Plechovite, ideally Ca2[K(H2O)]KBe3Si12O30, a new mineral of the milarite supergroup from Jordanów Śląski, Lower Silesia, Poland: Description, crystal structure, and relationships to agakhanovite-(Y)

Abstract Plechovite, Ca2[K(H2O)]KBe3Si12O30, is a new silicate species belonging to the milarite group of the milarite supergroup. The mineral was identified within a granitic pegmatite linked to rodingite-like calc-silicate rocks and metasomatized granite formations in a serpentinite quarry at Jordanów Śląski, in Lower Silesia, southwestern Poland. It occurs as a well-defined zone, approximately 30 × 10 μm in size, inside a crystal of agakhanovite-(Y). This crystal developed in a small cavity filled with clinochlore, embedded in a fractured quartz-feldspar pegmatite matrix. Plechovite is translucent, yellowish in color. It has a white streak, vitreous luster, a Mohs hardness of ∼6, brittle tenacity, and no visible fracture, cleavage or parting. The calculated density is 2.626 g/cm3, and its calculated mean refractive index is ∼1.547. The plechovite holotype contains (wt%): 70.78(43) SiO2, 0.31(20) Al2O3, 1.21(7) Y2O3, 0.16(3) Ce2O3, 0.23(4) Pr2O3, 0.16(9) Nd2O3, 0.09(5) Sm2O3, 9.91(9) CaO, 0.14(2) BaO, 0.19(8) Na2O, 6.67(15) K2O, 6.51 BeO(calc.), 0.73 B2O3(calc.), and 1.72 H2O(calc.); total 98.86. The empirical formula of plechovite, normalized by omitting SiO2 to 6 O apfu, 5 [A + T(2)] apfu and 3 T2(Be + B + Al) apfu, is: A(Ca1.845Y0.112Pr0.014Ce0.010Nd0.010Sm0.006La0.003)Σ2.000B[(K0.489Na0.065(H2O)0.944□0.502]Σ2.000C(K0.991Ba0.009)Σ1.000T(2)(Be2.719B0.218Al0.063) T(1)Si12.000 O30.000. It suggests a simplified formula (Ca,Y,Ln)2[(K,Na),H2O,□]2(Be,B,Al)3Si12O30, and the ideal formula Ca2[K(H2O)]KBe3Si12O30. The H2O groups from the B site are mostly bound to the structural framework viaDOw–H···AO1 and possibly also DOw–H···AO3 hydrogen bonds, marked in the Raman spectrum as a dominant O–H stretching band at 3430 cm-1. Three secondary bands at 3660–3730 cm–1 most likely correspond to H2O groups located at the B site but not connected to the plechovite framework in any way. Plechovite is replaced by agakhanovite-(Y) via the substitution AY3+ + B□ → ACa2+ + BK+, and the two species form a continuous solid-solution series. The crystal structure of plechovite has been refined to R1 = 0.0405 for 688 reflections with F > 2σF. Plechovite has a hexagonal P6/mcc structure with the following unit-cell parameters: a = 10.4014(2) Å, c = 13.6982(3) Å, V = 1283.45(6) Å3, and c : a = 1.3170; Z = 2. In the pegmatite from Jordanów Śląski, agakhanovite-(Y) and plechovite are retrograde minerals that formed under greenschist-facies conditions. They are late-stage, subsolidus hydrothermal phases that crystallized from Ca–Na–Cl-rich fluids infiltrating the pegmatite and circulating through its fractures. These fluids may have leached beryl, micas, K-feldspar, and REE-bearing phases in an Al-poor alkaline environment, supplying the Be2+, K+ and (Y + Ln)3+ necessary for the crystallization of plechovite and agakhanovite-(Y), which precipitated with decreasing temperature and increasing pH.