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Itoigawaite, a new mineral, the Sr analogue of lawsonite, in jadeitite from the Itoigawa-Ohmi district, central Japan

Published online by Cambridge University Press:  05 July 2018

H. Miyajima ,
S. Matsubara ,
R. Miyawaki  and
K. Ito
H. Miyajima
Affiliation:
Fossa Magna Museum, Itoigawa, Niigata 941-0056, Japan
S. Matsubara
Affiliation:
Department of Geology, National Science Museum, Tokyo 169-0073, Japan
R. Miyawaki
Affiliation:
Department of Geology, National Science Museum, Tokyo 169-0073, Japan
K. Ito
Affiliation:
Membership of Fossa Magna Museum, Itoigawa, Niigata 941-0056, Japan
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Abstract

Itoigawaite, SrAl2Si2O7(OH)2·H2O, is a new member of the lawsonite group found in the Itoigawa-Ohmi district, central Japan. It is orthorhombic, Cmcm, a = 6.031(2), b = 8.945(2), c = 13.219(4) Å, Z = 4. The strongest lines in the X-ray powder diffraction pattern are 4.68 (s) (111), 4.26 (vs) (021), 3.31 (vs) (113), 2.75 (vs) (114, 202), 2.68 (vvs) (130), 2.63 (s) (131), 2.50 (s) (220, 203, 132), 2.45 (s) (221). Electron microprobe analysis gave SiO2 32.98, TiO2 0.87, Al2O3 27.67, Fe2O3 0.39, MgO 0.27, CaO 0.45, SrO 27.71, H2O (diff.) 9.66, total 100.00 wt.%, corresponding to (Sr0.97Ca0.03Mg0.02)Σ1.02 (Al1.97Ti0.04Fe0.02)Σ2.03Si1.99H3.89O10 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 α = 1.664 (2), β = 1.674 (2), γ = 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 µm across and minute jadeite crystals with interstitial natrolite aggregates. Itoigawaite is considered to be crystallized from fluid during later stage activity of high-P/T metamorphism. The name is for the locality.

Keywords

itoigawaitenew mineralstrontiumjadeitejadeititelawsoniteRenge metamorphic beltItoigawa-Ohmi district, Japan
Type
Research Article
Information
Mineralogical Magazine , Volume 63 , Issue 6 , December 1999 , pp. 909 - 916
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1999

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