Microtexture investigation of amblygonite-montebrasite series with lacroixite: Characteristics and formation process in pegmatites

Yohei Shirose, Seiichiro Uehara

Research output: Contribution to journalArticle

Abstract

Amblygonite-montebrasite series and lacroixite from Nagatare Li-Cs-Ta (LCT) pegmatite, Fukuoka Prefecture, Japan, were investigated by powder X-ray diffraction (XRD), electron microprobe analyses, and transmission electron microscope (TEM)/scanning transmission electron microscope (STEM) analyses. Scattered patchy or lamellar lacroixite was contained in montebrasite and amblygonite in all observed specimens. TEM/STEM observations revealed that the patchy and lamellar texture comprised lacroixite and low-fluorine montebrasite having same crystal orientations as that of host montebrasite and the boundaries corresponded to well-developed {110} planes. The observed microtexture was newly discovered, and it is an important evidence of the exsolution process. In XRD experiments conducted at high temperature, the unit-cell parameters of amblygonite were closer to that of monoclinic structures such as lacroixite with increasing temperature. Results suggested that scattered patches or lamellae of lacroixite were exsolution textures from a high-temperature phase. Montebrasite and amblygonite specimens from other localities involved varying textures corresponding to their occurrence. The amblygonite-montebrasite series from petalite-bearing pegmatite included low to high lacroixite contents and that from lower-temperature pegmatite with spodumene either did not possess or involved low lacroixite contents. Gem-quality montebrasite from drusy vugs formed at low temperature did not include any exsolution texture or lacroixite. The variety of texture of the amblygonite-montebrasite series indicated in this study generated new possibilities as the indicator of pegmatite-forming process.

Original languageEnglish
Pages (from-to)75-84
Number of pages10
JournalAmerican Mineralogist
Volume103
Issue number1
DOIs
Publication statusPublished - Jan 26 2018

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pegmatite
textures
Textures
texture
exsolution
Electron microscopes
electron microscopes
electron
Bearings (structural)
Temperature
spodumene
X-ray diffraction
Gems
Scanning
scanning
Fluorine
fluorine
lamella
Crystal orientation
electron probe analysis

All Science Journal Classification (ASJC) codes

  • Geophysics
  • Geochemistry and Petrology

Cite this

Microtexture investigation of amblygonite-montebrasite series with lacroixite : Characteristics and formation process in pegmatites. / Shirose, Yohei; Uehara, Seiichiro.

In: American Mineralogist, Vol. 103, No. 1, 26.01.2018, p. 75-84.

Research output: Contribution to journalArticle

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