Annite is the Fe rich end-member of the phlogopite-biotite-annite series of micas. The ideal structural formula of annite is: $\{K\sp+\}\lbrack Fe\sbsp{3}{2+}\rbrack\langle Al\sp{3+}Si\sbsp{3}{4+}\rangle O\sbsp {2-}(OH\sp-)\sb2.$ Two samples of annite have been studied, a synthetic, true end-member (in the form of a fine powder) and a natural, near end-member (in the form of single crystal). The samples have been studied using SQUID magnetometry and Mossbauer spectroscopy at temperatures ranging from 4.2 K to 295 K using a purpose-built Mossbauer cryostat and insert. In disagreement with earlier predictions it has been determined that the magnetic ground state of annite consists of planes of predominantly ferromagnetic spins stacked antiferromagnetically along the c* axis. In further contrast to earlier predictions, it has been found that, for annite, the ordering temperature, T$\sb{\rm N}$, is greater than 10 K, with the synthetic sample having T$\sb{\rm N}$ = 58 K. It is proposed that the planar antiferromagnetic structure is part of the intrinsic zero-field magnetic domain structure and that this structure is stabilized by inter-layer dipole-dipole forces. Mossbauer spectroscopy measurements indicate some striking features of the temperature development of magnetic state of both annite samples. In particular, the results indicate that there is a persistent paramagnetic Fe$\sp{2+}$ contribution at temperatures as low as 0.1 T$\sb{\rm N}$; and that the Fe$\sp{3+}$ moments disorder at temperatures significantly below T$\sb{\rm N}.$ Plausible causes for these features are discussed. The first documented attempt to fit the low temperature spectrum of annite is reported. A method for stripping the Fe$\sp{3+}$ contribution from the liquid helium spectrum of synthetic annite is discussed and a hierarchy of simple models is constructed and applied in attempt to fit the remaining Fe$\sp{2+}$ subspectrum. An acceptable fit is not obtained. The consequences of the failure of the simple models are examined.
| Identifer | oai:union.ndltd.org:uottawa.ca/oai:ruor.uottawa.ca:10393/6646 |
| Date | January 1994 |
| Creators | Christie, Iain A. D. |
| Contributors | Rancourt, Denis, |
| Publisher | University of Ottawa (Canada) |
| Source Sets | Université d’Ottawa |
| Detected Language | English |
| Type | Thesis |
| Format | 258 p. |
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