Natural pseudowollastonite: Crystal structure, associated minerals, and geological context

• Published in: Lithos 2012-03, Vol.134-135, p.75-90
• Main Authors: Seryotkin, Yurii V., Sokol, Ella V., Kokh, Svetlana N.
• Format: Article
• Language: English
• Subjects: Combustion metamorphism; Mottled Zone; Parawollastonite; Pseudowollastonite; Wollastonite
• ISSN: 0024-4937; 1872-6143
• DOI: 10.1016/j.lithos.2011.12.010

Pseudowollastonite, an extremely rare constituent of ultrahigh-temperature combustion metamorphic and igneous rocks, has been found as a rock-forming mineral in Ca-rich paralava veins of Nabi Musa fossil mud volcano (Dead Sea area). Pseudowollastonite-bearing paralavas are the products of combustion metamorphism associated with spontaneous burning of methane. The melt began to crystallize at 1480–1500°C about the ambient pressure. Pseudowollastonite enters two mineral assemblages: (1) rankinite, larnite, nagelschmidtite, wollastonite (1T), gehlenite-rich melilite, Ti-rich andradite, cuspidine, and fluorapatite; (2) parawollastonite (2M), wollastonite (1T), gehlenite-rich melilite, Ti-rich andradite, fluorellestadite. In this study we present the first single-crystal structure determination of natural pseudowollastonite. Pseudowollastonite from Nabi Musa dome is stoichiometric CaSiO3 and belongs to the most widespread four-layer polytype: a=6.83556(10) Å, b=11.86962(18) Å, c=19.6255(3) Å, β=90.6805(13)°, V=1592.21(4) Å3, space group C2/c. We argue that pseudowollastonite is so scarce in nature because its formation requires joint action of several uncommon factors: availability of hot melts of T>1200°C that bear free calcium but are poor in Mg and Fe (mostly as Fe3+) and their crystallization in the shallow crust followed by quenching. ► Pseudowollastonite has been found as a rock-forming mineral in paralava veins. ► Pseudowollastonite-bearing paralavas resulted from natural combustion of methane. ► Crystallization was in shallow crust at T=1265–1500°C and was followed by quenching. ► The parent melts were larnite-normative, rich in Ca and depleted in both Mg and Fe. ► The mineral is very scarce in nature as it forms by joint action of the rare factors.