In the Fregeneda area different pegmatitic types can be distinguished on the basis of their mineralogy, internal structure and field relationships. The most common type corresponds with simple pegmatites with a homogeneous internal structure, but Li and Sn-bearing pegmatites are also relatively widespread, besides a minority group of Fe-Mn phosphate-bearing pegmatites that has recently been characterized. These pegmatites are located in an intermediate zone, between the barren pegmatites and the most evolved Li and Sn-bearing bodies, and they carry a complex association of phosphate minerals. The study of these phosphates has allowed the identification of the primary phases as wyllieite, graflonite, sarcopside, triplite-zwieselite and ferrisicklerite; the secondary phosphates are rosemaryite, heterosite-purpurite, alluaudite and v/iyrynenite. In this study, the main characteristics of these phosphate minerals are reported, including their chemical composition, analysed by electron microprobe, and their unit-cell parameters, calculated using X-ray powder diffraction techniques.A common transformation mechanism in this phosphate association is the oxidation of the transition metal cations at the same time as Na-leaching in wyllieite to generate rosemaryite, and Li-leaching in ferrisicklerite to generate heterosite. The occurrence of sarcopside lamellae in ferrisicklerite and heterosite is evidence of the replacement processes of the former by the latter. A Na-metasomatic replacement of the early phosphates as ferrisicklerite and graftonite, producing alluaudite, is also a well developed process.Phosphate minerals occur in pegmatites with an intermediate degree of fractionation, appearing between the barren and the more evolved pegmatites with Li and Sn, which is in agreement with the pegmatite field zonation established in the literature.

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The Cañada pegmatite (Salamanca, Spain), a Li-P-(Sn-Nb ± Ta)-bearing granitic pegmatite, is intruded partly into a gabbro and partly into a leucogranite. Three phosphate associations have been distinguished, based on paragenesis, texture, and chemistry: (1) ferrisicklerite-magniotriplite-johnsomervilleite association (in the border zone), in which Mg-bearing phosphates and coexisting Fe-Mg silicates are abundant; (2) ferrisicklerite-graftonite association (in the transition zone) characterized by higher values of Fe/(Fe + Mg) than in the border zone; and (3) triphylite-sarcopside association (in an inner zone), which also includes minor montebrasite, ferrocolumbite, and cassiterite, typical of an evolved pegmatite facies. As a result of the decrease of Mg, the Fe/(Fe + Mg) ratios for phosphates, biotite, and tourmaline increase from the border to the inner association (e.g., for ferrisicklerite and graftonite, from 0.67 and 0.85 in the border to 0.94 and 0.98 in the inner association, respectively). This difference is particularly evident for biotite and tourmaline; for example, the Fe/(Fe + Mg) ratios for tourmaline range from 0.59 in the border to 0.86 in the inner zone. These variations seem to reflect contamination of marginal zones of the pegmatite by some type of reaction with the host gabbro. Thus, an evolutionary trend involving inward crystallization from the margins and contamination of fluids from wallrocks into pegmatite-forming melt may be a plausible genetic model. The occurrence of phosphates along with Fe-Mg silicates would indicate that the melt contained on the order of 1.3-2.4 wt% P 2 O 5 , based on experimental silicate-phosphate equilibria.

Micas of the muscoviteÀlepidolite series are main constituents of the evolved pegmatites from the Okatjimukuju-Kaliombo portion of the Karibib belt, Namibia. The compositional variations shown by the micas from the intermediate zones are mainly controlled by the Li 3 Al À1 & À2 and SiLi 2 Al À2 & À1 substitution schemes, whereas for the micas from the core margins and the replacement bodies, only the first of these two exchange vectors seems to operate. The chemical composition of the micas not only depends on the degree of pegmatite evolution, but also on the position in the internal zonation of the pegmatite. Micas from the core margins and the replacement units are generally richer in F, Li, Rb, Cs and Zn than those from the intermediate zones. In general, the contents of these elements increase with decreasing K/Rb ratio. However, some data departing from this general trend are also observed, which could be related to subsolidus processes. Some pegmatite bodies show a complete internal evolution, developed from the margins to the core zone, which is reflected in the chemical composition of the micas. The regional distribution of pegmatites does not define a zonation, because an overlapping of pegmatites with different degrees of evolution occurs. This could be due to the high level of evolution attained by most of the rare-element pegmatites, and to their topography with respect to a dome structure of the basement.