Bull. Minéral. (1981), 104, 578-581.

The refinement of the crystal structure of carboborite MgCa2[C03]2[B(0H)4]2 .4H20

Affinement de la structure cristalline de carboborite MgCa2[C03]2\_B(0H)�]2.4H20.

1. Experimental

Carboborite is a new hydro-bearing borate mineral discovered in China by Xie Xiande et al. (1964). It was determined to be monoclinic system by single crystal X-ray analysis : a = 11.32Â, b = 6.68 Â, c = 18.59 Â, (3 = 91° 41'. An unit cell contains :

4{MgO ,2CaO .C02 B203 . 10 H20} (1).

The crystal forms of carboborite are well-developed, being a sharp pseudo-rhombohedro-shaped, with developing forms e{101}, m{110}, n{121}, x{01 1 }, a{100}, s{101}. An elongated crystal measuring 0.2 mm x 0.15 mm x 0.15 mm-was selec¬ ted for the X-ray diffraction study. Initially, the Weis-senberg photographs were adopted to confirm that it was really a single crystal, with monoclinic system, P2l/n and approximate cell dimensions a = 11.05Â. b = 6.68 À, c = 10.69 À, (3 = 116.8° (2). While, accor¬ ding to the new data of chemical analysis result obtained the chemical formula has been corrected as : MgO . 2CaO . 2C02 . B203 . 8H20.

For the intensity data collection the crystal was mounted along its long dimension on Rigaku, RASA-IIS 4-circle automatic diffractometer equipped with

Table I. — Carboborite unit cell parameters.

a graphite monochromator (MoKa radiation). The positions of twenty reflections with 7° < 20 < 13° were measured exactly and the 20 values were refined by least squares method ; the lattice parameters obtained are listed in table I. The refraction intensities of 1964 hkl reflections were measured. A co — 20 scan, 3°-65°, Aco = (1.63 + 0.5 tan 0) and a speed of 4°/min for co scan. The background measurement time is 5 sec. The Lorentz and polarization corrections were applied.

2. Determination of the structure and its description

In order to explore the distributions of the heavy atoms Ca, Mg in the structure, first, the Patterson functions P(UW), P(UOW), P(U1/2W) were calcula-