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Clintonite-1M: Crystal Chemistry and Its Relationships to Closely Associated Al-Rich Phlogopite

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Clintonite-1M: Crystal Chemistry and Its Relationships to Closely Associated Al-Rich Phlogopite American Mineralogist, Volume 82, pages 936±945, 1997 Clintonite-1M: Crystal chemistry and its relationships to closely associated Al-rich phlogopite ELISA ALIETTI,MARIA FRANCA BRIGATTI, AND LUCIANO POPPI Dipartimento di Scienze della Terra, UniversitaÁ Degli Studi di Modena, Via S. Eufemia, 19, I-41100, Modena, Italy ABSTRACT Crystal-structure re®nements were performed on clintonite-1M crystals [ideal compo- [6] [6] [4] sition Ca (Mg2Al) (SiAl3)O10(OH)2] from skarns of the Predazzo-Monzoni area and Adamello Massif (northern Italy) with the aim of characterizing some aspects of their crystal chemistry and their relationships with closely associated phlogopite-1M. In the clintonite samples examined, the tetrahedral composition ranges from Si1.19Al2.78Fe0.03 to [4] [6] [4] [6] Si1.28Al2.70Fe0.02, indicating that the extent of the exchange vector Al21 Mg22 Si (Al,M), which links trioctahedral with dioctahedral Ca-bearing brittle micas, was very limited. Single-crystal X-ray diffraction data were collected and structure re®nements completed [4] 31 in space group C2/m converging to Robs from 0.027 to 0.037 for six samples. The Al for [4]Si41 substitution, which is close to 70%, produces more regular and ¯atter tetrahedra than in the case of phlogopite, together with an increase in the thickness and in the lateral dimensions of the sheet; the presence of Al31 in octahedral coordination, on the other hand, reduces the dimensions of both M1 and M2 sites with a consequent decrease in the thick- ness of the sheet. The volume, the ¯attening angle C, and the central cation off-center shift (BLD) of the trans M1 octahedral site are greater than those of the cis M2 site, thus indicating a normal octahedral ordering. The high mis®t value (from 1.187 to 1.326 AÊ ) between tetrahedral and octahedral sheets is mostly compensated by the distortion of the tetrahedral ring (tetrahedral rotation angle a: 23.1 # a # 24.98). Relative to phlogopite, the interlayer separation in clintonite is reduced by about 0.6 AÊ . INTRODUCTION requiring Al31 substitution in both tetrahedral and octa- hedral sites. The trioctahedral brittle mica clintonite, ideally The X-ray structure and the crystal chemical complex- [6]Ca[6](Mg Al)[4](SiAl )O (OH) in composition, is not 2 3 10 2 ity of natural clintonites was reviewed by MacKinney et very common, but it is of mineralogical and petrological al. (1988). In this paper, we explore aspects of clintonite 31 interest. Clintonite has a high Al content, which violates crystal chemistry not previously treated comprehensively, the aluminium-avoidance principle of Loewenstein such as the relationship between clintonite and closely (1954), and it has important petrogenetic implications for associated phlogopite. thermally metamorphosed Ca- and Al-rich and Si-poor rocks (Olesch 1975; Olesch and Seifert 1976; Bucher- Nurminen 1976). The formation of clintonite in natural SAMPLE OCCURRENCE environments appears to be a function of both bulk rock composition and CO (a ) and potassium(a ) activi- The crystals examined occur in very small lenses of 2 CO21/2 KO contact metamorphic siliceous, argillitic marbles of the ties, which must be low. An increase ina leads to KO1/2 Predazzo-Monzoni Hills area and of the Adamello Mas- clintonite-phlogopite assemblages (Bucher-Nurminen sif. Both localities are in the Italian Alps (northern Italy). 1976). Polished thin sections of rock were ®rst examined on a In an experimental study (Olesch 1975), it was reported wavelength-dispersive electron microprobe (ARL- that the ®eld of homogeneous clintonite at constant H2O SEMQ) and on the scanning electron microscope (Philips pressure (P 5 2 kbar) occurs at temperatures up to 870 HO2 SEM XL40 with an EDAX energy dispersive detector) 8C and that the tetrahedral Si/Al ratio varies between 0.18 by backscattered-electron imaging and X-ray maps to test and 0.54. However, these relationships are not straight- crystal homogeneity. Homogeneous clintonite crystals forward because the [4]Al31 for [4]Si41 substitution is het- occur in samples Cli5a, Cli7c, Cli8a, and Cli8d from Toal erovalent and involves other heterovalent substitutions in de la Foja (Monzoni Hills) and in samples Cli9a and the structure to maintain electroneutrality. The primary Cli9b from Farinas di Stabio (Adamello Massif ), whereas [4] [6] [4] [6] exchange component in clintonite is Si21 Mg21 Al Al, in some samples from Toal de la Foja and in samples 0003±004X/97/0910±0936$05.00 936 ALIETTI ET AL.: CLINTONITE-1M 937 TABLE 1. Selected structural parameters and crystal chemical formulas of the Al-rich phlogopite crystals Phl1a Phl1b Phl2a Phl3a Phl4a Unit-cell parameters a (AÊ ) 5.306(1) 5.309(2) 5.305(2) 5.299(1) 5.307(2) b (AÊ ) 9.135(3) 9.180(5) 9.189(3) 9.179(2) 9.199(1) c (AÊ ) 10.272(3) 10.291(4) 10.286(3) 10.279(3) 10.291(2) b (8) 100.01(2) 100.00(4) 99.96(2) 99.90(2) 99.89(2) V (AÊ 3) 493.5 493.9 493.9 492.5 494.9 Tetrahedral and interlayer parameters ^T-O& (AÊ ) 1.660 1.662 1.663 1.666 1.661 Ê ^T-O&basal (A) 1.657 1.661 1.662 1.663 1.663 T-O3 (AÊ ) 1.667(2) 1.663(2) 1.665(2) 1.673(3) 1.656(2) V (AÊ 3) 2.346 2.355 2.359 2.371 2.352 a (8) 10.2 10.7 11.1 12.5 10.7 t (8) 110.1(1) 110.3(1) 110.1(1) 109.6(1) 110.3(1) TAV (8)2 0.48 0.89 0.51 0.03 0.91 BLDT (%) 0.261 0.086 0.096 0.210 0.226 ELDT (%) 0.515 0.551 0.439 0.221 0.418 Sheet thickness (AÊ ) 2.253 2.257 2.259 2.250 2.254 Interlayer separation (AÊ ) 3.480 3.487 3.484 3.515 3.481 Octahedral parameters ^M1-O& (AÊ ) 2.066 2.066 2.067 2.067 2.072 ^M2-O& (AÊ ) 2.065 2.065 2.064 2.066 2.070 Ê 3 VM1 (A ) 11.561 11.576 11.571 11.542 11.668 Ê 3 VM2 (A ) 11.536 11.556 11.514 11.351 11.644 BLDM1 (%) 0.644 0.639 0.658 0.553 0.600 BLDM2 (%) 0.260 0.383 0.305 0.326 0.514 ELDM1 (%) 5.095 4.999 5.116 5.435 4.845 ELDM2 (%) 5.061 4.970 5.041 5.190 4.816 cM1 (8) 58.99 58.92 59.00 59.28 58.75 cM2 (8) 58.97 58.89 58.93 59.06 58.74 eu/esM1 1.1074 1.1053 1.1077 1.1147 1.1016 eu/esM2 1.1068 1.1047 1.1062 1.1095 1.1011 2 OAVM1 (8) 37.22 35.95 37.87 42.55 33.67 2 OAVM2 (8) 37.10 35.95 37.42 39.55 34.12 Sheet thickness (AÊ ) 2.129 2.134 2.130 2.112 2.149 Ê DTM (A) 0.580 0.585 0.606 0.668 0.574 Crystal chemical formulas [4] [6] 31 21 [12] Phl1a (Si2.74Al1.26) (Al0.24Fe0.09 Fe 0.12 Mg2.48Mn0.01Ti0.02) (Na0.04K0.93)O9.99[(OH)1.95F0.06] [4] [6] 31 21 [12] Phl1b (Si2.65Al1.35) (Al0.24Fe0.07 Fe 0.11 Mg2.55Mn0.01Ti0.02) (Na0.04K0.93)O9.96[(OH)1.95F0.09] [4] [6] 31 21 [12] Phl2a (Si2.60Al1.40) (Al0.18Fe0.15 Fe 0.03 Mg2.63Ti0.01) (Na0.02K0.95Ba0.01)O9.93[(OH)1.96F0.11] [4] [6] 31 21 [12] Phl3a (Si2.50Al1.50) (Al0.47Fe0.15 Fe 0.07 Mg2.23Mn0.04Ti0.01) (Na0.02K0.95Ba0.01)O10.02[(OH)1.94F0.04] [4] [6] 31 21 [12] Phl4a (Si2.60Al1.40) (Al0.20Fe0.11 Fe 0.04 Mg2.64Mn0.01) (Na0.02K0.90Ca0.02Ba0.02)O9.92[(OH)2.02F0.06] Note: The de®nitions of the different parameters are reported in the footnote of Table 6. from Toal del Mason (Monzoni Hills) X-ray maps of (and or [6]Fe31 ) for [6]Mg21 substitutions (exchange vector [4] [6] [4] [6] some mica crystals, polished parallel to [001] show two Si21 Mg21 Al Al). sets of distinct layers perpendicular to (001) whose com- position corresponds to phlogopite and clintonite, respec- ANALYTICAL METHODS tively (Alietti 1996). These samples were not investigated Chemical analysis further. In samples Cli5a, Cli7c, Cli8a, and Cli8d clintonite oc- The compositions of clintonite crystals were deter- curs as a major green to dark-green replacement mineral mined using an ARL-SEMQ wavelength-dispersive elec- in addition to spinel, fassaite, and calcite (Morandi et al. tron microprobe. Polished thin sections from about 30 1984). In samples Cli9a and Cli9b clintonite crystals oc- rock samples and the crystals used for structure re®ne- cur as coarse, dark-green ¯akes with fassaitic pyroxene, ments were analyzed (Table 2). A minimum of 20 mica olivine, calcite, spinel, chlorite, and andraditic garnet (Ul- crystals were analyzed in each section, multiple point mer 1982). analyses being performed for each crystal to test for Crystal chemical data on phlogopite crystals (referred chemical homogeneity. to as Al-rich phlogopite: Phl1a, Phl1b, Phl2a, Phl3a, and The variation in the composition of individual crystals Phl4a) have been published previously by Alietti et al. selected for X-ray study was below 3% for Mg21,Al31, (1995) and are summarized in Table 1. Compared to the Si41, and Ca21 and typically below 2%.
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