1. INTRODUCTION
In the literature there are many suggestions how to make 
distinction between different potassium feldspar “mineral 
species”, but the International Mineralogical Association 
(IMA) recommends three terms: “sanidine”, “orthoclase” and 
“microcline” (BARTH, 1934). “Microcline” (K>>Na)AlSi3O8 
is the low-temperature phase. The crystal structure is triclinic 
C1̄ with four crystallographic tetrahedral (T) sites with an 
ordered Si, Al distribution. Orthoclase has monoclinic C2/m 
symmetry, according to the disordered distribution of Al and 
Si between the four T sites. Mineral species and mineral 
varieties can be characterized by various techniques. Under 
equilibrium conditions, LAVES & GOLDSMITH (1961) 
distinguishes the following K-feldspars: low microcline, 
intermediate microcline and high microcline. Based on X-ray 
powder diffraction (XRPD), RIBBE (1983) classifies them as 
low microcline, intermediate microcline and orthoclase. Using 
selected area (electron) diffraction (SAED), BAMBAUER et 
al. (1989) identified K-feldspars based on their scattering 
characteristics in [001] zone projection, as a) regular microcline 
twinning, A (Albite law) and P (Pericline law) twins spot 
splitting; b) irregular twinning microcline (A and P twins spot 
splitting and diffuse streaks; and c) tweed (domains) orthoclase 
(A and P twins diffuse streaks). STEWART & RIBBE (1983) 
distinguished optical microcline (dd0d) and optical orthoclase 
(d=0d) by polarized light optical microscopy (PLOM) 
according to the extinction angle d in (001) sections. According 
to SÁNCHEZ-MUÑOZ et al. (2012) microcline is characterized 
by a low crystallization temperature, triclinic symmetry and 
ordered Si, Al distribution, while orthoclase is metastable 

 
Microstructures of feldspar crystals from Pakra Creek  
valley granite (Papuk Mt., Croatia)
Marija Horvat1,*, Viktória Kovács Kis2 and Jasmina Martinčević Lazar1

1  Croatian Geological Survey, Department of Geology, Sachsova 2, 10000 Zagreb, Croatia;  
(jmartincevic@hgi-cgs.hr; *corresponding author: mhorvat@hgi-cgs.hr)

2 HUN-REN Centre for Energy Research, Konkoly-Thege Miklós út 29-33, H-1121, Budapest, Hungary; (kis.viktoria@ek.hun-ren.hu)

doi: 10.4154/gc.2025.06 

Abstract
Optical polarizing microscopy, X-ray powder diffraction and transmission electron micro-
scopy with energy-dispersive spectroscopy have been used to study microstructures in 
feldspars from two structurally and texturally different granite types from Pakra Creek valley 
(Papuk Mt., Croatia). The porphyritic texture granite contains pink potassium feldspar mega-
crysts with cross-hatched extinction and tweed pattern, which proved to be intermediate 
microcline to orthoclase. These crystals have Or > 90% and An=0 composition, and coexist  
with low albite Or(1.2)Ab(98.8)An(0). The feldspars from the granite with homogenous fine-
grained microstructure are white in colour and proved to be mixture of lamellar and zoned 
plagioclase and intermediate microcline. The plagioclase has a Na:Ca ratio close to 3:1 and 
minor K content (72.2%<Ab<76.5%, 23%<An<26.8%, Or<1.4%). Pure albite (Ab=97-99%) 
and anorthite (An=99%) grains were also detected. The higher content of plagioclase in the 
granite with homogenous fine-grained microstructure indicates a higher crystallization 
 temperature compared to the porphyritic type. The microstructure of the feldspar crystals 
suggests a parent melt close to the eutectic composition for the studied granites.

having triclinic-like symmetry, with partially ordered Si, Al 
distribution, which leads to the formation of modulations in 
structural strain. The same authors highlight the significance 
of a high content of chemical hinderers in orthoclase, which 
act as a barrier to structural ordering. The fine grainsize of 
granite WEBBER et al. (1999) connected to a drop in pressure 
and rapid cooling at high crustal levels (ROCKHOLD et al., 
1987). WILLAIME et al. (1976) described a complete list of 
potassium feldspar microstructures depending on the melt 
composition and cooling rate.

Potassium feldspar megacrysts are very common in gra-
nitic rocks, being either triclinic microcline or monoclinic or-
thoclase. KERRICK (1969) firstly expressed an opinion that 
potassium feldspar megacrysts crystallize from the melt in the 
early phase of magma solidification compared to the ground-
mass microcline. VERNON (1986), BATEMAN (1992) and 
COX et al. (1996) later confirmed this theory. VERNON (1986) 
and COX et al. (1996) linked Ba-oscillatory zoning in potas-
sium feldspar megacrysts to their igneous crystallization. 
DICKSON & SABINE (1967) and JOHNSON et al. (2006a, b) 
attributed their crystallization to a late water-rich fluid phase 
under subsolidus conditions as porphyroblasts and replacement 
of other mineral grains. COLLINS & COLLINS (2002, and 
references therein) suggested microcline replacements of pla-
gioclase and formation of myrmekite due to the influence of 
potassium bearing fluids. COLEMAN et al. (2005) and 
GLAZNER et al. (2004) reported that a porphyroblastic origin 
is necessary in the post solidification phase, and that the igne-
ous origin of the megacrysts is compatible with the host pluton. 
GLAZNER et al. (2004) also introduced the term “big tank” 

2025 | 78/1 | 45–60 | 7 Figs. | 2 Tabs. | 2 Suppls. | www.geologia-croatica.hr
 Journal of the Croatian Geological Survey 
and the Croatian Geological Society 

Article history:
Manuscript received: November 15, 2024 
Revised manuscript accepted: January 21, 2025 
Available online: February 27, 2025

 

 
Keywords: microstructure, feldspar, granite, 
Pakra Creek valley, Papuk Mt., Croatia

mailto:jmartincevic@hgi-cgs.hr
mailto:mhorvat@hgi-cgs.hr
mailto:kis.viktoria@ek.hun-ren.hu


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crystallizing reservoir of magma and gave examples that the 
potassium feldspars mostly grow after the magma crosses the 
rheological lock-up threshold of ≈ 50% crystals (GLAZNER & 
JOHNSON, 2013). Further, the same authors stated that the 
high potassium composition of megacrysts and their large size 
possibly required growth temperatures below the nominal soli-
dus temperatures of ≈ 650 °C and exsolution of Ab (albite) com-
ponent down to temperatures of ≈ 400 °C. BROWN & PAR-
SONS (1989) and PARSONS et al. (2015), discusses exsolution 
and twin microstructures in alkali feldspars in relation to their 
thermal history including behaviour during diagenesis and we-
athering. They also emphasize that phase equilibria and ther-
modynamics of reactions should be considered as appropriate, 
i.e. taking into account the effect of interfaces and particle size 
and stability in oxide-bearing systems (NAVROTSKY et al., 
2008; NAVROTSKY, 2011; KONTONIKAS-CHAROS et al., 
2018). The first report about alkali feldspar megacrysts from 
Pakra Creek (Papuk Mt., Croatia) was given in KUČAN & 
KRMPOTIĆ (1911). They described them in a gneiss host as 
"microcline-microperthites". POLJAK (1952) mentioned up to 
5 cm feldspar crystals in the Pakra Creek valley granites. The 
"large pink microcline" crystals from pegmatitic pockets 
(Sloboština Creek) and granitic veins (Pakra Creek) were later 
studied by TAJDER (1957). The megacrysts in adamellite and 
granodiorite of West Papuk Mt. as porphyritic and vein filling, 
diffuse, metasomatic K-feldspar megacrysts were described by 
VRAGOVIĆ (1965).

Two types of megacrysts, vein filling (Pakra Creek, Papuk 
Mt.) and pocket forming (Sloboština Creek, Papuk Mt.), were 

studied by KOVÁCS KIS et al. (2004), and proved to be low 
microcline in association with low albite and quartz. The alkali 
feldspar megacryst from Pakra Creek was characterised by the 
absence of twinning, while the megacryst from Sloboština 
Creek showed a tweed-like texture (KOVÁCS KIS et al., 
2004). The deduced crystallization temperatures were below 
460 °C, and near to but above 460 °C, respectively. These low 
microcline megacrysts are characterised by a continuously 
modulated lattice on the submicroscopic scale.

The main aim of this paper is determining the type of 
feldspars and their microstructure in two different granite 
types (sample 2PPG-4, the porphyritic, and PPG-24 the 
homogeneous type), both from Pakra Creek valley, Papuk Mt. 
(Fig. 1). These two different types of feldspar host rocks were 
chosen for this study, which are representative of granites in 
the Pakra Creek valley and associated with pegmatite occur-
rence described in KOVÁCS KIS et al. (2004). These results 
will be useful for the further study of a series of granites and 
associated migmatite and metamorphic samples, and will 
provide details of the geological history of the area.

2. METHODS
The chemical and structural inhomogeneity of the feldspar 
grains were measured using a polarizing microscope (PM), 
X-ray powder diffractometer (XRPD) and a transmission 
electron microscope (TEM).

The modal composition was determined using a polarizing 
microscope Zeiss Axio Lab A1 equipped with an ocular net 
and Zeiss Axiocam 105 digital camera with software ZEN at 

Figure 1. The sampling localities (blue circles) in the Pakra Creek valley (Papuk Mt.) on the compiled geological map of Slavonian Mts. (JAMIČIĆ, 2001) 
and their position within the Tisza mega-unit on the tectonic map of SCHMID et al. (2020). Lithology legend for the geological map: Pk – chlorite-seric-
ite schist, metagabbro, marble, amphibolite, amphibole schist, phlaseride granitoid, garnet-staurolite gneiss; Mi – migmatite; Gbm – garnet-biotite-
muscovite gneiss; + granitoids (symbol from HORVAT, 2004); D,C – graphitic schist, conglomerate, sandstone; PT – quartz sandstone, conglomerate; T1 
– sandstone, siltstone, shale; T2-3 – dolomite, dolomitic limestone; J1-3 – limestone; M2 – conglomerate, sand, gravel; M3 – conglomerate, sand, clay and 
marl; M4 – conglomerate, limestone, marl; M5-6 – marl and limestone; M7 – sand, marl and clay; c – albite rhyolite, andesite, basalt; Pl,Q – gravel and sands; 
lQ1 – loess; dprQ2 – deluvial-proluvial deposits; aQ2 – alluvium of creeks.



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the Department of Geology of Croatian Geological Survey. 
The value of standard deviations of repeated measurements 
was below 3%.

The X-ray powder diffraction (XRPD) measurements of 
crystals extracted from the granite rock (grains a and b, sample 
2PPG-4) and non-magnetic, low density fraction of whole rock 
PPG-24 sample, were performed on a SIEMENS 0500 powder 
diffractometer (Cu radiation, 40kV, 20 mA, analogous regi-
stration at 0.5° 2q/min goniometer speed), at the Department 
of Mineralogy of Eötvös Loránd University. For the d-value 
measurements, the reflections of accompanying quartz (PDF 
00-033-1161) were used as an inner standard. The X-ray 
powder diffraction (XRPD) measurements were performed 
additionally for megacrysts (grain c) and a whole rock sample 
2PPG-4 with a PANalytical X' Pert PRO diffractometer at the 
Department of Geology, Croatian Geological Survey (Cu-tube, 
graphite monochromator and Pixel detector; experimental 
conditions: 45 kV, 40 mA, PW 3018/00 PIXcel detector, 
primary beam divergence 1°, and continuous scan step 0.02° 
2θ/s). The program X’Pert HighScore Plus (PANalytical, 2016) 

was used, which contains powder diffraction database PDF 4/
Minerals (International Centre for Diffraction data, 2024).

Powder samples for TEM were prepared from large sized 
crystals which appear to be homogeneous both with the naked 
eye and under an optical microscope. Special attention was 
paid to select pieces that could be considered representative of 
the large, macroscopic crystals. However, the presence of a 
minor, non-representative sample volume in the TEM spe ci-
men cannot be ruled out. Therefore, for SAED and (HR)TEM 
analysis, only such grains were used, which proved to be the 
major component based on the comparison of TEM-EDS and 
XRD data.

The samples for TEM observations were obtained by 
grinding the sample under ethanol and mounting a drop from 
the suspension onto a Cu-grid covered by amorphous carbon 
supporting film.

TEM study was done at the HUN-REN Centre for Energy 
Research, Budapest, Hungary, using a Themis (ThermoFisher) 
TEM, operated at 200 kV and equipped with Cs (spherical 
aberration) correction in the imaging system (spatial resolution 
in HRTEM mode 0.8 Å). The microscope is equipped with a 

Figure 2. a) The sample 2PPG-4 image; b, c) Photomicrographs of porphyritic structure sample 2PPG-4, XPL – cross-polarized light; b) The microcline, 
plagioclase, quartz, biotite, muscovite presence and myrmekite occurrence; c) alkali feldspar cross-hatched extinction pattern; perfect cleavage along 
{001}, good cleavage along {010} are observed. Right angle cleavage is characteristic; both twin systems extinct simultaneously; d) The sample PPG-24 
image; e, f) Photomicrographs of the homogeneous structured PPG-24 sample, XPL – cross-polarized light; e) The orthoclase, plagioclase, quartz,  biotite 
and muscovite presence; f) Plagioclase exhibits compositional zonation as well. Abbreviations are according WARR (2021): Bt – biotite, Mcc – microcline, 
Ms – muscovite, Or – orthoclase, Pl – plagioclase and Qz – quartz. Diameter of the coin is 1.6 cm.



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Schottky field emission gun (FEG) having an energy spread 
of 0.7 eV. SAED patterns were taken from an area of 600 nm 
diameter and recorded using a 4kx4k Ceta camera and Velox 
software (Thermo Fisher). Offline analysis of SAED patterns 
and Fast Fourier Transform (FFT) analysis of high resolution 
(HR)TEM images was undertaken with Velox software 
(Thermo Fisher) and CRISP software (Calidris, Sollentuna, 
Sweden) (HOVMÖLLER, 1992). Calculation of SAED patterns 
was done using the SingleCrystal 4.1.7 software (CrystalMaker 
Software Limited, Begbroke, UK).

3. RESULTS
The sample 2PPG-4 contains pink feldspar megacrysts 
measuring up to 2x1 cm size, and this granite has a porphyritic 
texture (Fig. 2a). Besides the pink megacrysts, a white mineral, 
hypidiomorphic to allotriomorphic in shape is also considered 
to be a feldspar. The sample contains biotite flakes and grey 
quartz grains. Biotite is a few mm and quartz is up to 1 cm in 
size (Fig. 2a). The second granite sample PPG-24 has a homo-
geneous structure and is composed of grains with a uniform 
grain size distribution measuring from 1 to 5 mm (Fig. 2d). 
The sample contains grey quartz, black biotite and white 
minerals, assumed to be feldspars of different cross-section 
sizes (Fig. 2d).

The porphyritic granite with pink megacrysts (sample 
2PPG-4) has the following modal composition: microcline 
32%, orthoclase 8%, plagioclase 43%, quartz 8%, biotite 5%, 
muscovite below 1% and chlorite below 1%. Polarized light 
microscope observations confirm two generations of potassium 
feldspar, the early phase being a feldspar within the potassium 

feldspar megacryst with cross-hatched extinction pattern (Fig. 
2b). This extinction pattern is characteristic for microcline 
twinning (Fig. 2c), which is regarded as a result of orthoclase-
to-microcline solid-state transformation (LAVES, 1950; 
McLAREN, 1984).

In the sample PPG-24, orthoclase grains can be distin-
guished besides plagioclase according to polarized light micro-
 scopy (Fig. 2e). This sample has the following modal com-
position: plagioclase 63%, orthoclase 16%, quartz 9%, biotite 
10% and chlorite under 2%. Besides lamellar plagioclase 
grains (Fig. 2e), zoned plagioclase also occurs (Fig. 2f).

Three pink megacrysts and whole rock of 2PPG-4 sample, 
and white feldspar from non-magnetic, low-density fraction 
of PPG-24 sample were analysed with XRPD. The reference 
diffraction data were: quartz (MORRIS et al., 1981), interme-
diate microcline (BORG & SMITH, 1969), orthoclase (BORG 
& SMITH, 1969), albite, calcian (BORG & SMITH, 1968), 
mica (GRIM et al., 1937) in BROWN (1961), albite (SMITH, 
1956), orthoclase (PRINCE et al., 1973), microcline (ALLAN 
et al., 1997) and quartz (PAKHOMOV et al., 1993). For the 
2PPG-4 sample grains XRPD patterns showed orthoclase and 
microcline, minor quartz, albite and mica (Table 1, Fig. 3). The 
diffraction pattern of the whole rock 2PPG-4 sample yielded 
microcline, minor quartz, albite and mica (Table 1). The 
triclinic symmetry of microcline manifests in the D131D 
reflection splitting into (131) and (13̄1) at the following d-values: 
3.015(3.025) Å and 2.980(2.996) Å (Fig. 3, Table 1). The value 
of triclinicity (D = d(131) – d(1-31) * 12.5) is 0.44 (grain a) and 
0.36 (grain c) calculated according to GOLDSMITH & 
LAVES (1954). Low albite peaks appear at the following 

Figure 3. XRPD analysis magnified detail of {131} diffraction region: a) pink megacryst extracted from the granite sample 2PPG-4 (grain a, this study); b) 
pink megacryst extracted from the granite sample 2PPG-4 (grain b, this study); c) pink megacryst extracted from the granite sample 2PPG-4 (grain c, 
this study); d) non-magnetic, low-density fractions of whole rock PPG-24 sample (taken from HORVAT et al., 2011). The related data set is in Table 1) 
IM – intermediate microcline; O – orthoclase; A – albite;  d values in Ångströms.



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49Horvat et al.: Microstructures of feldspar crystals from Pakra Creek valley granite (Papuk Mt., Croatia)

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50 Geologia Croatica 78/1
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4



G
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51Horvat et al.: Microstructures of feldspar crystals from Pakra Creek valley granite (Papuk Mt., Croatia)

d-values: 4.04 Å, 3.87(3.86) Å, 3.64(3.68) Å, 3.19 Å, 2.93(2.94) 
Å and 2.85(2.86) Å (Table 1). One pink grain was determined 
as orthoclase (grain b). These results, together with the tri cli-
nicity values obtained and calculated in the previous study 
(HORVAT et al., 2011) indicate lower triclinicity values for 
potassium feldspars from porphyritic and high(er) tri cli nicity 
values for the potassium feldspars from homogeneous granite 
types (Table 2).

According to XRPD analysis, the white feldspar from 
PPG-24 sample proved to be mixture of intermediate microcline 

and plagioclase with a composition of Ab=71%, An=29% 
(BORG & SMITH, 1968) albite, Ca-bearing, ordered (Table 1). 
Besides feldspar phases, minor mica and quartz were detected. 
Triclinicity of the microcline component is 0.32 (Table 2).

By using TEM methods, crystal chemical details averaged 
out by XRD can be studied, which potentially provides a more 
precise crystallization history of these feldspar crystals. 
During the TEM EDS study of the 2PPG-4 sample mostly 
crystals with Or > 90% and An=0 composition were measured 
(for detailed measurement data see Suppl. 1), besides that only 
one grain with albite composition Or(1.2)Ab(98.8)An(0), and 
low albite structure was found. Figure 4 shows the characteristic 
microstructure of crystals with high potassium content.

Low magnification TEM images of the feldspar megacrysts 
in the 2PPG-4 sample exhibit a tweed pattern (Fig. 4a). On the 
[001] zone axis SAED pattern (Fig. 4a), diffuse scattering is 
observable along the two marked reciprocal axes, which makes 
a star-like shape of the diffraction spots (Fig. 4c). Such a 
distortion of diffraction spots indicates the simultaneous 
presence of albite and pericline twins (SANCHEZ-MUNOZ 
et al., 2012). The angle between [010]* and [100]* is 90°, which 
suggests monoclinic structure, however, careful observation 
of the diffraction spots at large scattering angles reveals that, 
while 90° is the dominant angle, the [010]* axis wobbles up to 
0.7° in the studied volume (Fig. 4b). In some cases, clear 
splitting of 0k0 spots can also be observed. An HRTEM image 
shows the two orthogonal lattice distortions in more detail 
(Fig. 4d). Intensity variation was measured along the [010] and 
[100] lattice directions, and lattice periodicity was found to be 
constant up to 100 nm scale. Undulations of intensity maxima 
were found on the 10 nm scale (Figs. 4e, f) in both directions, 

Table 2. List of triclinicity values (a=12.5*ad(131)-d(13̄1a) calculated ac-
cording to GOLDSMITH & LAVES (1954), for potassium feldspars from non-
magnetic, low density fractions of whole rock (homogeneous rock type) 
and megacrysts (porphyritic rock type) of granites from Pakra Creek valley 
(Papuk Mt., Croatia).

Sample name Rock type
D
calculated 
triclinicity 

1 2PPG-4 (a) porphyritic 0.44

2 2PPG-4 (b)* porphyritic 0

3 2PPG-4 (c) porphyritic 0.36

4 PPG-24* homogeneous 0.32

5 2PPG-3* porphyritic 0

6 2PPG-5* porphyritic 0.29

7 2PPG-6 (a)* porphyritic 0

8 2PPG-6 (b)* porphyritic 0

9 PPG-19* homogeneous 0

10 PPG-18* homogeneous 0.87

11 PPG-20* homogeneous 0.70

12 PPG-23* homogeneous 0.56

*triclinicity values from HORVAT et al. (2011)

Figure 4. a) TEM bright field image of a feldspar crystal of Or(91.9)Ab(8.1)An(0) composition exhibiting tweed pattern with its [001] zone axis SAED pat-
tern in the inset. Indexing is according to orthoclase structure; b,c) enlarged diffraction spots showing their typical shapes; d) HRTEM image of the same 
crystal; and e,f) intensity profiles along lines A and P in a 100 px wide band. Profile A and P show banding related to albite and pericline twinning.



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52 Geologia Croatica 78/1

these correspond to the thickness of albite and pericline twin 
lamellae. (During this analysis, a uniform sample thickness 
was supposed, supported by the same range of intensity 
variation measured in the two perpendicular directions on Fig. 
4e, f).

According to TEM EDS, the feldspar from the PPG-24 
sample is a plagioclase with a Na:Ca ratio close to 3:1 and 
minor K content (72.2%<Ab<76.5%, 23%<An<26.8%, 
Or<1.4%), which is in agreement with XRD results (for 
detailed measurement data see Suppl. 2). Besides, two low 
albite grains (with Ab 97 and 99%) and an anorthite grain 
(An=99%) were detected.

The microstructure of a plagioclase grain is presented in 
Figure 5. [1̄02̄] zone axis SAED pattern is indexed according 
to low albite structure (Fig. 5a), however, in case of low albite, 
the [020]*^[201̄]* calculated angle is 87.5° (Fig. 5c), while the 
measured angle on the experimental SAED pattern is 90°, 
which implies monoclinic symmetry. Besides, a diffuse 

streaking between diffraction spots parallel to [2̄01]* axis is 
observed (Fig. 5a), which implies a more complex, disordered 
microstructure. For a better understanding of the structural 
disorder, FFT analysis of HRTEM images has been performed 
(Fig. 5b). According to FFT analysis, the [020]*^[201̄]* angle 
exhibits certain variation as a function of location, it ranges 
between 86.1° and 90° (Fig. 5d). Additionally, at some 
locations, concentration of the diffuse scattering is observed 
on the FFTs halfway between the spots parallel to the [2̄01]* 
axis (Fig. 5b red dotted arrows). At these locations, reflections 
only appear if the unit cell is larger, i.e. the c-axis is doubled 
with respect to albite structure. Based on these observations, 
we believe that the SAED represents an average structure with 
orthogonal (monoclinic) axes and additionally, disorder along 
[2̄01]* axis. On the nano scale, at the same time, structural and/
or chemical ordering has been initiated, and triclinic domains 
with doubled c-axis developed.

Figure 5. a) SAED pattern of a feldspar crystal of Or(1.4)Ab(72.2)An(26.4) composition (PPG-24 sample), indexed as [1̄02̄] zone axis orientation of low al-
bite. In the inset, the bright field image of the crystal is given. White dotted arrows indicate diffuse scattering between diffraction spots parallel to [2̄01]* 
axis; b) HRTEM image of the same crystal. The FFT of the area indicated by a white circle is in the upper left insert. The red dotted arrows on the FFT 
show that half way between the spots, the diffuse scattering is concentrated. Intensity profile along the A-B band is shown in the upper right insert. Red 
dotted arrows indicate the half way reflections; c) Calculated [1̄02̄] SAED pattern of low albite (S.G. C1̄, a=8.142Å, b= 12.785 Å, c= 7.159 Å, a= 94.19°, a= 
116.61°, a= 87.61°); and d) calculated SAED pattern of a high Ca containing plagioclase (S.G. I1̄, a= 8.188 Å, b= 12. 116.04°Å, c= 14.196 Å, a= 93.37°, a= 
116.04°, a= 90.87°). Red dotted arrows on (d) indicate extra spots with respect to (c); e) Experimental SAED pattern on (a) with the simulation shown on 
(d). Note a good agreement of distances and angles between simulation and SAED.



G
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53Horvat et al.: Microstructures of feldspar crystals from Pakra Creek valley granite (Papuk Mt., Croatia)

4. DISCUSSION
Although the XRPD profiles of the feldspar samples from the 
studied granites are quite similar, and proved to be intermediate 
microcline and orthoclase, (in the case of 2PPG-4) mixed with 
albite (2PPG-24) or albite-Ca (PPG-24), the studied feldspars 
show microstructural and microtextural heterogeneities under 
the polarized light microscope (development of twin lamellae 
and twin boundaries, compositional zonation), and a complex, 
disordered nanostructure. TEM EDS measurements of both 
samples are in agreement with expectations based on the 
XRPD results, and in the case of the PPG-24 sample, they 
allow the specified composition to be related to the nano struc-
ture features.

The presence of orthoclase in the XRPD patterns of the 
2PPG-4 sample indicates a minimum value for the 
crystallization temperature of approx. 530 C° (Fig. 6). The 
studied grains that showed triclinicity in their XRPD patterns, 
based on the splitting of (131) and (13̄1) reflections, reveal 
deviation from the monoclinic structure. Obtained values of 
triclinicity that reflect an ordered Al-Si distribution in the 
tetrahedral sites are small (Δ = 0.44, and 0.36). However, 
besides triclinic symmetry, the HRTEM revealed tweed 
nanostructure. Tweed structure forms by the simultaneous 
development of two independent twin systems, albite and 
pericline, as indicated by SAED patterns and HRTEM images 
(XU et al., 2000). This allows the relaxation of structural 
strain, which arises from the Al-Si ordering with cooling. The 
presence of tweed structure provides a lower limit of feldspar 
crystallization for deduced temperature of approximately 
460 C° (GRIFFEN, 1992).

Based on TEM EDS, the PPG-24 sample contains several 
feldspar phases, albite anorthite and Na-Ca plagioclase (Ab:An 
≈ 3:1), the latter being the most frequently measured (for 
detailed measurement data see Suppl. 2). This composition is 
in agreement with the composition of the plagioclase phase 

Ab=71%, An=29% (BORG & SMITH, 1968), which was iden-
tified on the XRPD pattern, besides the intermediate microcline.

Based on the HRTEM study, the plagioclase composition 
can be related to nanostructure characteristics, which help the 
understanding of crystal formation. Diffuse streaking and 
intensity concentration parallel to [2̄01]* of Na-Ca plagioclase 
indicates a complex, disordered nanostructure and an incipient 
structural and/or chemical ordering in a unit cell with c≈14Å 
(Fig. 5e). Ordering cannot be completed because of kinetic 
reasons, but the triclinic structure is evident on HRTEMs 
(Fig. 5b).

According to the phase diagram (Fig. 7), at the Ab:An = 
3:1 ratio measured in the PPG-24 sample, the plagioclase solid 
solution disproportionated to low albite and I1̄ anorthite at 
400 C°. The I1̄ structure is consistent with the FFT shown on 
Figure 5b. The fact that no incommensurate structures have 
been found, suggests that the average composition of the start-
ing material was close to the eutectic. Regarding the tempera-
ture of crystallization, we can only estimate the lowest value 
as 400 C°, but not the possible highest temperature at which 
granite crystallization started. The zonation observed under 
polarized light is probably related to changing com position, as 
supported by the different Ab:An ratios measured by TEM EDS.

The “chemically” delayed kinetics of ordering are com-
parable to a relatively fast cooling rate, for instance in epizonal 
granitoids in an anorogenic or post-tectonic environment, 
where the emplacement of magma in a relatively cool environ-
ment produces rocks mostly consisting of XRPD proved ortho-
clase (MARFUNIN, 1966; MARMO, 1971). It is in connection 
with the temperature decrease, when the triclinic–monoclinic 
transition occurs (McKENZIE, 2018).

VERNON (1986, 1990, 1999, 2004) and VERNON & 
PATERSON (2002) reviewed the subject of the potassium 
feldspar megacrysts in granite, with the conclusion that the 
phenocrysts evidence is favoured. The same authors suggested 

Figure 6. Estimated average composition of the starting material (arrow) and chemical composition of a feldspar megacryst (asterisks) from 2PPG-4 
sample plotted on the phase diagram of alkali feldspars (after SMITH, 1974; modified by BROWN, 1981; in GRIFFEN, 1992).



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that K-feldspar megacrysts in granites grow late in the cooling 
history, even at subsolidus conditions. The experimental 
evidence indicates that potassium feldspar is one of the last 
minerals to become saturated in felsic magmas (PIWINSKII, 
1968; PIWINSKII & WYLLIE, 1968; WHITNEY, 1975; 
CLEMENS & WALL, 1981). However, experiments also 
indicate that when potassium feldspar begins to crystallize, the 
magma typically contains 60–70% liquid (CLEMENS & WALL, 
1981; WINKLER & SCHULTES, 1982). Though potassium 
feldspar is commonly the last mineral to begin crystallizing in 
granitic magmas, abundant melt is still present at that stage, 
allowing sufficient space for the megacrysts to grow according 
to VERNON (2004, and references therein). Myrmekite 
develops at around 450-500 ?C (TRIBE & D’LEMOS, 1996) and 
500–670 °C (WIRT & VOLL, 1987), and relatively calcic 
plagioclase (oligoclase) in myrmekite, replaces potassium feld-
spar (VERNON et al., 1983) implying relatively high tempera-
tures. The minimum temperature estimated for crystal growth 
is 653–439 °C for two-mica granite as given by LIU et al. (2023).

The modal composition of the two granite samples 
corresponds to a monzogranite–granodiorite composition 
according to IUGS (1973). The homogeneous structure and 
high plagioclase content of the sample PPG-24 most probably 
corresponds to higher crystallization temperatures compared 
to the porphyritic sample 2PPG-4, while the microstructure of 
the feldspars suggests parent melt close to eutectic composition 
for studied granites.

5. CONCLUSION
The feldspars from two different granite varieties from Pakra 
Creek valley (Papuk Mt.), one with a porphyritic texture and 
one with a homogeneous structure, were studied by optical 
pola rizing microscopy, X-ray powder diffraction and trans mis-
sion electron microscopy with energy-dispersive spectroscopy. 
The obtained results enable the following conclusions:

1) The porphyritic granite variety has a monzogranite 
com position and contains pink potassium feldspar megacrysts. 
Pink potassium feldspars are revealed to be orthoclase and 
intermediate microcline (Or > 90% and An=0). The tweed-like 
nanostructure provides evidence for the lower limit of potas-
sium feldspar crystallization temperature of approximately 
460 C°.

2) The homogeneous granite variety has a granodiorite 
composition and medium granular texture. It is abundant in 
plagioclase. It contains several feldspar phases: albite 
(Or0.3Ab98.7An1), anorthite (Or0Ab0.5An99.5) and Na-Ca plagio-
clase (Or0.8Ab74.1An25.1). Under the polarizing microscope the 
observed potassium feldspar is orthoclase, showed as inter-
mediate microcline in XRPD pattern, beside albite. The diffuse 
streaking and intensity concentration parallel to [2̄ 01]* of 
Na-Ca plagioclase indicates structural modulations.

3) The microstructure of the feldspars suggests a parent 
melt close to a eutectic composition for the studied granites.

Figure 7. Estimated average composition of the starting material (arrow) and chemical composition of feldspars (asterisks) from PPG-24 sample plotted 
on the phase diagram of plagioclase (after CARPENTER & McCONNELL, 1984). Abbreviations: Pe – peristerite intergrowth, Bø – Bøgglid intergrowth, Hü 
– Hüttenlocher intergrowth, ss – solid solution. Structure types: e, C1̄, I1̄, P1̄.



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ACKNOWLEDGMENT
The presented work is the first result in the mineralogical frame 
of the internal research project “Complementary petro chrono-
logical characterization of granites, progressively metamorphic 
rocks and migmatites in the Papuk metamorphic complex (Sla-
vo nian Mts., Croatia) (PAPUKRON)” at Croatian Geological 
Survey, funded by National Recovery and Resilience Plan 
2021–2026 of the European Union – NextGenerationEU, and 
monitored by the Ministry of Science, Education and Youth of 
the Republic of Croatia. The Authors extend their gratitude to 
the Department of Geology, Croatian Geological Survey’s 
colleagues who helped in any way during the preparation of 
this work. The authors acknowledge the remarks and suggestions 
of the reviewers and associate editor, which greatly contributed 
to the quality of the manuscript.

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Supplement 1. – TEM EDS measurements of grains from sample 2PPG-4.
2PPG-4 sample, pink grain 2024/08/01 analysis number 0928 – Or(1.2)Ab(98.8)An(0)

Z Element Family Atomic Fraction (%) Atomic Error (%) Mass Fraction (%) Mass Error (%) Fit Error (%)

8 O K 61.45 3.06 48.65 3.31 0.38

11 Na K 7.34 1.41 8.35 1.63 1.66

13 Al K 7.63 1.44 10.19 1.93 0.96

14 Si K 23.49 3.49 32.65 4.31 0.68

19 K K 0.09 0.02 0.17 0.03 2.54

2PPG-4 sample, pink grain 2024/08/01 analysis number 1030 – Or(91.8)Ab(8.2)An(0)

Z Element Family Atomic Fraction (%) Atomic Error (%) Mass Fraction (%) Mass Error (%) Fit error (%)

8 O K 60.69 6.70 45.40 3.21 2.11

11 Na K 0.63 0.14 0.67 0.14 1.03

13 Al K 8.34 1.88 10.52 2.20 1.76

14 Si K 23.41 5.18 30.74 6.29 1.58

19 K K 6.93 1.43 12.67 2.39 0.40

20 Ca K 0.00 0.00 0.00 0.00 0.00

2PPG-4 sample, pink grain 2024/08/01 analysis number 1045 – Or(89.5)Ab(10.5)An(0)

Z Element Family Atomic Fraction (%) Atomic Error (%) Mass Fraction (%) Mass Error (%) Fit error (%)

8 O K 60.78 6.90 45.83 3.32 2.03

11 Na K 0.65 0.15 0.70 0.15 1.11

13 Al K 8.30 1.89 10.56 2.22 1.51

14 Si K 24.77 5.52 32.78 6.72 1.33

19 K K 5.50 1.15 10.13 1.92 0.34

20 Ca K 0.00 0.00 0.00 0.00 0.00

2PPG-4 sample, pink grain 2024/08/01 analysis number 1047 – Or(93.3)Ab(6.7)An(0)

Z Element Family Atomic Fraction (%) Atomic Error (%) Mass Fraction (%) Mass Error (%) Fit Error (%)

8 O K 61.66 2.98 46.31 3.03 0.38

11 Na K 0.51 0.11 0.55 0.12 5.92

13 Al K 7.90 1.49 10.01 1.90 2.29

14 Si K 22.87 3.42 30.16 4.15 1.04

19 K K 7.07 1.20 12.97 2.14 0.16

20 Ca K 0.00 0.00 0.00 0.00 515.82

2PPG-4 sample, pink grain 2024/08/01 analysis number 1136 – Or(86.6)Ab(13.4)An(0)

Z Element Family Atomic Fraction (%) Atomic Error (%) Mass Fraction (%) Mass Error (%) Fit Error (%)

8 O K 61.00 3.02 45.84 3.05 0.74

11 Na K 1.00 0.20 1.08 0.23 1.14

13 Al K 7.95 1.49 10.07 1.90 1.27

14 Si K 23.54 3.50 31.05 4.22 1.35

19 K K 6.51 1.12 11.95 2.00 0.07

20 Ca K 0.00 0.00 0.01 0.00 19.49

2PPG-4 sample, pink grain 2024/09/24 analysis number 1259 – Or(91.9)Ab(8.1)An(0)

Z Element Family Atomic Fraction (%) Atomic Error (%) Mass Fraction (%) Mass Error (%) Fit error (%)

8 O K 61.42 6.56 46.18 3.13 1.58

11 Na K 0.59 0.13 0.64 0.13 2.18

13 Al K 8.43 1.89 10.69 2.23 0.93

14 Si K 22.83 5.02 30.13 6.14 1.11

19 K K 6.73 1.38 12.36 2.32 0.41

20 Ca K 0.00 0.00 0.00 0.01 0.00



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2PPG-4 sample, pink grain 2024/09/24 analysis number 1231 – Or(94.8)Ab(5.2)An(0)

Z Element Family Atomic Fraction (%) Atomic Error (%) Mass Fraction (%) Mass Error (%) Fit error (%)

8 O K 60.09 6.72 44.97 3.14 0.42

11 Na K 0.33 0.08 0.36 0.07 1.33

13 Al K 8.41 1.91 10.61 2.22 0.96

14 Si K 25.09 5.59 32.96 6.76 1.20

19 K K 6.07 1.27 11.11 2.10 0.58

20 Ca K 0.00 0.00 0.00 0.00 0.00

2PPG-4 sample, pink grain 2024/09/24 analysis number 1135 – Or(91.1)Ab(8.9)An(0)

Z Element Family Atomic Fraction (%) Atomic Error (%) Mass Fraction (%) Mass Error (%) Fit Error (%)

8 O K 61.64 2.99 46.47 3.04 0.85

11 Na K 0.62 0.13 0.68 0.14 0.75

13 Al K 8.41 1.57 10.69 2.00 1.15

14 Si K 22.88 3.42 30.28 4.16 0.74

19 K K 6.45 1.10 11.88 1.98 0.35

20 Ca K 0.00 0.00 0.00 0.00 0.00



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Supplement 2. – TEM EDS measurements of grains from sample PPG-24.
PPG-24 sample, white grain 2024/08/01 analysis number 1316 – Or(0.5)Ab(76.5)An(23)

Z Element Family Atomic Fraction (%) Atomic Error (%) Mass Fraction (%) Mass Error (%) Fit Error (%)

8 O K 61.08 2.80 47.67 2.94 0.59

11 Na K 6.63 1.28 7.44 1.45 1.88

13 Al K 9.85 1.80 12.96 2.35 1.43

14 Si K 20.40 3.16 27.94 3.96 0.95

19 K K 0.05 0.01 0.09 0.02 5.31

20 Ca K 1.99 0.28 3.90 0.55 0.28

PPG-24 sample, white grain 2024/08/01 analysis number 1322 – Or(0)Ab(0.7)An(99.3)

Z Element Family Atomic Fraction (%) Atomic Error (%) Mass Fraction (%) Mass Error (%) Fit Error (%)

8 O K 56.85 2.68 40.80 2.49 0.63

11 Na K 0.08 0.02 0.08 0.02 7.85

13 Al K 16.88 2.83 20.44 3.35 1.26

14 Si K 15.61 2.59 19.67 3.18 0.33

19 K K 0.00 0.00 0.00 0.00 0.00

20 Ca K 10.57 1.34 19.01 2.30 0.22

PPG-24 sample, white grain 2024/08/01 analysis number 1412 – Or(1.4)Ab(72.2)An(26.4)

Z Element Family Atomic Fraction (%) Atomic Error (%) Mass Fraction (%) Mass Error (%) Fit Error (%)

8 O K 57.37 3.00 43.71 3.01 1.27

11 Na K 5.66 1.11 6.20 1.24 1.11

13 Al K 10.98 2.00 14.11 2.55 0.67

14 Si K 23.80 3.54 31.82 4.29 0.45

19 K K 0.11 0.02 0.21 0.04 1.37

20 Ca K 2.07 0.29 3.95 0.57 0.39

PPG-24 sample, white grain 2024/09/24 analysis number 1355 – Or(0.6)Ab(99.4)An(0)

Z Element Family Atomic Fraction (%) Atomic Error (%) Mass Fraction (%) Mass Error (%) Fit Error (%)

8 O K 62.80 2.97 50.10 3.26 0.60

11 Na K 7.28 1.39 8.34 1.62 0.90

13 Al K 7.71 1.45 10.37 1.95 1.22

14 Si K 22.06 3.33 30.90 4.18 0.24

19 K K 0.04 0.01 0.08 0.01 2.73

PPG-24 sample, white grain 2024/09/24 analysis number 1409 – Or(0.3)Ab(97.5)An(2.3)

Z Element Family Atomic Fraction (%) Atomic Error (%) Mass Fraction (%) Mass Error (%) Fit error (%)

8 O K 61.46 6.62 48.60 3.28 0.09

11 Na K 7.01 1.58 7.96 1.67 0.59

13 Al K 8.08 1.82 10.77 2.25 0.07

14 Si K 23.27 5.13 32.30 6.59 0.06

19 K K 0.02 0.00 0.03 0.01 7.48

20 Ca K 0.17 0.03 0.33 0.05 0.90

PPG-24 sample, white grain 2024/09/24 analysis number 1427 – Or(0)Ab(0.3)An(99.7)

Z Element Family Atomic Fraction (%) Atomic Error (%) Mass Fraction (%) Mass Error (%) Fit Error (%)

8 O K 57.79 2.77 41.82 2.62 2.91

11 Na K 0.03 0.01 0.03 0.01 10.30

13 Al K 16.18 2.75 19.75 3.28 1.51

14 Si K 16.08 2.66 20.42 3.27 0.26

19 K K 0.00 0.00 0.00 0.00 0.00

20 Ca K 9.92 1.28 17.99 2.21 0.09



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PPG-24 sample, white grain 2024/10/09 analysis number 1030 – Or(0.2)Ab(76.5)An(23.3)

Z Element Family Atomic Fraction (%) Atomic Error (%) Mass Fraction (%) Mass Error (%) Fit error (%)

8 O K 62.41 6.36 49.34 3.20 1.53

11 Na K 6.47 1.44 7.35 1.53 0.22

13 Al K 9.19 2.04 12.25 2.54 0.35

14 Si K 20.87 4.53 28.96 5.87 0.14

19 K K 0.01 0.00 0.03 0.01 7.70

20 Ca K 1.05 0.18 2.08 0.31 0.11

PPG-24 sample, white grain 2024/10/09 analysis number 1103 – Or(0.8)Ab(72.4)An(26.8)

Z Element Family Atomic Fraction (%) Atomic Error (%) Mass Fraction (%) Mass Error (%) Fit error (%)

8 O K 61.27 6.16 47.73 3.02 0.86

11 Na K 5.41 1.21 6.06 1.26 2.06

13 Al K 10.44 2.31 13.72 2.84 0.68

14 Si K 20.81 4.52 28.46 5.76 0.17

19 K K 0.07 0.01 0.12 0.02 2.57

20 Ca K 2.00 0.33 3.91 0.57 0.32

PPG-24 sample, white grain 2024/10/09 analysis number 1140 – Or(0.9)Ab(73)An(26.1)

Z Element Family Atomic Fraction (%) Atomic Error (%) Mass Fraction (%) Mass Error (%) Fit Error (%)

8 O K 59.78 3.08 46.18 3.19 2.60

11 Na K 4.42 0.88 4.91 0.99 0.27

13 Al K 11.00 2.01 14.33 2.59 1.32

14 Si K 23.15 3.48 31.40 4.27 0.95

19 K K 0.06 0.01 0.11 0.02 4.95

20 Ca K 1.58 0.22 3.06 0.45 0.17