations and geometries for marshallsussmanite*.

* Angle variance (a.v.) and quadratic elongation (q.e.) parameters defined by Robinson et al. (Reference Robinson, Gibbs and Ribbe1971).

Fig. 4.

The marshallsussmanite crystal structure: (a) perspective view along [010]; (b) a tetrahedral layer and an octahedral layer viewed down face pole (101), shown as cyan Na atoms, green CaO₆ octahedra, magenta MnO₆ octahedra and dark blue SiO₄ tetrahedra.

Crystal structure

The crystal structure of marshallsussmanite consists of single chains of corner linked SiO₄ tetrahedra running along [010], connected by double ribbons of edge-sharing CaO₆ and MnO₆ octahedra also along [010], alongside a ribbon of irregular Na sites (Fig. 4a). Alternatively, the marshallsussmanite structure may be considered layered with separate octahedral and tetrahedral layers (Fig 4b). Marshallsussmanite is isostructural with other members of the pectolite group (Table 1). Using the site nomenclature of Ohashi and Finger (Reference Ohashi and Finger1978), the marshallsussmanite crystal structure has Na in the A site, Ca in the M1 site and Mn in the M2 site. Sample SCH of Ohashi and Finger (Reference Ohashi and Finger1978) corresponds to marshallsussmanite, containing 64% of the ideal marshallsussmanite component. Holotype marshallsussmanite has refined chemistry (Na_0.948Li_0.052)(Ca_0.793Mn_0.207)(Mn_0.937Ca_0.063)Si₃O₈(OH), corresponding to 79% of the idealized end-member NaCaMnSi₃O₈(OH), with minor LiNa_–1, MnCa_–1 and CaMn_–1 substitutions.

The A site (Na) is an irregular coordination polyhedron of 7 O atoms within a radius of 3 Å. The A site bond-valence sum for all seven Na–O contacts gives too large of a sum (1.32 e ^–). In order to analyse the bonding environment, a procrystal electron density distribution was generated by superimposing spherically averaged Roothan-Hartree-Fock wave functions (Downs et al., Reference Downs, Andalman and Hudacsko1996) onto the atomic positions of the marshallsussmanite crystal structure. In the procrystal electron density model, two Bader (Reference Bader1990) criteria were sought between Na and each nearby O with a cutoff distance of 5 Å. First a bond path is sought, which is a continuous path of local maxima of electron density between the two atoms; and second, a bond critical point along the bond path is sought, which is a (3,$\bar{1}$) saddle in electron density. The modelling software demonstrated four bonded interactions, specifically to the O2, O3, O4 and O9 atoms. For these bonded interactions, <R(Na–O)> = 2.234(2) Å (Table 6). The bond-valence sum of these four interactions is 0.97 e ^– (Table 7), in good agreement with the expected valence for Na. The A site, with the Na displacement ellipsoid within a cage of 7 O atoms, including its four bonded interactions is shown in Fig. 5. Ostensibly, the displacement ellipsoid shows its greatest amplitude normal to these four interactions. The crystal structure refinement yielded an A-site scattering factor of 10.56 e ^–, significantly less than 11 e ^– expected for atomic Na. Assuming full occupancy and the presence of Li in the A site, the site occupancy refined to [Na_0.948Li_0.052].

Table 7.

Bond-valence sums for marshallsussmanite*.

* Calculations based on refined chemistry and parameters from Brese and O'Keeffe (Reference Brese and O'Keeffe1991) and Brown and Altermatt (Reference Brown and Altermatt1985). The low sums for O4 and O3 are consistent with an H atom position between O3 and O4, located nearer to O4.

Fig. 5.

The bonding environment of the A site in marshallsussmanite viewed along [011]. Procrystal electron density modelling indicates bond paths and critical points between Na and each of O2, O3, O8 and O9. These four interactions have a bond valence sum of 0.97 e ^–. The long axis of the displacement ellipsoid lies roughly normal to these bonded interactions. Thus, the A-site with refined chemistry [Na_0.948(5)Li_0.052(5)] in marshallsussmanite is 4-coordinate with <R(Na–O)> = 2.234(2) Å.

The M1 (Ca) site in the marshallsussmanite structure has octahedral coordination with <R(M1–O)> = 2.341 Å and V(M1O₆) = 16.29 Å³ (Table 6). The quadratic elongation (Robinson et al., Reference Robinson, Gibbs and Ribbe1971) of M1O₆ is 1.0336, which indicates a rather distorted site. The refined occupancy of M1 is [Ca_0.793Mn_0.207]. The mean bond length corresponds well with <R(Ca1–O)> = 2.327 Å for an octahedral site in pectolite with occupancy [Ca_0.73Mn_0.27] (Arakcheeva et al., Reference Arakcheeva, Pattison, Meisser, Chapuis, Pekov and Thélin2007).

The M2 (Mn) site in the marshallsussmanite structure has octahedral coordination, and is a bit smaller and more regular than M1. Type marshallsussmanite has <R(M2–O)> = 2.231 Å, which compares favourably with <R(Mn–O)> for two separate sites in serandite (Jacobsen et al., Reference Jacobsen, Smyth, Swope and Sheldon2000), with <R(Mn–O)> = 2.261 Å and <R(Mn–O)> = 2.361 Å. The two unique MnO₆ octahedra in serandite have V = 14.81 Å³ and 14.36 Å³. In marshallsussmanite, V(M2O₆) = 14.42 Å³, with a refined M2 site occupancy of [Mn_0.937Ca_0.063]. The MnO₆ octahedron in natural johannsenite (Capitani and Mellini, Reference Capitani and Mellini2000) has <R(Mn–O)> = 2.164 and V = 13.42 Å³. Compared to johannsenite, the greater <R(Mn–O)> in marshallsussmanite is consistent with minor Ca substitution. The quadratic elongation (Robinson et al., Reference Robinson, Gibbs and Ribbe1971) of M2O₆ is 1.0182, which shows significantly less distortion than M1O₆.

There are three unique Si sites in marshallsussmanite: Si1, Si2, Si3. Each Si atoms shows tetrahedral coordination with O, with typical silicate bond distances (Table 6). Quadratic elongation and angle variance parameters (Robinson et al., Reference Robinson, Gibbs and Ribbe1971) indicate these silicate tetrahedra are quite regular. Each SiO₄ groups shows rigid body behaviour (Downs, Reference Downs, Hazen and Downs2000), indicating complete cation ordering. Bond-valence sums for each of the Si atoms approach 4 (Table 7).

In pyroxenes, a repeat period of two silicate tetrahedra leads to the definition of a single chain angle (∠O3–O3–O3). However, as marshallsussmanite has three distinct bridging O atoms (O7, O8, O9), there are three different chain angles ∠O7–O8–O9 = 155.95°; ∠O9–O7–O8 = 139.77°; ∠O8–O9–O7 = 116.25°. The mean bridging oxygen angle <∠O–O–O> = 137.32° (Fig. 6), corresponding to a highly compressed silicate chain (Thompson and Downs, Reference Thompson and Downs2004), in contrast to a fully extended chain with <∠O–O–O> = 180°. The presence of a strong hydrogen bond between two non-bridging O atoms (O3 and O4) contributes to significant compression of the silicate chain (Fig. 7).

Fig. 6.

Comparison of the cation ribbons viewed along face pole (101) of the pectolite, marshallsussmanite and serandite crystal structures. Atoms shown as cyan Na ellipsoids, green CaO₆ octahedra and magenta MnO₆ octahedra.

Fig. 7.

The hydrogen bonding environments in the pectolite-group minerals, viewed along [413]. In marshallsussmanite, O4 acts as the donor atom, while in both pectolite and serandite, O3 acts as the donor atom. The H position acts a proxy for the relative saturation of the O3 and O4 atoms by nearby cations, in accord with bond-valence summations.

A hydrogen atom was located within the difference-Fourier maps; its positional and isotropic displacement parameters converged without restraint. The hydrogen atom is located between non-bridging O3 and O4 atoms in the silicate chain, which have a separation of 2.458(2) Å. Bond valence calculations (Table 7) indicate sums of 1.38 e ^– and 1.61 e ^– for O4 and O3, respectively. As O4 is less saturated than O3, H is expected nearer O4, as corroborated by the crystal structure refinement (Table 6).

Spectroscopic study

Raman spectra were collected on a Nicolet Almega microRaman spectrometer. Sample excitation wavelengths of 532 nm and 780 nm were used. The Raman spectra were collected with a 1800 lines cm^–1 grating with an average resolution of 2.1 pixels/cm^–1. Marshallsussmanite spectra collected with 532 nm excitation showed moderate fluorescence whereas spectra collected at 780 nm excitation had low backgrounds with excellent signal-to-noise ratios. Figure 8 compares the Raman spectra of marshallsussmanite at 780 nm (R120109) with pectolite at 532 nm (R050396) and serandite at 532 nm (R040056). Complete spectral data are available from the RRUFF database (Lafuente et al., Reference Lafuente, Downs, Yang, Stone, Armbruster and Danisi2015).

Fig. 8.

(a) Raman spectra of R120109 marshallsussmanite, R050396 pectolite and R040056 serandite. Peak positions in these spectra have sufficient separation to permit identification solely by Raman spectroscopy. (b) Hydrogen bond stretching modes of R120109 marshallsussmanite, R050396 pectolite and R040056 serandite. The relatively strong H-bond stretching modes are expected for very short O3–O4 separations ~ 2.5 Å.

The most prominent feature of the Raman spectrum of marshallsussmanite is an Si–O–Si bending mode at 659 rel. cm^–1 (Fig. 8a). The Raman spectrum is similar to those of isostructural pectolite and serandite. The peak at 1010 rel. cm^–1 corresponds to a stretching of non-bridging Si–O bonds. Peaks in the 200–400 rel. cm^–1 range relate to rotational modes and bending modes of the octahedral cations. The H-bond stretching modes for marshallsussmanite, pectolite and serandite in the frequency range (1370–1390 rel. cm^–1) are shown in Fig. 8b, which are consistent with short d(H⋅⋅⋅O) ≈ 1.4–1.6 Å separations (Libowitzky, Reference Libowitzky1999).

Distinguishing marshallsussmanite, pectolite and serandite

Marshallsussmanite has properties similar to both pectolite and serandite. However, despite similarities in their powder X-ray diffraction profiles, unit-cell refinement to four significant figures can reliably distinguish these species (Table 1). Single-crystal methods can reliably distinguish these species. Standardized chemical analysis also affords reliable discrimination of pectolite-group species, and in many cases, semi-quantitative SEM-EDS analysis is sufficient. Raman spectroscopy also distinguishes the species; for instance, a prominent Si–O–Si bending mode occurs at 667 rel. cm^–1 in serandite, 650 rel. cm^–1 in pectolite and 659 rel. cm^–1 in marshallsussmanite (Fig. 8).