Sborgit und β-Sborgit — eine zweite synthetische Modifikation von Na[B5O6(OH)4] · 3 H2O / Sborgit and β-Sborgit – a Second Synthetic Modification of Na[B5O6(OH)4] · 3 H2O

Abstract

In crystallization experiments for silver borate hydrates from solutions of borax, boric acid, and silver nitrate, a mixture of sborgite and a second modification — named by us “β-sborgite” — have been found besides excess H3BO3 and a microcrystalline silver borate. Both modifications contain the same distorted pentaborate ion [B5O6(OH)4]-. Both Na+ ions in the asymmetric unit are in special positions along the binary axis; one, Na(1), is octahedrally coordinated by four H2O molecules and two OH groups, the other one, Na(2), is tetrahedrally coordinated by two H2O molecules, two OH groups and — at longer distances — with two bridging oxygen atoms of the pentaborate unit. The different angles — only a small rotation on the b-axis — of the pentaborate anions between an identical matrix of Na-cations give rise to the two different modifications.