Abstract
The phase region surrounding the composition (Mg,Mn) 3BO 5, the nominal formula of the phases ludwigite, orthopinakiolite, takéuchiite, and pinakiolite, has been investigated by powder X-ray diffraction and transmission electron microscopy. It has been found that in samples prepared at 1000°C or higher the phases lie in the sequence ludwigite, takéuchiite, and orthopinakiolite as the composition increases in oxygen and Mn 3+ content. All compounds appear to be nonstoichiometric and the composition (Mg,Mn) 3BO 5 has no special significance at 1000°C. The commonest defect microstructures observed were wider or narrower than normal twin plane spacings and coherent intergrowths of one structure type in another. Pinakiolite only formed at temperatures up to approximately 900°C. An alternative twinned form of pinakiolite, utilizing (100) twin planes, not previously observed before in synthetic material, has been characterized. It is suggested that takéuchiite is correctly regarded as a 1:1 intergrowth of ludwigite and orthopinakiolite and that pinakiolite and ludwigite are high and low temperature polymorphs with similar compositions.
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