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

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.