The locality for macraeite is a phosphate pegmatite within a pegmatite formation now called Cubos–Mesquitela–Mangualde. It is located near the village of Mesquitela close to Mangualde, in the district of Viseu in the northern centre of Portugal (40°35^′13.7^′′ N, 7°45^′04.3^′′ W). A mining concession at the pegmatite, Chão do Castanheiro, has been operated by the Carolino family since 1935, mining for quartz and feldspar and also Be, Li, and Ta. Specimens containing macraeite were collected in 1984 on the dumps of the mine site. The specimens were in the form of phosphate nodules of weathered triplite, heterosite, and lithiophilite. Associated minerals are deep reddish-purple crystals of strengite, tan deeply etched triplite, reddish-brown bermanite, inky-blue phosphosiderite, and occasional switzerite.

Macraeite occurs as light-greenish-yellow equant euhedral crystals with relatively uniform sizes from sub-100 to ∼ 150 µm. Individual crystals have a pseudo-rhombic dodecahedral shape (Fig. 1) although the crystals are more commonly aggregated and form complex intergrowths (Figs. 2, 3) with dimensions of up to 0.5 mm. The crystal forms are {010}, {001}, {111}, and {$\stackrel{\mathrm{‾}}{\mathrm{1}}$11} as shown in Fig. 1. The calculated density, for the empirical formula and single-crystal unit-cell volume, is 2.39 g cm⁻³.

Figure 1(a) Isolated crystal of macraeite associated with red-brown bermanite on specimen MSM38050. Field of view (FOV) 0.55 mm. Photo by Christian Rewitzer. (b) Crystal drawing of macraeite. Clinographic projection.

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Figure 2Macraeite crystals associated with red-brown bermanite and purple strengite on specimen MSM38050. FOV 1.3 mm. Photo by Christian Rewitzer.

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Optically, macraeite crystals are biaxial (+), with α=1.605(3), β=1.611(3), and γ=1.646(3) (measured in white light). The measured 2V from extinction data analysed with EXCALIBR (Gunter et al., 2004) is 45(3)°, and the calculated 2V is 25.8°. Dispersion is strong with r<v. The optical orientation is X=b, Y=c, Z=a. The Gladstone–Dale compatibility index (Mandarino, 1981) is 0.020 (excellent) based on the empirical formula and the calculated density.

Crystals of macraeite were analysed using wavelength-dispersive electron microprobe (EMP) spectrometry on a JEOL JXA-8500F Hyperprobe operated at an accelerating voltage of 15 kV and a beam current of 2.2 nA. The beam was defocused to 10 µm. Analytical results (average of 17 analyses on 17 crystals) are given in Table 1. There was insufficient material for direct determination of H₂O, so it was based upon the crystal structure. The analyses were affected by dehydration of the specimen in the high vacuum of the microprobe. The dehydration was accompanied by strong cracking of the crystals, as shown in Fig. 3. This resulted in increasing analysis totals with increasing time in the vacuum. The totals shown in Table 1 cover the range 96.4 to 108.8. This behaviour is common to highly hydrated paulkerrite-group minerals, as noted by Sejkora et al. (2006), who reported EMP analysis totals in the range of 115 % to 119 % for their analyses of benyacarite.

Table 1Analytical data (wt %) for macraeite.

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Figure 3Scanning electron microscope back-scattered electron image of intergrown macraeite crystals, showing crystallographically controlled parallel cracking due to dehydration in the microscope vacuum.

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The empirical formula, normalised to 4P and 34 (K + O + F), is
K_0.84Mn_1.98Mg_0.08Al_0.31Fe${}_{\mathrm{1.42}}^{\mathrm{3}+}$Ti${}_{\mathrm{1.18}}^{\mathrm{4}+}$P_4.00F_0.53O_32.63H_30.92.

The least-squares programme OccQP (Wright et al., 2000) was used to obtain optimised site assignments for the M1 to M3 sites based on the refined bond distances and site scattering and the chemical analyses, giving the empirical formula in structural form as ^A1[(H₂O)_0.83K_0.17]_Σ1.00 ^A2[K_0.65(H₂O)_0.35]_Σ1.00 ^M1(Mn${}_{\mathrm{1.98}\square \mathrm{0.02}}^{\mathrm{2}+}$)_Σ2.00 ^M2(Fe${}_{\mathrm{1.09}}^{\mathrm{3}+}$Al_0.31Ti${}_{\mathrm{0.52}}^{\mathrm{4}+}$Mg_0.08)_Σ2.00 ^M3(Ti${}_{\mathrm{0.66}}^{\mathrm{4}+}$Fe${}_{\mathrm{0.34}}^{\mathrm{3}+}{)}_{\mathrm{\Sigma }\mathrm{1.00}}$ (PO₄)₄ ^X[O_0.87F_0.53(OH)_0.60]_Σ2.00 (H₂O)₁₀ ⋅ 4H₂O, where the A-site occupancies are from the structure refinement.

The simplified formula is
[(H₂O),K] [K,(H₂O)] (Mn²⁺)₂(Fe³⁺,Al,Ti⁴⁺,Mg)₂
(Ti⁴⁺,Fe³⁺) (PO₄)₄[O,(OH),F]₂(H₂O)₁₀ ⋅ 4H₂O.

The ideal end-member formula based on the dominant constituents at each site is [(H₂O)K]Mn₂(Fe₂Ti)(PO₄)₄[O(OH)](H₂O)₁₀ ⋅ 4H₂O, which requires K₂O 4.75, MnO 14.31, Fe₂O₃ 16.11, TiO₂ 8.06, P₂O₅ 28.63, and H₂O 28.14, with total 100 wt %.

Note that the (OH) is only marginally higher than F in the empirical formula calculated from the mean F content. The individual F analyses span a wide range from 0.39 wt % to double the mean F content. The higher-F analyses would give F > OH in the empirical formula, corresponding to a fluor-macraeite.

The empirical formula shows considerable mixing of the major constituents, Ti and Fe³⁺, between the crystallographically similar M2 and M3 sites such that changes in the location of minor constituents between the sites can change the dominant constituents. This problem is common to paulkerrite-group minerals, and to overcome it the compositions at the M2 and M3 sites are merged and an unambiguous end-member can be obtained using the site-total-charge method (Bosi et al., 2019a, b). This approach has been approved by the IMA CNMNC, revised proposal 22-K-bis. It is illustrated for macraeite in Fig. 4, which shows possible end-member compositions and their phase-fields based on occupation of the merged (M2₂M3) sites by Al, Ti, and Fe³⁺. For macraeite, the composition at the merged sites is ^M2+M3(Fe${}_{\mathrm{1.43}}^{\mathrm{3}+}$Ti_1.18Al_0.31Mg_0.08). Removing the minor amount of Mg and scaling to 3(Fe${}^{\mathrm{3}+}+$ Ti + Al) gives Fe_1.46Ti_1.22Al_0.32. This composition, shown by the cross in Fig. 4, sits in the Fe₂Ti end-member compositional field. Thus, for macraeite, the dominant-constituent approach for individual M2 and M3 sites using OccQP (Wright et al., 2000) and the merged-site approach give the same end-member (M2₂M3) composition. The empirical M2₂M3 compositions for other paulkerrite-group minerals are shown in Fig. 4.

Figure 4Ternary diagram for (M2)₂M3 site Al–Ti–Fe³⁺ compositions, showing end-member compositions (Al₂Ti, Ti₂Al, etc.) and location of the empirical composition for macraeite. For comparison, the published empirical compositions are shown for the paulkerrite-group minerals benyacarite (Demartin et al., 1997), paulkerrite (Peacor et al., 1984), mantienneite (Fransolet et al., 1984), rewitzerite (Grey et al., 2023b), pleysteinite (Grey et al., 2023c), and hochleitnerite (Grey et al., 2023d). Red crosses correspond to minerals with Mn at M1, and blue crosses correspond to minerals with Mg at M1.

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Raman spectroscopy was conducted on a Horiba XploRA PLUS spectrometer using a 532 nm diode laser, 200 µm slit, and 1800 grooves mm⁻¹ diffraction grating and a 100× (0.9 numerical aperture) objective. The spectrometer was calibrated using the 520.7 cm⁻¹ band of silicon. The sample was susceptible to thermal damage at high laser power, so the spectrum was recorded at a laser power of 4 mW. The sample was examined after the spectrum was recorded to verify that no thermal damage had occurred. The spectrum is shown in Fig. 5. The O–H stretch region has a broad band centred at 3460 cm⁻¹ with shoulders at 3580, 3460, and 3250 cm⁻¹. The H–O–H bending-mode region for water has a band at 1645 cm⁻¹. The P–O stretching region has a band at 955 cm⁻¹ with shoulders at 1005 and 980 cm⁻¹ corresponding to symmetric stretching modes and two weaker bands at 1125 and 1077 cm⁻¹ corresponding to antisymmetric stretching modes. Bending modes of the (PO₄)³⁻ groups are manifested by a band centred at 595 cm⁻¹ and a pair of bands at 465 and 450 cm⁻¹. Peaks at lower wavenumbers are related to lattice vibrations. An intense pair of bands at 830 and 780 cm⁻¹ can be assigned to Ti–O stretch vibrations for short M2–O bonds that occur in linear trimers of corner-connected octahedra M2–M3–M2 in the structure. The same bands are present in the Raman spectra of all paulkerrite-group minerals that have had spectra reported (Grey et al., 2023b, c, d), and they are consistent with published Raman data for titanates that have short Ti–O distances (Bamberger et al., 1990; Tu et al., 1996; Silva et al., 2018).

Figure 5Raman spectrum for macraeite.

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Crystallographic data for macraeite are available in the Supplement.

The supplement related to this article is available online at: https://doi.org/10.5194/ejm-36-267-2024-supplement.

IEG oversaw the research and wrote the paper. CR field-collected the specimen. RH and CR obtained preliminary EDS analyses, and CR obtained optical images of the specimen. WGM assisted in the diffraction data analysis. ARK measured the optical properties, Raman spectrum, powder XRD, and crystal morphology. SB collected and processed the single-crystal diffraction data, and NCW performed the site assignment analysis.

The contact author has declared that none of the authors has any competing interests.

Publisher’s note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors.

This paper was edited by Sergey Krivovichev and reviewed by Sergey Britvin and Lyudmila Lyalina.