Crystal Chemical Variations in Li- and Fe-Rich Micas from Pikes Peak Batholith (Central Colorado)

American Mineralogist, Volume 85, pages 1275–1286, 2000 Crystal chemical variations in Li- and Fe-rich micas from Pikes Peak batholith (central Colorado) MARIA FRANCA BRIGATTI,*,1 CRISTINA LUGLI,1 LUCIANO POPPI,1 EUGENE E. FOORD,† AND DANIEL E. KILE2 1Department of Earth Sciences, University of Modena and Reggio Emilia, 41100 Modena, Italy 2United States Geological Survey, Denver, Colorado 80225, U.S.A. ABSTRACT The crystal structure and M-site populations of a series of micas-1M from miarolitic pegmatites that formed within host granitic rocks of the Precambrian, anorogenic Pikes Peak batholith, central Colorado, were determined by single-crystal X-ray diffraction data. Crystals fall in the polylithionite- [6] siderophyllite-annite field, being 0 ≤ Li ≤ 2.82, 0.90 ≤ Fetotal ≤ 5.00, 0.26 ≤ Al ≤ 2.23 apfu. Ordering of trivalent cations (mainly Al3+) is revealed in a cis-octahedral site (M2 or M3), which leads to a lowering of the layer symmetry from C12/m(1) (siderophyllite and annite crystals) to C12(1) diperiodic group (lithian siderophyllite and ferroan polylithionite crystals). On the basis of mean bond length, the ordering scheme of octahedral cations is mostly meso-octahedral, whereas the mean electron count at each M site suggests both meso- and hetero-octahedral ordering, the calculated mean atomic numbers being M1 = M3 ≠ M2, M2 = M3 ≠ M1 and M1 ≠ M2 ≠ M3. As the siderophyllite content increases, so do the a, b, and c unit-cell parameters, as well as the refractive indices, primarily nβ. The tetrahedral rotation angle, α, is generally small (1.51 ≤ α ≤ 5.04°) and roughly increases with polylithionite content, whereas the basal oxygen out-of-plane tilting, ∆z, is sensitive both to octahedral composition and degree of order (0.0 ≤ ∆z ≤ 0.009 Å for siderophyllite and annite, 0.058 ≤ ∆z ≤ 0.144 Å for lithian siderophyllite and ferroan polylithionite crystals). INTRODUCTION (1993), and Rieder et al. (1996). An interesting feature of these This study concerns the crystal chemistry of trioctahedral trioctahedral micas is the octahedral ordering pattern. The ideal micas from pegmatites associated with granitic units of the layer symmetry reduces from C2/m to C2 space group, more anorogenic Pikes Peak batholith (PPB), central Colorado. The precisely, referring to diperiodic groups, from C12/m(1) to PPB is a result of a complex magmatic history of crystalliza- C12(1) (Dornberger-Schiff et al. 1982), as the result of a dif- tion and crustal assimilation, beginning with a mantle-derived ferent cation ordering in cis-octahedral sites. basaltic magma and culminating with the miarolitic cavity stage Depending on the cation distribution of octahedral sites, ˇ of pegmatite evolution that represents the final product of crys- Durovicˇ (1994) subdivided micas in: (1) homo-octahedral (all tallization (Simmons et al. 1987; Wobus and Hutchinson 1988; three octahedral sites are occupied by the same kind of ions), Cˇern´y, 1991; Foord et al. 1995; Kile and Foord 1998). Micas (2) meso-octahedral (one site is occupied by a cation different widely vary in composition, from annite in the host granite from that in the other two sites), (3) hetero-octahedral (each of through siderophyllite and ferroan phlogopite in graphic the three sites is differently occupied). The location of the ori- pegmatites, and finally to lithian siderophyllite and ferroan gin of the octahedral sheet corresponds: (1) to the M1 site for polylithionite in miarolitic cavities (Foord et al. 1995; Kile and homo-octahedral micas, (2) to the site with different occupa- Foord 1998). In miarolitic pegmatites, micas, which are fre- tion for meso-octahedral micas, and (3) to the site with the ˇ quently Li- and Fe-enriched, have been used as markers of lowest electron density in hetero-octahedral micas (Durovicˇ et chemical fractionation and as petrogenetic indicators, as well al. 1984). Consequently, two kinds of layers exist (Zvyagin 1997; Nespolo et al. 1999): the M1 layer has an origin of the as for characterizing relative HF, HCl, O2, and H2O fugacities in fluids (Foord et al. 1995; Kile and Foord 1998). Therefore, octahedral sheet in M1 site, whereas the M2 layer originates in the knowledge of crystal structure and chemistry, as well as of either M2 or M3 site. The M1 layer is far more common. Meso- octahedral cation partitioning, can further our understanding trioctahedral crystals along the trilithionite-polylithionite join, of pegmatite evolution and conditions of crystallization. solved in the space group C2/m for 1M [Takeda and Burnham The crystal structure of micas in the siderophyllite- 1969, Guggenheim 1981 (lepidolite-1M from Radkovice)] and polylithionite join was studied, for example, by Takeda et al. C2/c for 2M1 (Swanson and Bailey 1981) or 2M2 polytypes (1971), Sartori et al. (1973), Sartori (1976, 1977), Guggenheim [Guggenheim 1981 (lepidolite-2M2 from Radkovice)], suggest and Bailey (1977), Brown (1978), Guggenheim (1981), the presence of M1 layers. Hetero-trioctahedral Li-rich micas- Swanson and Bailey (1981), Backhaus (1983), Weiss et al. 1M (zinnwaldite-1M) refined by Guggenheim and Bailey (1977) and by Backhaus (1983) (lepidolite-1M) in C2 subgroup were built up of M2 layers and M1 layers, respectively. Furthermore, *E-mail: [email protected] the refinement of a 2M1 polytype (zinnwaldite-2M1), in the space †Deceased on January 8, 1998. group Cc (Rieder et al. 1996) indicates an octahedral cation 0003-004X/00/0009–1275$05.00 1275 1276 BRIGATTI ET AL.: CRYSTAL CHEMISTRY OF Li- AND Fe-RICH MICAS ordering similar to that reported by Guggenheim and Bailey (OH)– was measured by thermogravimetric analysis in Ar gas – (1977) for 1M polytype. flow to minimize the reaction 2FeO + 2(OH) → Fe2O3 + H2 + However, data are lacking on changes in geometrical and O2–, using a Seiko SSC 5200 thermal analyzer (heating rate 10 ordering parameters induced by variation in composition and °C/min and flow rate 200 mL/min). The determination based degree of order for crystals in the K-Li-Fe-Al-Si trioctahedral on the weight loss observed in the temperature range 500–1100 mica system. °C was adjusted for the F content. The Fe2+ amount (estimated Here, we attempt to (1) characterize the crystal structure of measure standard deviation σ < 4%) was estimated by a semi- 17 mica-1M crystals in the K-Li-Fe-Al-Si trioctahedral mica micro-volumetric method (Meyrowitz 1970). The chemical composition plane; (2) clarify the mechanisms of Li incorpo- formulae were based on O24–x–y(OH)xFy. Chemical composition ration in the layer; (3) identify the ordering pattern of the octa- (Table 2) was determined by combining the microprobe results hedral sites. with structure refinement (for Li, OH, and Fe2+). Optical data (Table 3) were determined using a conventional EXPERIMENTAL METHODS petrographic microscope and calibrated immersion oils. Two of the refractive indices, nβ and nγ, were determined by grain Samples mount on an approximately centered Bxa figure; nα was deter- The mica crystals (Table 1) represent the binary joins mined using spindle stage methods to attain proper orientation siderophyllite-polylithionite and annite-siderophyllite. All crys- of the X vibration direction. Determination of the refractive tals are from the shallow-seated to subvolcanic pegmatites of indices for lighter-colored micas was relatively straightforward, Precambrian Pikes Peak batholith, located in the southern Front and the error is given as ±0.002, whereas determination of the Range of Colorado, west of Colorado Springs and south and indices of the darker and more pleochroic samples, i.e., those southwest of Denver (Foord et al. 1995; Unruh et al. 1995; having indices above approximately 1.650, have errors of Kile and Foord 1998). The pegmatites in the southern part of ±0.004. The optic axial angle was determined by Mallard’s the batholith host miarolitic cavities where micas were often method, using an ocular micrometer. The 2VX was calculated associated with amazonite, smoky quartz, goethite, topaz, and (D = KnβsinV). For further details see Kile and Foord (1998). fluorite (Foord et al. 1995). X-ray single-crystal diffraction Chemical and optical determination Crystals were examined by the precession method, and those Significant compositional and distinct, sharp color zoning, belonging to 1M polytype were selected for cell dimensions resulting from primary growth fluctuations, are present in some and intensity data. In general, crystal quality was good, but in mica crystals formed in miarolitic cavities (samples 140, 177, some cases [310] twinning, 1Md sequences and different 54, 55, 114). Therefore, the crystals were initially examined by polytypes (mostly 3T) were found. scanning electron microscope (Philips SEM XL-40 with an X-ray diffraction data were collected on a Siemens auto- EDAX energy dispersive detector) by backscattered-electron mated four-circle diffractometer with rotating anode (graph- imaging and X-ray maps to distinguish homogeneous portions. ite-monochromated MoKα radiation of λ = 0.71073 Å at 52 Electron microprobe analyses were performed using a wave- kV and 140 mA operating conditions) using X-SCAN software length-dispersive ARL-SEMQ microprobe (operating condi- (Siemens 1996). Reflection intensities (+h, ±k, ±l) were col- tions: 15 kV accelerating voltage, 15 nA sample current, and lected using the ω scan mode (scan window 2.2–3.8°) and cor- defocused electron beam of about 3 µm spot size). The F con- rected for Lorentz-polarization effects and for absorption using tent was determined by the method reported by Foley (1989). a complete ψ scan (0–360° at 10° intervals in ϕ) with more Analyses and data reduction were done using the PROBE soft- than five uniformly distributed reflections with regard to 2θ (χ ware package (Donovan 1995). Li content was determined by > 80°). The unit-cell parameters were determined from the set- Emission Spectrometry Plasma (ICP, Varian Liberty 200). Then ting angles of more than 50 reflections in the range 15 ≤ 2θ ≤ 25 mg of each sample were digested with a mixture of HF (38%) 30°.

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