The layer structure of HBi differs from that of TcBi by the presence of octahedral [[Manganese|Mn<sup>4+</sup>]] vacancies (Vac). The chemical formula of synthetic HBi depends on [[pH]]. The generic formula is H<sup>+</sup>''<sub>x</sub>'' Mn<sup>3+</sup>''<sub>y</sub>'' Mn<sup>2+</sup>''<sub>z</sub>'' (Mn<sup>4+</sup>''<sub>u</sub>'' Mn<sup>3+</sup>''<sub>v</sub>''Vac''<sub>w</sub>'')O<sub>2</sub>, with ''x'' + 3''y'' + 2''z'' = ''v'' + 4''w'' for neutrality.<ref name=":3" /><ref name=":4" /><ref name=":5" />
The MnO<sub>2</sub> layers are stacked periodically in synthetic [[Triclinic crystal system|triclinic]] and [[Hexagonal crystal family|hexagonal]] birnessite [[Crystal|crystals]]. It is, however, rarely the case in natural materials.<ref>{{Cite journal|last=Manceau|first=A|display-authors=1|last2=Marcus|first2=M. A.|last3=Grangeon|first3=S.|last4=Lanson|first4=M.|last5=Lanson|first5=B.|last6=Gaillot|first6=A.-C.|last7=Skanthakumar|first7=S.|last8=Soderholm|first8=L.|date=2013|title=Short-range and long-range order of phyllomanganate nanoparticles determined using high-energy X-ray scattering|url=http://dx.doi.org/10.1107/s0021889812047917|journal=Journal of Applied Crystallography|volume=46|pages=193–209|doi=10.1107/s0021889812047917}}</ref> In addition to being chemically complex, natural birnessite [[Crystal|crystals]] are structurally disordered with respect to the layer stacking and the flatness of the layers.<ref>{{Cite journal|last=Manceau|first=A|display-authors=1|last2=Marcus|first2=Matthew A.|last3=Grangeon|first3=S.|last4=Lanson|first4=M.|last5=Lanson|first5=B.|last6=Gaillot|first6=A.-C.|last7=Skanthakumar|first7=S.|last8=Soderholm|first8=L.|date=2013-02-01|title=Short-range and long-range order of phyllomanganate nanoparticles determined using high-energy X-ray scattering|url=http://scripts.iucr.org/cgi-bin/paper?S0021889812047917|journal=Journal of Applied Crystallography|volume=46|issue=|pages=193–209|doi=10.1107/S0021889812047917|issn=}}</ref> A natural birnessite crystal may contain only a few layers, and they are often bent and always imperfectly stacked with orientational and translational loss of registry. The stacking disorder is referred to as “turbostratic” when the layers are oriented completely at random. Natural birnessite with turbostratically stacked layers is named vernadite,<ref>{{Cite journal|last=Giovanoli|first=R.|date=1980|title=Vernadite is random-stacked birnessite|url=http://dx.doi.org/10.1007/bf00206520|journal=Mineralium Deposita|volume=15|pages=251-253|doi=10.1007/bf00206520}}</ref><ref>{{Cite journal|last=Manceau|first=A.|display-authors=1|last2=Combes|first2=J. M.|date=1988|title=Structure of Mn and Fe oxides and oxyhydroxides: A topological approach by EXAFS|url=http://dx.doi.org/10.1007/bf00307518|journal=Physics and Chemistry of Minerals|volume=15|pages=283–295|doi=10.1007/bf00307518}}</ref><ref>{{Cite journal|last=Hochella|first=M. F.|date=2005|title=Environmentally important, poorly crystalline Fe/Mn hydrous oxides: Ferrihydrite and a possibly new vernadite-like mineral from the Clark Fork River Superfund Complex|url=http://dx.doi.org/10.2138/am.2005.1591|journal=American Mineralogist|volume=90|pages=718–724|doi=10.2138/am.2005.1591}}</ref> and the synthetic analog is named δ-MnO<sub>2</sub>. The layer spacing of vernadite can be also ~7 [[Angstrom|Å]] or ~10 [[Angstrom|Å]], and interstratification of the two types of layers has been observed on quartz coatings<ref name=":1" /> and in ferromanganese crusts.<ref>{{Cite journal|last=Lee|first=S|display-authors=1|last2=Xu|first2=Hfang|last3=Xu|first3=Wenqian|last4=Sun|first4=Xiaoming|date=2019|title=The structure and crystal chemistry of vernadite in ferromanganese crusts|url=https://scripts.iucr.org/cgi-bin/paper?S2052520619006528|journal=Acta Crystallographica Section B Structural Science, Crystal Engineering and Materials|volume=75|pages=591–598|doi=10.1107/S2052520619006528}}</ref>
