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Crystal chemistry of synthetic Ca₂Al₃Si₃O₁₂OH–Sr₂Al₃Si₃O₁₂OH solid-solution series of zoisite and clinozoisite

¹ Technische Universität Berlin, Fachgebiet Mineralogie, ACK 9, Ackerstr. 76, D-13355 Berlin, Germany
² GeoForschungszentrum Potsdam Telegraphenberg 14473 Potsdam, Germany

Correspondence: ^* E-mail: guido.doersam{at}web.de

Coexisting solid-solution series of synthetic zoisite-(Sr) and clinozoisite-(Sr) were synthesized in a 1 M (Ca,Sr)Cl₂ solution at 2.0 GPa, 600 °C for 6 days in a piston cylinder press. Solid solutions were synthesized from X_Sr^Zo = Sr/(Ca + Sr) = 0.06 to 1 and X_Sr^Czo = 0.08 to 0.5 in zoisite and clinozoisite, respectively. The products were characterized with SEM, EMP, and powder-XRD. Zoisites form crystals up to 30 µm in size. Lattice parameters of zoisite increase linearly with increasing Sr content. For synthetic zoisite-(Sr) lattice parameters are a = 16.3567(5) Å, b = 5.5992(2) Å, c = 10.2612(5) Å, and V = 939.78(7) ų in space group Pnma. Volume of clinozoisite (P2₁/m) increases with increasing X_Sr^Czo , but the lattice parameter a collapses, and b, c, and ß have a discontinuity at X_Sr^Czo 0.25. The decrease in angle ß of clinozoisite results in compression of M3 and T3 polyhedra and increase of the A2 polyhedron. A1-O7 distance of 2.12 Å in clinozoisite is extremely short at X_Sr^Czo 0.25, but with further Sr incorporation on A2 this distance relaxes quickly to 2.24 Å, combined with a torsion of T3. In zoisite, Sr incorporation leads to an opposite movement of neighboring octahedral chains parallel a and causes changes in the linked T3, and angle O5-T3-O6 increases with X_Sr from 96.3 to 101°. The intra-crystalline distribution of Sr shows that A2 is the favored position and continuous incorporation on A1-position starts above X_Sr^Zo 0.35 for zoisite and above X_Sr^Czo 0.45 for clinozoisite.

Key Words: Zoisite • clinozoisite • strontium • synthesis • solid-solution series • hydrothermal • EMP data • XRD data

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