Abstract
The crystal chemistry of inderite, a hydrous borate with known ideal formula MgB 3 O 3 (OH) 5 ·5H 2 O from the Kramer deposit, was re-investigated by electron probe micro-analysis in wavelength dispersive mode, laser ablation-(multi collector-)inductively coupled plasma-mass spectrometry and single-crystal neutron diffraction. The chemical data prove that the real composition of the investigated inderite is substantially identical to the ideal one, with insignificant content of potential isomorphic substituents, so that, excluding B, inderite does not contain any other industrially-relevant element (e.g., Li concentration is lower than 2.5 wt ppm, Be or REE lower than 0.1 wt ppm). The average δ 11 B NIST951 value of ca. − 7 ‰ lies within the range of values in which the source of boron is ascribable to terrestrial reservoirs (e.g., hydrothermal brines), rather than to marine ones. Neutron structure refinements, at both 280 and 10 K, confirm that the building units of the structure of inderite consist of: two BO 2 (OH) 2 tetrahedra (B-ion in sp 3 electronic configuration) and one BO 2 (OH) triangle (B-ion in sp 2 electronic configuration), linked by corner-sharing to form a (soroborate) B 3 O 3 (OH) 5 ring, and a Mg-octahedron Mg(OH) 2 (OH 2 ) 4 . The B 3 O 3 (OH) 5 ring and the Mg-octahedron are connected, by corner-sharing, to form an isolated Mg(H 2 O) 4 B 3 O 3 (OH) 5 (molecular) cluster. The tri-dimensional edifice of inderite is therefore built by heteropolyhedral Mg(H 2 O) 4 B 3 O 3 (OH) 5 clusters mutually connected by H-bonds, mediated by the zeolitic (“interstitial”) H 2 O molecules lying between the clusters, so that the correct form of the chemical formula of inderite is Mg[B 3 O 3 (OH) 5 ](H 2 O) 4 ·H 2 O, rather than MgB 3 O 3 (OH) 5 ·5H 2 O. All the thirteen independent oxygen sites of the structure are involved in H-bonding, as donors or as acceptors. This confirms the pervasive nature and the important role played by the H-bonding network on the structural stability of inderite. The differences between the crystal structure of the two dimorphs inderite and kurnakovite are discussed.