Itoigawaite, a New Mineral, the Sr Analogue of Lawsonite, in Jadeitite from the Itoigawa-Ohmi District, Central Japan

Abstract

Itoigawaite, SrAI2Si207(OH)2.H20 , is a new member of the lawsonite group found in the ItoigawaOhmi district, central Japan. it is orthorhombic, Cmcm, a = 6.031(2), b = 8.945(2), c = 13.219(4) A, Z = 4. The strongest lines in the X-ray powder diffraction pattern are 4.68 (s) (111) on the basis of O -10. It is transparent, blue in colour with vitreous lustre. The streak is white and the cleavage good in one direction. The hardness is 5-5.5. The calculated density is 3.3 g cm 3. It is colourless in thin section and optically biaxial positive with ~ = 1.664 (2), [3 = 1.674 (2), 7 -1.688 (2) and 2V(calc.) = 81 ~ It occurs in a thin veinlet cutting a lavender-coloured Ti-bearing jadeitite. The veinlet includes irregularly-shaped aggregates composed of minute tabular crystals of itoigawaite up to 50 ~tm across and minute jadeite crystals with interstitial natrolite aggregates. Itoigawaite is considered to be crystallized from fluid during later stage activi~ of high-P/T metamorphism. The name is for the locality.

Phase relation in the NaAlSiO₄–SiO₂–H₂O system for the hydrothermal precipitation of jadeite, albite, natrolite, and analcime in jadeitite of the Itoigawa–Omi area, Japan

“…4b) accompanied by itoigawaite (Sr analog of lawsonite) in the host jadeitite (Miyajima et al 1999). In this area, new minerals such as euhedral rengeite and euhedral matsubaraite, together with euhedral jadeite, also occur in veins and cavities filled with natrolite in the host jadeitite.…”

“…Albite has polysynthetic twins. Natrolite jadeitite and analcime jadeitite have also been described in this area (e.g., Miyajima et al, 1999;Ishida et al, 2005;Morishita et al, 2007).…”

“…The new minerals containing Sr, Ti, and Zr as major elements often have euhedral shapes, and hereafter will be referred to as new Sr-Ti-Zr minerals. In veins and patches, jadeite also shows euhedral shapes (Miyajima et al, 1999).…”

“…Since then, the hydrothermal origin of jadeitite has been reported in other localities such as Cuba (e.g., Cárdenas-Párraga et al, 2012), Myanmar (e.g., Shi et al, 2012), and Japan (Miyajima et al, 1999). Jadeitite occurs generally as tectonic blocks or veins in a serpentinite matrix; therefore, the hydrothermal origin is discussed in relation to serpentinization of mantle wedge peridotite and/or high-pressure, low-temperature metamorphism at subduction zones.…”

“…The jadeitite of this area was once considered as metasomatic remnant of jadeite-quartz rocks (e.g., Shido, 1958;Chihara, 1971), formed during the high P/T type metamorphism of~300 Ma (Banno, 1958;Tsujimori, 2002;Kunugiza et al, 2004;Matsumoto et al, 2011). However, this jadeitite is also considered to be hydrothermal in origin (Miyajima et al, 1999;Kunugiza et al, 2000;Morishita et al, 2007;Harlow et al, 2015). Tsujimori and Harlow (2012) proposed that the jadeitite of this area consists of both P-and R-types, with the latter being based on the presence of large amphibole laths in omphacite-bearing jadeitite.…”