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In this first single-crystal Mössbauer study of mica we have measured the room-temperature spectra for different orientations (with respect to the -γ-ray flux) of a well-characterized monocrystalline wafer of natural phlogopite. We apply a new method (D. G. Rancourt, Nucl. Instrum. Methods Phys. Res. B44, 199 (1989). of performing thickness corrections, which yields the intrinsic absorber cross section. The simplest physical model that is consistent with all the spectra is described as follows. The Fe2+ spectral contribution is due to correlated distributions of hyperfine electric-field gradient (efg) magnitudes and directions that can be understood in terms of two families of octahedral sites: a cis-like family having a narrower distribution of larger efg magnitudes and directions in the ab-plane and probably along the b axis, and a trans-like defect family having a broader distribution of smaller efg magnitudes and more out-of-plane efg directions. The Fe3+ spectral component is also due to broad and correlated distributions of efg magnitudes and directions. Its efg components have signs opposite to those of Fe2+, with e2qQ > 0 and mostly out-of-plane directions. Contrary to many claims in the literature, we show that: (i) thickness effects are not negligible, (ii) meaningful cis/trans spectral-area ratios cannot be obtained by attempting to resolve the relevant overlapping spectral components, (iii) information concerning the hidden Fe3+ spectral lines (i.e., the low-energy components of Fe3+ quadrupole doublets) is impossible to obtain, and (iv) resolving the Fe3+ into cis and trans components is, at best, questionable.

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