T118 Petrologic Mineralogy—The Study of Minerals in Context (Posters): In Honor of Charles V. Guidotti

HIGH-PRESSURE RAMAN SPECTROSCOPIC STUDY OF PREHNITE, CA2AL(ALSI3O10)(OH)2

Paper number: T118-10
Program: Technical Sessions
Time: 1:30 PM – 5:30 PM

Authors

Abstract

The mineral prehnite, Ca2Al(AlSi3O10)(OH)2 , is an important secondary mineral in mafic igneous rocks and is paired with pumpellyite as a metamorphic grade indicator between the zeolite and greenschist facies. Prehnite is a layer silicate, having alternating sheets of AlO6 octahedra and double sheets of AlO4/SiO4 tetrahedra. The calcium occupies cages created by the sheets and the H atoms are bonded to the apical oxygen atoms of the AlO6 octahedra. The average structure of prehnite, determined by Papike and Zoltai (1967), was found to be Pncm. This structure has been confirmed in a single crystal X-ray diffraction study, using a sample of prehnite from Sterzing, Tyrol. Different space group symmetries, including P2cm, P2/n, and the average structure, Pncm, arise from differences in Al/Si ordering on the tetrahedral sites. Raman experiments are underway on a suite of prehnite samples from a range of localities to determine differences that might arise from different Al/Si ordering schemes.

In addition, we report results from the first high-pressure Raman spectroscopic study of prehnite. A single crystal of prehnite from Bealetan Quarry, Fauquior County, Virginia was loaded in a diamond anvil cell along with a ruby sphere for pressure calibration. A 4:1 mixture of methanol:ethanol was used as the pressure-transmitting medium. Raman spectra were collected at ~1 GPa increments between 1 bar and 10 GPa. Fourteen peaks between 200 and 1200 cm-1 were tracked as a function of pressure and all show a slight increase in frequency as pressure increases. No discontinuities in peak positions were observed. Three peaks between 3200 and 3600 cm-1 assigned to O-H stretching modes were also tracked as a function of pressure. Between 1 bar to 8 GPa, the Raman frequencies of the O-H stretching modes increase with increasing pressure, but above 8 GPa, the O-H stretching modes display a decrease in frequency. Further work is underway to confirm and explore this intriguing result.