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The real structure of tobermorite 11A: normal and anomalous forms, OD character and polytypic modifications

2001; Elsevier BV; Volume: 13; Issue: 3 Linguagem: Inglês

10.1127/0935-1221/2001/0013-0577

1617-4011

Stefanο Merlino, Elena Bonaccorsi, Thomas Armbruster,

X-ray Diffraction in Crystallography

Specimens of tobermorite 11A from Wessels mine, South Africa, an 'anomalous' tobermorite, and from BaIscenov, Urals, Russia, a 'normal' tobermorite, have been studied through X-ray diffraction. Their structuraldisorder, evidencedby the diffusenessof the reflectionswithk odd, has beendealtwiththrough an OD approach, which allowed us to derive a reliable model of the structure, to define the two main polytypes (MDO structures),which were subsequently refined, revealingthe distinctive structural details of the two specimens. The MDO1polytype oftobermorite11A fromWesselsmine (orthorhombic, spacegroup F2dd,a=11.265(2), b = 7.386(1), c = 44.970(9) A) was refinedto R = 0.128, whereas the refinement carriedon the MDO2 polytype (monoclinic, space group B11m, a = 6.735(2), b = 7.385(1), c = 22.487(4) A, = 123.25(1)°) converged to R = 0.051. Two refinements were carried out for the MDO2 polytype of the specimen from the Urals (space group B11m, a = 6.732(2), b = 7.368(1), c = 22.680(4) A, = 123.18(1)°) with data collectedwith conventional source (R = 0.089) and with synchrotron radiation(R = 0.110). Common structural features are the infinite layers of calcium polyhedra, parallel to (001), with tetrahedral chainsof wollastonite-type running along b and attachedon both sides of these layers.The composite layers are stacked along c and connected through formation of double tetrahedral chains, with 'ideal' composition (Si6O15(OH)2)and(Si6O16(OH))inthespecimensfromWesselsmineandUralsrespectively.Themostdistinctive featureof the two specimensisthe content of the structuralcavities:whereasonly watermoleculeswerefound in the specimen from Wessels mine, 'zeolitic'calcium cations and water molecules were found in that from Urals. Thesestructuralaspectsarecomparedandtheirrelevanceinexplaining thedifferentthermalbehaviour ofthe two types of tobermorite 11A is indicated.