13-horvat.indd 153 � AB STRA CT Potassium feldspars from different granitoids and gneisses of Papuk Mt. (Slavonia, Croatia) have been investigated by X-ray powder diffraction. Diffraction patterns classically observed as well as patterns calculated by Rietveld re- fi nement were compared and discussed. Triclinicity was calculated according to GOLDSCHMIDT & LAVES (1954) while the structural state of the feldspars was determined using the methods of KROLL & RIBBE (1983) and GO- DINHO & JALECO (1973). Results showed that the type of potassium feldspar depend on the investigated host-rock, indicating variation in the structural state from orthoclase, intermediate microcline to highly ordered microcline. Po- tassium feldspar megacrysts in biotite-granodiorites and monzogranites are intermediate microcline or orthoclase, while two-mica monzogranites contain low microcline. Gneisses contain low microcline and orthoclase in Brzaja Creek and low microcline in Djedovica Quarry. Classically observed and digital diffraction patterns calculated by the Rietveld refi nement method produced comparable results and provided a very good correlation of the results ob- tained by different methods. High triclinicity values of feldspars from investigated granitoid and gneiss samples from Papuk Mt. (Slavonia, Croatia) are in accordance with a high Al content in the T1o site and their fully ordered state indicates a slow(er) cooling-rate. Low triclinicity values, an Al content in T1o site around 0.60 and ordering index smaller than 0.80 can be interpreted as a result of relatively fast(er) cooling which allowed lower ordering of the po- tassium feldspar. Keywords: X-ray powder diffraction, potassium feldspars, triclinicity, structural state, ordering index, granitoids and gnesses, Papuk Mt., Croatia X-ray study of potassium feldspars from diff erent granitoid types and gneisses of Papuk Mt. (Slavonia, Croatia) � Marija Horvat1, Darko Tibljaš2, György Buda3 and György Lovas3 1Vukomerečka cesta 55A, 10040 Zagreb, Croatia; (horvatmarija@hotmail.com) 2Department of Geology, Faculty of Science, Horvatovac 95, HR-10000 Zagreb, Croatia; (dtibljas@geol.pmf.hr) 3Department of Mineralogy, Eötvös Loránd University, Pázmány Péter st. 1/C, H-1171 Budapest, Hungary; (buda@ludens.elte.hu; lovas@ludens.elte.hu) doi: 104154/gc.2011.13 Geologia Croatica 64/2 153–162 4 Figs. 2 Tabs. Zagreb 2011 Geologia CroaticaGeologia Croatica called triclinicity (∆) (GOLDSCHMIDT & LAVES, 1954). WRIGHT & STEWART (1968), developed a method which was revised by STEWART & WRIGHT (1974) that uses b and c cell edges and α* and β* cell angles for estimations of obliquity. KROLL (1971, 1973 and 1980) and KROLL & RIBBE (1983) developed the method for calculating Al occupancy of tetrahedral sites based on lattice translations along [110] 1. INTRODUCTION X-ray powder diffraction is the most common method for determining symmetry and ordering of potassium feldspars. For monoclinic crystals the hkl and hk̄l refl ections show a single sharp peak on the diffraction pattern. For triclinic crystals the hkl and hk̄l peaks have different 2θ values and form a double peak. In the case of alkali feldspars, the split- ting of the 131 and 13̄1, that occur at ~29.9° 2θ CuKα, is Geologia Croatica 64/2Geologia Croatica 154 and [11̄0] directions: named tr [110] and tr [11̄0], respec- tively. Assuming that the angular difference of 2̄04 and 060 peaks, in the 29–31° region changes linearly with degree of order, GODINHO & JALECO (1973) defi ned an ordering index (∆Sm) based on this difference. Various potassium feldspar polymorphs and their struc- tural states, from the different granitoid types and gneisses found at Papuk Mt. in Croatia are presented (Fig. 1). Results obtained from the classically observed and digital X-ray dif- fraction patterns calculated by Rietveld refi nement are com- pared. For both of these patterns GOLDSCHMIDT & LA VES’s (1954) triclinicity and KROLL & RIBBE’s (1983) ordering path calculations were undertaken and are discuss ed. Re sults for the T1o site occupancy estimated by KROLL & RIBBE’s (1983) method, are compared with the results calculated by NEVES & GODINHO’s (1995) for- mulae. Finally, the structural state of these feldspars derived from triclinicity and Al occupancy of tetrahedral sites is dis- cussed. 2. GEOLOGICAL SETTING Papuk Mt. is part of the Slavonian Mountains in Croatia which are located in the southernmost part of the Panno- nian Basin in the Bihor nappe system (Fig. 1) of Tisza Mega-Unit (SCHMID et al., 2008). The published geolog- ical maps 1:100 000, Orahovica sheet (JAMIČIĆ & BR- KIĆ, 1987) and Daruvar sheet JAMIČIĆ (1989), report that it is primarily composed of metamorphic and granitoid rocks. The main mi neralogical features, geochemistry and detailed structural-tectonic investigations of granitoids and related gneisses and pegmatites in the area are discussed in many papers, including; TAJDER (1957), RAFFAELLI (1965), VRAGOVIĆ (1965), TAJDER (1969), SLOVE- NEC (1976, 1978, 1982, 1984), JAMIČIĆ (1983, 1995, 2001), PAMIĆ & LANPHERE (1991), PAMIĆ et al. (1988, 1996), HORVAT et al. (2002), HORVAT (2004), HORVAT & BUDA (2004), BALEN et al. (2006), BIŠEVAC et al. (2009), BIŠEVAC et al. (2010), HOR VÁTH et al. (2010). Feldspars are the most abundant minerals in these rocks. Figure 1: (A) Segment of the map (SCHMID et al., 2008) showing the major tectonic units of the Alps, Carpathians, Dinarides and Hellenides with the po- sition of Papuk Mt. within the Tisza Mega-Unit composed of the Mecsek, Bihor and Codru nappe systems. (B) A section of the Geological Map of the Sla- vonian Mts. (Papuk, Krndija, Ravna gora and Psunj) (JAMIČIĆ, 2001) with sampling localities (Table 1). Legend: 1 – Main tectonic lines; 2 – Alluvium of creeks; 3 – Deluvial-proluvial deposits; 4 – Loess; 5 – Pliocene-Quaternary: gravel and sands; 6 – albite rhyolite, andesite, basalt; 7 – Pontian: sand, marl and clay; 8 – Sarmatian-Pannonian: marl and limestone; 9 – Badenian: conglomerate, limestone, marl; 10 – Karpatian: conglomerate, sand, clay and marl; 11 – Ottnangian: conglomerate, sand, gravel; 12 – Upper Cretaceous: sandstone and limestone; 13 – Granite of Požeška gora; 14 – Jurassic: limestone; 15 – Middle and Upper Triassic: dolomite, dolomitic limestone; 16 – Lower Triassic: sandstone, siltstone, shale; 17 – Permotriassic: quartz sandstone, conglomerate; 18 – Devonian-Carboniferous: graphitic schist, conglomerate, sandstone, siltstone; 19 – Gran- itoids; 20 – Gneiss; 21 – Migmatite; 22 – (Precambrian): chlorite-sericite schist, metagabbro, marble, amphibolite, amphibole-schist, phlaseride granitoid, garnet-staurolite gneiss. Horvat et al.: X-ray study of potassium feldspars from diff erent granitoid types and gneisses of Papuk Mt. (Slavonia, Croatia) Geologia Croatica 155 A short review about the previous study of feldspars in this area can be found in and KOVÁCS KIS et al. (2004) and references within. 3. MATERIALS AND METHODS Various types of granitoid and gneiss rock samples (Table 1) were collected from several valleys on Papuk Mt. Sampling localities are shown on the compiled geological map in Fi- gure 1. The potassium feldspars from the aforementioned rocks were differentiated by X-ray powder diffraction mea- surements. X-ray investigation included measurement and indexing of the X-ray powder diffraction patterns of two kinds of materials: (1) non-magnetic, low density separates of fi ner-grained rocks (a mixture of quartz, alkali feldspar and plagioclase, i.e. felsic components that were impossible to separate from each other by hand-picking under the ste- reomicroscope) and (2) feldspar megacrysts picked out from four samples that are representative for different granitoid types and localities. (1) The separates were prepared by crushing and wet sieving followed by heavy liquid (bromoform) separation of either the 0.25–0.125 mm or 0.125–0.063 mm fractions. The light fraction was further cleaned by a Frantz isodynamic magnetic separator (model LI). Separates that contained K- feldspar, plagioclase and quartz grains, were pulverised in an agate mortar. X-ray powder diffraction patterns were ob- tained on an analogue Siemens D500 powder diffractometer in the 5° to 65° 2θ range. Instrumental parameters were: CuKα radiation, 40 kV, 20 mA, Ni fi lter, registration at 0.5° 2θ/min goniometer, 1 cm/min chart speed, 2×103 sensitivity and 0.1 mm detector aperture. NaCl was used as zero shift internal Table 1: List of investigated samples showing locality, rock type and triclinicity values (Δ = 12.5*[d(131)-d(13̄1)]) calculated according to GOLDSCHMIDT & LAVES (1954), for potassium feldspars from: (1) non-magnetic, low density fractions of bulk rock (mixture of quartz, alkali feldspar and plagioclase) and (2) megacrysts from porphyric rock types. (a,b) two megacrysts from one rock sample. Determination of mineral phases was according to the following reference data: quartz (Q) – JCPDS 33-1161; low albite (LA) – JCPDS 20-0554; albite, calcian (A) – JCPDS 20-0548 ; low microcline (LM), intermediate mi- crocline (IM) and orthoclase (O) from BORG & SMITH (1969); muscovite (Ms) from GRIM, BRAY & BRADLEY (1937) in BROWN (1961) and chlorite (Chl) from SHIROZU (1958) in BROWN (1961). Locality (Fig. 1) Sample name Rock type (based on mineralogical and/or geochemical data) Determined mineral phases in classical X-ray pattern (1) Δ Calculated from classical diff raction pattern (1) Δ Calculated from Rietveld refi nement diff raction pattern (2) Brzaja Creek PPM-3 gneiss LM, O, LA, Q, Ms 0.92 Brzaja Creek PPG-4 granodiorite LM, LA, Q, Ms 0.87 Brzaja Creek PPM-5 gneiss O, LM, LA, Q 0 Brzaja Creek PPG-8 two-mica monzogranite LM, LA, Q 0.81 0.96 Brzaja Creek PPM-9 gneiss O, A, Q, Ms 0 Brzaja Creek PPG-12 two-mica monzogranite LM, LA, Q, Ms 0.87 Djedovica Quarry PP-13/1 gneiss LM, LA, Q, Ms, Chl 0.92 Djedovica Quarry PP-13/2 gneiss LM, LA, Q, Ms, Chl 0.96 Djedovica Quarry PP-13/4 gneiss LM, A, Q, Ms, Chl 0.97 Djedovica Quarry PP-13/5 gneiss LM, LA, Q, Ms 1 Djedovica Quarry PP-13/6 gneiss LM, LA, Q, Ms 0.90 Pakra Creek 2PPG-3 porphyric granodiorite O, A, Q 0 Pakra Creek 2PPG-4 porphyric granodiorite O, A, IM, Q, Ms 0 Pakra Creek 2PPG-5 porphyric granodiorite IM, A, Q, Ms 0.29 0.33 Pakra creek 2PPG-6 (a) porphyric granodiorite O/IM, A, Q, Ms 0 Pakra Creek 2PPG-6 (b) porphyric granodiorite O/IM, A, Q, Ms 0 Pakra Creek PPG-19 granodiorite O, LA, Q, Ms 0 0 Pakra Creek PPG-24 porphyric granodiorite IM, A, Q 0.32 0.29 Pakra Creek PPG-18 two-mica monzogranite LM, O, LA, Q, Ms 0.87 Pakra Creek PPG-20 monzogranite LM, A, Q, Ms, Chl 0.70 0.72 Pakra Creek PPG-23 two-mica granodiorite LM, LA, Q, Ms 0.56 Šandrovac Creek 2PPG-32 two-mica monzogranite LM, O, LA, Q, Ms 0.83 Rajčevica Creek 2PPG-33 monzogranite LM, LA, Q, Ms, Chl 0.68 Kišeljevac Creek HEG-31 biotite monzogranite LM, O, LA, Ms, Chl 0.88 Geologia Croatica 64/2Geologia Croatica 156 Figure 2: The {131} diff raction region of studied feldspars obtained by the classical method. The sample name is on the right side of the pattern’s seg- ment. Peaks are assigned according to the following reference data: quartz (Q) – JCPDS 33-1161; low albite (LA) – JCPDS 20-0554; albite (A) – JCPDS 20- 0548; low microcline (LM), intermediate microcline (IM) and orthoclase (O) from BORG & SMITH (1969). standard. Powder patterns were recorded on the paper and the peak positions were determined manually. The potas- sium feldspar polymorph(s) have been identifi ed with the help of the JCPDS database. UnitCell software (HOLLAND & REDFERN, 1997) was used for unit cell parameters cal- culation. (2) Data for megacrysts were collected in 5–70° 2θ ran- ges on a Siemens D5000 theta-theta diffractometer equipped with graphite secondary beam monochromator. Conditions were: CuKα1 radiation, 0.02° 2θ step size and 5 seconds count- ing time per step. The Rietveld analyses were performed by the DBWS-9006 PC program package (YOUNG et al., 1994). Triclinicity and ordering were calculated for both type of patterns (classical and digital) i.e. for both type of sam- ples (separeates and megacrysts). Triclinicity was calculated using the formula ∆ = 12.5*[d(131)-d(13̄1)] of GOLD- SCHMIDT & LAVES (1954). Degree of ordering was de- termined according to [110] method (KROLL & RIBBE, 1983) and ∆Sm method (NEVES & GODINHO, 1995). Lattice translations tr [110] = ½ (a2 + b2 + 2ab cosγ)1/2 and tr [11̄0] = ½ (a2 + b2 – 2ab cosγ)1/2 were used for graphi- cal estimation of t1o values (KROLL & RIBBE, 1983) while the ordering index ∆Sm = 15.32 − ∆2θ / 0.608 (NEVES & GODINHO, 1995) is applied for calculation the percentage of T1o sites occupied by Al: ΣAl(T1) = 45 ∆Sm + 55. 4. RESULTS The powder diffraction patterns revealed that different poly- morphs of feldspar are present in the rocks that differ both Horvat et al.: X-ray study of potassium feldspars from diff erent granitoid types and gneisses of Papuk Mt. (Slavonia, Croatia) Geologia Croatica 157 by rock type and sampling locality. X-ray powder patterns revealed that monzogranites from Rajčevica and Pakra Creeks (Figs. 1 and 2), granitoids from Brzaja Creek and gneisses from Djedovica Quarry contain only low microcline and low albite. Gneisses from Brzaja Creek, monzogranite from Ki- šeljevac and Šandrovac Creeks and fi ne to medium-grained granitoid samples from Pakra Creek valley also contain or- thoclase. Porphyric and slightly porphyric granodiorites are typical of the Pakra Creek valley. Orthoclase is ubiquitous in these samples. If microcline appears, it is intermediate mi- crocline. The potassium feldspar polymorph in the PPM-9 gneiss sample (Brzaja Creek) is monoclinic orthoclase. Rietveld refi nement for four feldspar megacrysts pro- duced identical results in determination of feldspar poly- morphs as the classical method, although small differences in Δ values were present (Table 1). Observed and calculated diffraction patterns of one of the feldspar megacryst samples (2PPG-5) are shown in Figure 3. Triclinicity (Δ) calculations showed that the majority of the potassium feldspars occurring in the Papuk granitoids have values higher than 0.70 on the scale, where a microcline with maximum triclinicity has value of 1. The exception is the intermediate microcline modifi cation that occurs in gran- odiorites of the Pakra Creek valley with triclinicity around or < 0.50. The lowest value is characteristic for the porphy- ric granodiorite of Pakra Creek valley (Δ=0.29). The highest values (Δ >0.90) are obtained on gneisses (Table 1). Pairs of triclinicity values obtained by two methods, classical (1) and Rietveld (2) are as follows: 0.81–0.96 for Brzaja Creek two- mica monzogranite, 0.29–0.33 and 0.32–0.29 for Pakra Creek porphyric granodiorites and 0.70–0.72 for Pakra Creek two- mica monzogranite (Tab. 1 and Fig. 2). Table 2 presents unit cell parameters, t1o occupancies and ordering for separates as well as for four potassium feld- spar megacrysts. Feldspars from granitoids 2PPG-3, PPG-19 (Pakra Creek) and gneisses PPM-5 and PPM-9 (Brzaja Creek) have lattice parameters that conform to a monoclinic cell. Two grains (patterns a and b) of the 2PPG-6 granitoid sam- ple show initial splitting of peak at 29.79 and 29.75 2°θ (Fig. 2). The splitting is not measurable so they are determined as orthoclase. Feldspars from PPG-4, PPG-8, PPG-20, 2PPG- 32, 2PPG-33, HEG-31 and PPG-12 granitoids have a0 be- tween 8.56 to 8.59Å, b0 from 12.92 to 13.01Å and c0 in 7.20– 7.22Å range, which correspond to the triclinic symmetry. Feldspar of PPG-18 and previously mentioned 2PPG-32 and HEG-31 samples shows a complicated {131} region with or- thoclase and low microcline peaks. These peaks prove a pres- ence of more than one potassium phase i.e. coexistence of pha ses having monoclinic and triclinic symmetries in various proportions within the same sample (Table 2). Overlapping of peaks on patterns (PPG-18, PPG-23 and PP-13/6 samples) meant that only a limited number of peaks could be used for calculation of unit cell parameters. Consequently the results for these samples are doubtful (Table 2, grey colour-coded samples). In contrast, feldspar from the 2PPG-5 sample shows intermediate microcline peaks, but those peaks are sharp with Figure 3: Observed and calculated diff raction pattern (plus signs and line, respectively) of 2PPG-5 sample feldspar megacryst. The lower curve shows the diff erence between the observed and calculated patterns. Tick marks indicate the positions of the allowed refl ections of microcline (upper) and albite (lower). Calculated triclinicity (Δ) is shown in Table 1 (last column). Geologia Croatica 64/2Geologia Croatica 158 fair intensity and they could be accurately read (Figure 2). Intermediate microcline of the 2PPG-5 sample have a0 = 8.56, b0 = 12.96 and c0 = 7.21Å, with α slightly different from 90°, which also revealed triclinic symmetry (Table 2). Lattice translation tr[11̄0] and cell volume results (Table 2) obtained from classically observed diffraction patterns gave an Al distribution in the T1o site as follows; granitoids (PPG-4, PPG-8 and PPG-12) and gneiss from Brzaja Creek (PPM-3), gneiss in Djedovica Quarry (PP-13/1), granitoids from Šandrovac (2PPG-32), Rajčevica (2PPG-33) and Ki- še ljevac locality (HEG-31) have high Al content in the T1o site. It is higher than 0.90, in most cases close to 1 (Table 2, Figure 4). However, granitoids in the Pakra Creek valley (2PPG-5, 2PPG-6, PPG-24) have an Al content around 0.60 in the T1o site. The exception is the PPG-20 Pakra granitoid sample with 0.96 t1o occupancy value. Lattice translation tr[11̄0] and cell volume results ob- tained from diffraction patterns calculated by Rietveld re- fi nement, (Table 2), showed that potassium feldspar from the Brzaja Creek granitoid (PPG-8R) has a high Al content in the T1o site (0.98), while megacrysts from Pakra Creek porphy- ric granodiorites (2PPG-5R, PPG-24R, PPG-20R) have lower values, around 0.60 up to 0.84. Comparison of the values obtained from the observed and calculated diffraction patterns showed that the results match in case of three samples (PPG-8, 2PPG-5 and PPG- 24). For the patterns of the PPG-20 sample there is an obvi- ous discrepancy between the t1o values obtained by classical and Rietveld refi nement methods (0.96 and 0.84, respecti- vely). This cannot be explained as uncertainty in reading the peak positions (see Figure 2), and is most probably the result of the small number of peaks that could be used for unit cell parameter calculations. Ordering index ∆Sm and ΣAl in the T1 sites calculated according to NEVES & GODINHO (1995) are also shown in Table 2. Investigated potassium feldspars have ∆Sm from 0.50 to 1 while ΣAl(T1) range from 79% to 99%. According to the fact that the total Al content of the T1 sites is 55% in Table 2: Unit cell parameters calculated with UnitCell software (HOLLAND & REDFERN, 1997) for classically observed diff raction patterns and calculated by Rietveld refi nement (signed by subscript R). Lattice translations tr[11̄0], Al content in the T1o site (according to KROLL & RIBBE, 1983 method), ordering index ΔSm and ΣAl(T1) (according to NEVES & GODINHO, 1995) for investigated potassium feldspars from Papuk Mt. Samples PPG-4 to HEG-31 are grani- toids, while samples PPM-3 to PP-13/6 are gneisses (see Table 1). The (doubtful) results for samples where overlapping of peaks allowed unequivocal in- dexing of limited number of peaks are shown in grey. Sample a0 (Å) b0 (Å) c0 (Å) a(°) b(°) g(°) V (Å3) tr[11̄0] (Å) t1o DSm SAl(T1) % PPG-4 8.577(5) 12.971(6) 7.220(3) 90.7(1) 115.87(6) 87.82(8) 722 (1) 7.6379 0.97 0.9 95 PPG-8 8.567(6) 12.95(1) 7.206(3) 90.71(6) 115.93(6) 87.74(6) 718(1) 7.6214 0.96 0.61 82 PPG-12 8.573(4) 13.01(1) 7.211(7) 90.55(8) 116.00(5) 87.83(7) 722(1) 7.6536 0.91 0.85 93 PPG-18 8.460(1) 12.776(5) 7.147(2) 90.56(4) 116.26(3) 87.88(4) 691(1) 7.5299 0.95 0.9 95 2PPG-3 8.574(7) 12.98(1) 7.21(1) 116.00(6) 721(1) 2PPG-4a 8.592(8) 12.98(1) 7.210(3) 90.02(9) 115.99(7) 89.36(9) 723(1) 7.7429 0.57 0.75 89 2PPG-5 8.568(8) 12.969(6) 7.210(3) 90.10(8) 115.90(6) 89.38(9) 720(1) 7.7309 0.59 0.73 88 2PPG-6a 8.599(5) 12.953(4) 7.219(3) 90.01(4) 116.41(6) 89.64(5) 720(1) 7.7427 0.56 0.53 79 2PPG-6b 8.577(6) 12.972(8) 7.21(1) 90.18(8) 115.93(6) 89.52(8) 721(1) 7.7515 0.53 0.53 79 PPG-19 8.594(6) 12.976(6) 7.209(5) 116.04(6) 722(1) PPG-20 8.582(4) 12.924(4) 7.223(3) 90.40(4) 116.07(5) 87.90(4) 719(1) 7.6248 0.96 0.93 97 PPG-23 8.505(4) 12.866(2) 7.145(1) 90.58(2) 115.78(2) 88.42(3) 703(1) 7.6130 0.79 0.83 92 PPG-24 8.575(2) 12.974(2) 7.205(1) 90.285(3) 116.14(2) 89.14(2) 719(1) 7.7220 0.6 0.71 87 2PPG-32 8.575(8) 12.958(7) 7.208(3) 90.751(6) 115.92(5) 87.74(5) 719(1) 7.6269 0.99 0.73 88 2PPG-33 8.572(4) 12.967(6) 7.216(5) 90.69(7) 115.80(6) 87.96(7) 722(1) 7.6438 0.95 0.71 87 HEG-31 8.591(6) 12.966(8) 7.221(5) 90.59(7) 115.91(7) 87.80(6) 721(1) 7.6382 0.98 0.85 93 PPM-3 8.595(7) 12.957(6) 7.217(2) 90.59(4) 115.94(3) 87.78(5) 722(1) 7.6343 0.97 0.98 99 PPM-5 8.59(1) 13.02(1) 7.20(1) 115.95(9) 724(1) PPM-9 8.552(8) 12.95(1) 7.179(8) 116.06(8) 714(1) PP-13/1 8.571(9) 12.96(2) 7.212(8) 90.716(9) 115.89(9) 87.66(4) 720(1) 7.6216 0.99 0.99 99 PP-13/6 8.571(4) 12.942(7) 7.202(3) 90.842(7) 115.85(5) 87.53(6) 718(1) 7.6059 out of diagram 0.81 91 PPG-8R 8.577 12.979 7.225 90.653 115.96 87.71 722(1) 7.6341 0.99 0.94 97 2PPG-5R 8.571 12.965 7.206 90.213 115.98 89.21 720(1) 7.7217 0.61 0.67 85 PPG-20R 8.577 12.966 7.212 90.520 115.98 88.25 720(1) 7.6629 0.84 0.83 92 PPG-24R 8.572 12.967 7.205 90.262 115.98 89.14 720(1) 7.7181 0.62 0.70 86 Horvat et al.: X-ray study of potassium feldspars from diff erent granitoid types and gneisses of Papuk Mt. (Slavonia, Croatia) Geologia Croatica 159 high sanidine and 100% in low microcline (KROLL & RIBBE, 1987) these estimated values correlate well with those de- termined from unit cell parameters. 5. DISCUSSION Polymorphic modifi cations of alkali feldspars are the result of the Al-Si distribution in the crystal structure. The way in which the Al content changes in each tetrahedral site (T1o, T1m, T2o and T2m), from the most disordered (sanidine), to the most ordered state (high triclinic/low microcline), is cal- led the ordering path in alkali feldspars (SMITH, 1974; GRIF FEN, 1992). Ordering is a two-step phenomenon: (1) the migration of Al from the T2 to T1 sites and (2) Al migra- tion from T1m, T2o and T2m to the T1o site (STEWART & WRIGHT, 1974). If the feldspar is monoclinic, the probabi- lity of fi nding Al at the T1o and T1m sites is equal (t1o = t1m). If the crystal is completely ordered, in the unit cell there will be 1.0 Al + 3.0 Si along b (KROLL & RIBBE, 1983). Dur- ing the ordering process, T-O bond lengths vary and O-T-O and T-O-T bond angles are affected (KROLL, 1973). The T-O bond lengths refl ect Al occupancy; therefore the Al oc- cupancy can be estimated by calculating T-O bond lengths. An increase in aluminium occupancy causes an increase in the mean bond length, whereas an increase in silicon occu- pancy causes their decrease (KROLL, 1973). During Al-Si ordering, triclinic feldspars expand along [110] and contract along [11̄0]. The translation tr[110] esti- mates t1o+t2o+t2m and tr [11̄0] estimates t1m+t2o+t2m. Be- cause the same amount of Al and Si atoms is present in the unit cell regardless of the structural state (GRIFFEN, 1992), unit cell volume (V) should be a function of composition but not of structural state. If tr[11̄0] is plotted versus cell volume (V), t1o can be estimated directly from the diagram of KROLL Figure 4: Plots of tr [11̄0] against V [Å3] showing the estimated t1o values for investigated feldspars. Values for end members are from KROLL & RIBBE (1987). Dots are from KROLL et al. (1986); crosses from WRIGHT & STEWART (1968). The kink at V = 695Å3 (Or~35) is due to the triclinic/monoclinic symme- try change. Geologia Croatica 64/2Geologia Croatica 160 & RIBBE (1983). Therefore the [110] method can be used for determining the (Al-Si) distributions among the non-equi- valent tetrahedral sites. If ordering proceeds as far as possi- ble, all of the aluminium is found in T1o, therefore t1o=1.0; t1m=t2o=t2m=0.0 and alkali feldspar is referred to as maxi- mum (or low) microcline. According to the results presented in Table 2 studied granitoids from Brzaja, Kišeljevac, Šan- drovac and Rajčevica Creek valleys and gneisses in Djedo- vica Quarry and one gneiss sample from Brzaja Creek valley contain highly ordered potassium feldspar, because most of the Al atoms are found in the T1o site (Fig. 4). These potas- sium feldspars have high triclinicity (0.81–0.99; Table 1) and correspond to the highly ordered microcline (low microcline) (SMITH, 1974; GRIFFEN, 1992). Al occupancy in the T1o site of Pakra Creek valley granitoid feldspars are between 0.53 and 0.62 (Table 2, Fig. 4); their triclinicity is low (around 0.30 or even 0.0) (Table 1) and they correspond to interme- diate microcline or orthoclase. Brzaja Creek valley gneiss feldspars, excluding low microcline in PPM-3 sample, with triclinicity value Δ = 0 revealed to be orthoclase. Results obtained by GOLDSCHMIDT & LAVES (1954), KROLL & RIBBE (1983) and NEVES & GODINHO (1995) methods indicate variation in the structural state, from or- thoclase, intermediate microcline to highly ordered micro- cline in the investigated rock samples. High triclinicity val- ues of feldspars from granitoid and gneiss samples from Pa puk Mt. (Slavonia, Croatia) are in accordance with high Al con- tents in the T1o site and their fully ordered state indicate a slow(er) cooling-rate. Low triclinicity values, Al content in T1o site of around 0.60, and an ordering index smaller than 0.80 can be interpreted as a result of relatively fast(er) cool- ing which allowed the existence of less ordered potassium feldspar. Classical and diffraction patterns calculated by the Ri- etveld refi nement method gave comparable results. Obtain- ing correct cell parameters depends primarily upon accurate measurement of the peak positions and the correct indexing of X-ray powder patterns. 6. CONCLUDING REMARKS According to X-ray powder diffraction results, the porphyric potassium feldspar in biotite-granodiorites and monzogran- ites is orthoclase or intermediate microcline, while two-mica monzogranite contains maximum microcline (low micro- cline). Orthoclase and intermediate microcline are typical for Pakra Creek valley granodiorites, while highly ordered microcline (low microcline) characterises monzogranite rock types in the Pakra, Šandrovac, Rajčevica, Kišeljevac and Br- zaja Creek valleys. Gneisses associated with granites contain low microcline and orthoclase (Brzaja Creek valley), and low microcline (Djedovica Quarry). Some granitoid and gneiss samples only contain orthoclase. Intermediate microcline or orthoclase from biotite-gran- odiorites and monzogranites compared with highly ordered, structured, potassium feldspars from two-mica monzogran- ites indicate differences in the rate of cooling, as the most important factor controlling the ordering of potassium feld- spars. The two-mica granitoids as host rocks, have a eutectic composition and peraluminous character. They are syn-col- lision granitoids according to HORVAT (2004), HORVAT & BUDA (2004) and their overall triclinic symmetry implies a slow cooling-rate and higher degree of order with the aid of fl uid activity or deformation in the case of the gneiss (BROWN & PARSONS, 1989). Porphyric biotite-granodiorite and monzogranite have a peraluminous-metaluminous character and were probably formed post-collision in an uplifted environment (HORVAT, 2004; HORVAT & BUDA, 2004). They were emplaced at shallower levels in the crust; thus cooling was relatively fast which prevented the symmetry inversion and retarded order- ing. Rapid cooling seems to be the most probable explana- tion for preservation of monoclinic structures and the mod- erate degree of Al-Si ordering of potassium feldspars in these rocks. Estimation of three different methods for K-feldspar cha- racterization: triclinicity calculations according to GOLD- SCHMIDT & LAVES (1954), structural state determination by the method of KROLL & RIBBE (1983) and the order- ing index method introduced by GODINHO & JALECO (1973) once again proved the very good correlation of results obtained by these methods. This fact gives greater weight to conclusions arising from them i.e. makes them more reliable. Simple and relatively fast standard methods for potassium feldspar determination and description produced results that correlate well with those obtained by more accurate but more time-consuming methods. ACKNOWLEDGMENT This study was carried out in the Department of Mineralogy, Eötvös Loránd University, Budapest and was supported by the Hungarian Ministry of Education through the Ph.D. scho larship and Hungarian National Research Fund (OTKA project no. K 67787). 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