TY - JOUR
T1 - Low-temperature evolution of OH bands in synthetic forsterite, implication for the nature of H defects at high pressure
AU - Ingrin, J.
AU - Liu, J.
AU - Depecker, C.
AU - Kohn, S. C.
AU - Balan, E.
AU - Grant, K. J.
PY - 2013/6
Y1 - 2013/6
N2 - We performed in situ infrared spectroscopic measurements of OH bands in a forsterite single crystal between -194 and 200 °C. The crystal was synthesized at 2 GPa from a cooling experiment performed between 1,400 and 1,275 °C at a rate of 1 °C per hour under high silica-activity conditions. Twenty-four individual bands were identified at low temperature. Three different groups can be distinguished: (1) Most of the OH bands between 3,300 and 3,650 cm-1 display a small frequency lowering (<4 cm-1) and a moderate broadening (<10 cm-1) as temperature is increased from -194 to 200 °C. The behaviour of these bands is compatible with weakly H-bonded OH groups associated with hydrogen substitution into silicon tetrahedra; (2) In the same frequency range, two bands at 3,617 and 3,566 cm-1 display a significantly anharmonic behaviour with stronger frequency lowering (42 and 27 cm-1 respectively) and broadening (~30 cm-1) with increasing temperature. It is tentatively proposed that the defects responsible for these OH bands correspond to H atoms in interstitial position; (3) In the frequency region between 3,300 and 3,000 cm-1, three broad bands are identified at 3,151, 3,178 and 3,217 cm-1, at -194 °C. They exhibit significant frequency increase (~20 cm-1) and broadening (~70 cm-1) with increasing temperature, indicating moderate H bonding. These bands are compatible with (2H)Mg defects. A survey of published spectra of forsterite samples synthesized above 5 GPa shows that about 75 % of the incorporated hydrogen belongs to type (1) OH bands associated with Si substitution and 25 % to the broad band at 3,566 cm-1 (type (2); 3,550 cm-1 at room temperature). The contribution of OH bands of type (3), associated to (2H)Mg defects, is negligible. Therefore, solubility of hydrogen in forsterite (and natural olivine compositions) cannot be described by a single solubility law, but by the combination of at least two laws, with different activation volumes and water fugacity exponents.
AB - We performed in situ infrared spectroscopic measurements of OH bands in a forsterite single crystal between -194 and 200 °C. The crystal was synthesized at 2 GPa from a cooling experiment performed between 1,400 and 1,275 °C at a rate of 1 °C per hour under high silica-activity conditions. Twenty-four individual bands were identified at low temperature. Three different groups can be distinguished: (1) Most of the OH bands between 3,300 and 3,650 cm-1 display a small frequency lowering (<4 cm-1) and a moderate broadening (<10 cm-1) as temperature is increased from -194 to 200 °C. The behaviour of these bands is compatible with weakly H-bonded OH groups associated with hydrogen substitution into silicon tetrahedra; (2) In the same frequency range, two bands at 3,617 and 3,566 cm-1 display a significantly anharmonic behaviour with stronger frequency lowering (42 and 27 cm-1 respectively) and broadening (~30 cm-1) with increasing temperature. It is tentatively proposed that the defects responsible for these OH bands correspond to H atoms in interstitial position; (3) In the frequency region between 3,300 and 3,000 cm-1, three broad bands are identified at 3,151, 3,178 and 3,217 cm-1, at -194 °C. They exhibit significant frequency increase (~20 cm-1) and broadening (~70 cm-1) with increasing temperature, indicating moderate H bonding. These bands are compatible with (2H)Mg defects. A survey of published spectra of forsterite samples synthesized above 5 GPa shows that about 75 % of the incorporated hydrogen belongs to type (1) OH bands associated with Si substitution and 25 % to the broad band at 3,566 cm-1 (type (2); 3,550 cm-1 at room temperature). The contribution of OH bands of type (3), associated to (2H)Mg defects, is negligible. Therefore, solubility of hydrogen in forsterite (and natural olivine compositions) cannot be described by a single solubility law, but by the combination of at least two laws, with different activation volumes and water fugacity exponents.
KW - Band shift
KW - Forsterite
KW - H defects
KW - Hydrogen
KW - Infrared
KW - OH bands
KW - Olivine
KW - Silicon vacancies
KW - Water solubility laws
UR - http://www.scopus.com/inward/record.url?scp=84878523712&partnerID=8YFLogxK
U2 - 10.1007/s00269-013-0587-3
DO - 10.1007/s00269-013-0587-3
M3 - Article
AN - SCOPUS:84878523712
SN - 0342-1791
VL - 40
SP - 499
EP - 510
JO - Physics and Chemistry of Minerals
JF - Physics and Chemistry of Minerals
IS - 6
ER -