ADVANCED MATERIALS & PROCESSES | NOVEMBER/DECEMBER 2023 26 The dark purple to purplish red color is shown in Fig. 1a. Table 1 shows the chemical composition of points A1 and A2 in Fig. 1c by EDS measurements. The mineral showed the presence of both Mn and Fe in the specimen. Purpurite is a gemstone because of the crystal structure and the purple color. It is also used in a wide array of applications as metal finishing for corrosion-resistant coating, improving lubrication, and wear reduction[17,18,22]. In general, MnPO 4 coatings are prepared by complex reactions[22]. Further exploration and utilization of naturally occurring minerals offer a new venue for utilizing chemicals from this system. Hydrated Yttrium Phosphate Mineral: Churchite. As shown in Fig. 2, churchite is a white phosphate mineral with the formula of Y(PO4) . 2H2O. EDS analysis indicates the possible chemical composition of the sample (points B1 and B2 in Fig. 2c). Churchite is a rare earth-based mineral with an acicular structure. Although it was described in 1865, the structure of phase pure Y(PO4) . 2H2O was analyzed in detail in a 2013 paper[23]. Churchite has a layered structure where each layer acts like a 2D polymer where Y and P are inter- connected with each other, and water molecules are located outside the layers[23]. Churchite-type REPO4·2H2O (RE = Gd to Lu and Y) materials also adopt a 2D layered structure with the water molecules occupying the interlayer regions. This group also showed that churchite structures are unstable upon the removal of water from the structure and transition into thermodynamically stable xenotime (REPO4) structure. From an application perspective, rare-earth phosphate materials have applications in nanocrystals, catalysts, photonic materials, bio- labeling studies, and as potential nuclear wasteforms[23–25]. Aluminum Hydrous Phosphate Minerals: Wavellite and Variscite with Crandallite. Figure 3 shows three types of aluminum-based phosphate minerals: wavellite, variscite, and crandallite. Wavellite occurs commonly in association with variscite and crandallite in fractures in phosphate rock deposits. Wavellite is used as a gemstone with yellowish-green color and beautiful radiating crystal shapes. It has an ideal formula of Al3(PO4)2(OH)3·5H2O. Similarly, variscite and crandallite can be used as gemstones and personal ornaments due to the beautiful and intense green color. Their chemical formulas are AlPO4·2H2O and CaAl3(PO4)2(OH, H2O)6, respectively[26-28]. EDS results are shown in Table 1 (Points C1, C2, D1, and D2). Crandallite has also been proposed as a potential for toxic storage due to its chemical stability[28]. Detailed follow up studies are recommended for understanding the mechanical and physical behavior of these solids. Hydrate Manganese Phosphate Mineral: Eosphorite with Roscherite. Unlike purpurite, both eosphorite and roscherite are hydrated manganese phosphate minerals. As Fig. 4 shows, eosphorite occurs in association with roscherite. EDS results are shown in Table 1 (Points E1, E2, and E3). Eosphorite is an Al-Mn based hydrous phosphate mineral with a chemical formula of MnAl(PO4)(OH)2·H2O. It is used as a gemstone due to the brown color and slender prismatic crystal shape, which can form radiating or spherical clusters. Like the anhydrous version discussed earlier, eosphorite also exists with Fe-containing endmember called childrenite due to hydrothermal phosphatization of metasediments and granite pegmatites[29]. Behal et al.[29] showed the antiferromagnetic nature of the eosphorite-childrenite series by neutron scattering at 3K. This study also emphasizes the fact that detailed follow-up examinations of these barely characterized complex phosphates may lead to design and development of materials with novel properties. Roscherite is a Ca-Be-Mn type phosphate with formula of Fig. 2 – (a) Digital picture of churchite: SEM micrographs of churchite in (b) SE and (c) BSE images of the same region. (a) Fig. 3 – (a) Digital picture of wavellite: SEM micrographs of wavellite in (b) SE and (c) BSE images of the same region. (d) Digital picture of variscite with crandallite: SEM image of variscite with carandallite in (e) SE and (f) BSE images of the same region. (b) (c) (d) (e) (f) (a) (b) (c)
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