First crystal-structure determination of natural lansfordite, MgCO₃·5H₂O

Abstract

This study presents the first crystal-structure determination of natural MgCO3·5H2O, mineral lansfordite, in comparison with previous structural works performed on synthetic analogues. A new prototype single-crystal X-ray diffractometer allowed us to measure an extremely small crystal (i.e. 0.020 mm × 0.010 mm × 0.005 mm) and refine anisotropically all non-hydrogen atoms in the structure and provide a robust hydrogen-bond arrangement. Our new data confirm that natural lansfordite can be stable for several months at room temperature, in contrast with previous works, which reported that such a mineral could be stable only below 10°C.

A new hydrate of magnesium carbonate, MgCO3·6H2O

“…MgCO 3 Á5H 2 O (mineral name: lansfordite), is known. Its crystal structure (monoclinic space group P2 1 /m) was determined by Liu et al (1990) from a synthetic sample and by Nestola et al (2017) from a mineral. Several possibilities are described to synthesize lansfordite (Ming & Franklin, 1985;Liu et al, 1990).…”

“…Besides that, there are contradictory statements about the temperature and the rate of conversion of the pentahydrate to the trihydrate. Some research groups have recorded a transition temperature between 283.15 and 288.15 K (Takahashi & Hokoku, 1927;Gloss, 1937;Yanaťeva & Rassonskaya, 1961;Hill et al, 1982;Langmuir, 1965;Ming & Franklin, 1985), while others observed the stability of synthesized and natural samples of lansfordite at room temperature over a period of a few months (Liu et al, 1990;Nestola et al, 2017). Neutral magnesium carbonates with a water content greater than five units per formula are not known up to now.…”