TY - JOUR

T1 - Natural Cr3+-rich ettringite

T2 - occurrence, properties, and crystal structure

AU - Seryotkin, Yurii V.

AU - Sokol, Ella V.

AU - Kokh, Svetlana N.

AU - Murashko, Mikhail N.

PY - 2018/3/1

Y1 - 2018/3/1

N2 - Cr3+-rich ettringite with Cr3+→Al substitution and Cr/(Cr + Al) ratios up to 0.40–0.50 was found in mineral assemblages of the Ma’aleh Adumim area of Mottled Zone (Judean Desert). The Cr3+-rich compositions were the latest in the thaumasite → ettringite–thaumasite solid solution → ettringite → ettringite–bentorite solid solution series. The mineral-forming solution was enriched in Cr3+ and had a pH buffered by afwillite at ~11–12. Chromium was inherited from larnite rocks produced by high-temperature combustion metamorphic alteration of bioproductive calcareous sediments. The Cr/(Cr + Al) ratios are within 0.10–0.15 in most of the analysed crystals. This degree of substitution imparts pink colouration to the crystals, but does not affect their habit (a combination of monohedra and a prism). The habit changes to pyramid faces in coarse and later Cr3+-bearing crystals as Cr/(Cr + Al) ratios increase abruptly to 0.40–0.50. Single-crystal XRD analysis of one Cr-free and two Cr3+-rich samples and their structure determination and refinement indicate that the Cr-rich crystals (with Cr/(Cr + Al) to 0.3) preserve the symmetry and metrics of ettringite. The Ca–O bonding network undergoes differentiation with increase of Cr3+ concentration at octahedral M sites. The compression of Ca2 and expansion of Ca1 polyhedra sub-networks correlates with the degree of Cr3+→Al substitution.

AB - Cr3+-rich ettringite with Cr3+→Al substitution and Cr/(Cr + Al) ratios up to 0.40–0.50 was found in mineral assemblages of the Ma’aleh Adumim area of Mottled Zone (Judean Desert). The Cr3+-rich compositions were the latest in the thaumasite → ettringite–thaumasite solid solution → ettringite → ettringite–bentorite solid solution series. The mineral-forming solution was enriched in Cr3+ and had a pH buffered by afwillite at ~11–12. Chromium was inherited from larnite rocks produced by high-temperature combustion metamorphic alteration of bioproductive calcareous sediments. The Cr/(Cr + Al) ratios are within 0.10–0.15 in most of the analysed crystals. This degree of substitution imparts pink colouration to the crystals, but does not affect their habit (a combination of monohedra and a prism). The habit changes to pyramid faces in coarse and later Cr3+-bearing crystals as Cr/(Cr + Al) ratios increase abruptly to 0.40–0.50. Single-crystal XRD analysis of one Cr-free and two Cr3+-rich samples and their structure determination and refinement indicate that the Cr-rich crystals (with Cr/(Cr + Al) to 0.3) preserve the symmetry and metrics of ettringite. The Ca–O bonding network undergoes differentiation with increase of Cr3+ concentration at octahedral M sites. The compression of Ca2 and expansion of Ca1 polyhedra sub-networks correlates with the degree of Cr3+→Al substitution.

KW - Bentorite

KW - Chromium

KW - Cr–ettringite

KW - Crystal structure

KW - Morphology

KW - Cr3+-ettringite

KW - PHASES

KW - MINERALS

KW - SOLUBILITY

KW - CR

KW - SOLID-SOLUTION

KW - THAUMASITE

KW - SULFATE

KW - MOTTLED ZONE COMPLEX

KW - CENTER-DOT 12H(2)O

KW - TETRACALCIUM ALUMINOFERRITE

UR - http://www.scopus.com/inward/record.url?scp=85028016773&partnerID=8YFLogxK

U2 - 10.1007/s00269-017-0917-y

DO - 10.1007/s00269-017-0917-y

M3 - Article

AN - SCOPUS:85028016773

VL - 45

SP - 279

EP - 292

JO - Physics and Chemistry of Minerals

JF - Physics and Chemistry of Minerals

SN - 0342-1791

IS - 3

ER -