Crystal chemistry of Zn-rich rhodonite ("fowlerite")

Abstract

Rhodonite (ideally MnSiO 3 ) is a triclinic pyroxenoid that typically contains signifi cant concentrations of Ca, Fe, and Mg. A variety called fowlerite contains concentrations of Zn up to 10 wt% and has been found only at Franklin, New Jersey. Data from electron microprobe analyses, single-crystal X-ray structure refi nements, and preliminary Mössbauer spectra suggest marked differences in the crystal chemistry of rhodonite and fowlerite. Cations in Zn-poor rhodonites and in fowlerites exhibit distinct substitutional trends. Substitution of Zn in fowlerite occurs entirely at M4, and Ca displays a stronger preference for M5 in fowlerite than in Zn-poor rhodonite. Further, the distinctive compositional variations in fowlerite lead to strongly mutually correlated lattice parameter variations distinct from those in Zn-poor rhodonite. Mean M-O distances in fowlerite, with the exception of <M2-O>, are strongly correlated with all unit-cell edges. For Zn-poor rhodonites, neither <M2-O> nor <M4-O> are signifi cantly correlated with cell edges, and the mean bond lengths for M1, M3, and M5 are correlated strongly with only a and b.The most striking distinctions between fowlerite and typical rhodonite occur in the geometrical details of M4 and M5. Distortion in the M4 site of magnesian rhodonite, typical Mn-rich rhodonite, and fowlerite increases with both increasing electron population and mean electronegativity. The M4 site assumes increasing amounts of tetrahedral character with increasing electron density (i.e., increasing Zn concentration). As M4 becomes more Zn-rich, tetrahedral, and covalent in character, M5 becomes more Ca-rich, less distorted, and more ionic in character. This suggests that "pure" fowlerite (nominally Ca 0.2 Zn 0.2 Mn 0.6 O 3 , or CaZnMn 3 O 15 ) may contain an unequivocally four-coordinated M4 site occupied solely by Zn, and with Ca largely restricted to M5.

Mossbauer spectroscopy of Zn-poor and Zn-rich rhodonite

“…Mössbauer spectra were obtained for specimens listed in Table 2 of Nelson and Griffen (2005). Localities and microprobe analyses are given in Table 1 of Nelson and Griffen (2005).…”

“…The structure is illustrated and described in detail in Nelson and Griffen (2005). The nontetrahedral cation sites are designated M1 through M5 (Fig.…”

“…M1, M2, and M3 are octahedra with differing degrees of distortion; M4 is a quite distorted octahedron with the cation displaced suffi ciently from the centroid that several workers have considered it to be 5-coordinated (e.g., Ohashi and Finger 1975;Peacor and Niizeki 1963;Peacor et al 1978;Pinckney and Burnham 1988); and M5 is usually considered an irregular 7-coordinated site. Nelson and Griffen (2005) refi ned the structures of several specimens of both "normal" (Zn-poor) rhodonite from various localities and Zn-rich rhodonite (variety name "fowlerite") from Franklin, New Jersey, its only known locality. They found that the structural differences between "normal" rhodonite (referred to hereafter simply as rhodonite) and Zn-rich rhodonite (fowlerite hereafter) occur mainly at the M4 and M5 sites.…”

“…Concomitantly, M5 becomes more Ca-rich and less distorted in terms of bond lengths (angular distortion being diffi cult to quantify for sevenfold coordination). Nelson and Griffen (2005) used site electronegativity values, based on cation distributions constrained by microprobe analyses and electron populations from X-ray site refi nements, to conclude that the increase in Zn and Ca in fowlerite, relative to rhodonite, leads to more covalent character for M4 and more ionic character for M5. Their preliminary Mössbauer spectra suggested that Fe is present in all M sites in both rhodonite and fowlerite, although not necessarily equally distributed among those sites.…”

“…Dickson interpreted his results to indicate that Fe distribution in rhodonite is essentially independent of bulk Fe concentration. Nelson and Griffen (2005) showed that rhodonite and fowlerite exhibit distinct crystal-chemical trends. The present study was undertaken to ascertain whether such differences are refl ected in the Mössbauer spectra.…”