• journal_title：American Mineralogist
• Contributor：Sarah E.M. Palaich ; Craig E. Manning ; Edwin Schauble ; Abby Kavner
• Publisher：Mineralogical Society of America
• Date：2013-08-01
• Format：text/html
• Language：en
• Identifier：10.2138/am.2013.4384
• journal_abbrev：American Mineralogist
• issn：0003-004X
• volume：98
• issue：8-9
• firstpage：1543
• section：Articles

The rare evaporite minerals hanksite, Na₂₂K(SO₄)₉(CO₃)₂Cl, and tychite, Na₆Mg₂(CO₃)₄(SO₄), are excellent case studies for the high-pressure behavior of ionic groups since their structures combine ionic complexity and high symmetry (hexagonal <em>Pem>6₃/<em>mem> and cubic <em>Fdem>3, respectively). Here we investigate the structure and compressibility of hanksite up to 20 GPa in the diamond-anvil cell using Raman spectroscopy and X-ray diffraction and of tychite up to 17 GPa in the diamond cell using X-ray diffraction and first-principles modeling. At ambient pressure, the Raman spectrum of hanksite has a single sulfate ν₁ frequency at 992 cm⁻¹ with a lower-frequency shoulder. As pressure is increased, this mode splits into two distinct peaks, which arise from two distinct local environments for the sulfate tetrahedra within the hanksite structure. Below 10 GPa, the mode Grüneisen parameter of the dominant sulfate ν₁ frequency is 0.27(1); the mode Grüneisen parameter of the lower frequency shoulder is 0.199(7). X-ray diffraction data of hanksite indicate a 5% volume drop between 8–10 GPa with no apparent change of symmetry. A Birch-Murnaghan fit to the data below 8 GPa yields an isothermal bulk modulus of 66(1) GPa for hanksite and 85(1) GPa for tychite, with <em>K′em> fixed at 4.