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Micas from mariupolite of the Oktiabrski massif (SE Ukraine): An insight into the host rock evolution – Geochemical data supported by Raman microspectroscopy
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 137 (2015) 817–826
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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Micas from mariupolite of the Oktiabrski massif (SE Ukraine): An insight into the host rock evolution – Geochemical data supported by Raman microspectroscopy Magdalena Duman´ska-Słowik a,⇑, Aleksandra Wesełucha-Birczyn´ska b, Adam Pieczka a a Faculty of Geology, Geophysics and Environmental Protection, Department of Mineralogy, Petrography and Geochemistry, AGH – University of Science and Technology, 30 Mickiewicza Av., 30-059 Kraków, Poland b Faculty of Chemistry, Jagiellonian University, 3 Ingardena Str., 30-060 Kraków, Poland
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Fresh annite, slightly weathered
biotite (annite–siderophyllite) and muscovite are found in mariupolite. Annite contains a wealth of inclusions: aegirine, albite, nepheline, pyrochlore, magnetite, natrolite, hematite. Fresh annite is a primary magmatic phase. Slightly altered dark mica is reequilibrated biotite or secondary post magmatic component of mariupolite. Muscovite is typical post-magmatic, hydrothermal phase free of inclusions.
a r t i c l e
i n f o
Article history: Received 23 May 2014 Received in revised form 30 July 2014 Accepted 24 August 2014 Available online 8 September 2014 Keywords: Biotite Muscovite Solid inclusions Oktiabrski massif Ukraine
a b s t r a c t Muscovite and two dark mica varieties (the coarse-crystalline, pegmatitic, and fine-crystalline with signs of early weathering) representing members of the biotite series, originating from mariupolite of the Oktiabrski massif, (Ukraine), were investigated along with their solid inclusions using electron microprobe and Raman micro-spectroscopy to discuss their genesis and relationship to the parental magma. 3+ The coarse-crystalline, pegmatitic biotite, (K1.90Rb0.02Na0.01)(Fe2+ 3.56Mg1.34Ti0.36Fe0.34Mn0.03)[(Si5.73Al2.10Fe3+ 0.17)O20](OH3.24 F0.76) represents the primary, magmatic annite that crystallized from an alkaline, Fe-rich and Mg-depleted host magma, whereas the fine-crystalline biotite, partly altered to vermiculite, (K1.752+ Rb0.03Na0.03)(Fe3+ 3.23Fe1.16Mg0.26Mn0.04Ti0.10)[(Si5.16 Al2.84)O20](OH)4.00, devoid of F, represents a re-equilibrated or secondary, post-magmatic Fe3+-bearing mica crystallized from alkaline to the subalkaline host magma. Muscovite, (K1.96Na0.06)(Al3.97Fe2+ 0.06)[(Si5.99Al2.01)O20](OH)4, with low Na/(Na + K) ratio, low Fe and devoid of Ti and also F, forms only tiny, subhedral flakes formed in the post-magmatic, hydrothermal stage. The primary, unaltered biotite contains numerous solid inclusions of primary origin (albite, aegirine, zircon, K-feldspar, nepheline, pyrochlore, magnetite) and secondary origin (natrolite, hematite, Ti-Mn oxides/hydroxides); most of them are accompanied by a carbonaceous substance, all confirmed by scanning electron microscopy and Raman microspectroscopy. Ó 2014 Elsevier B.V. All rights reserved.
⇑ Corresponding author. E-mail address: [email protected] (M. Duman´ska-Słowik). http://dx.doi.org/10.1016/j.saa.2014.08.127 1386-1425/Ó 2014 Elsevier B.V. All rights reserved.
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Introduction Micas are one of the most common group of sheet silicates in the Earth. They are widespread in a variety of geological environments from igneous, sedimentary to metamorphic rocks. Their simplified formula, end-members and species for the true, brittle and interlayer-deficient micas are presented by Rieder et al. [1] and Tischendorf et al. [2]. The specific mica structure enables them to incorporate a large number of main and trace elements of different radii and charges [3], so the concentrations of some of the elements can be sensitive indicators of the physiochemical environment, in which a host rock crystallized [e.g. 4,5]. The compositions of the mica-group minerals are mainly dependent on geochemical and physical parameters as well as temperature, pressure, oxygen-, fluorine- and chlorine fugacities, whole rock composition and the coexisting mineral association [6]. Abdel-Rahman [7], Shabani et al. [8] and Moazamy [9] noticed that compositions of dark micas, representing the biotite-subgroup minerals (a series including phlogopite, siderophyllite, annite and eastonite), depend largely upon the nature of the magmas, from which they have crystallized. Hence, the compositions of mica-group minerals usually bear an important imprint on the petrogenetic processes. Micas, mainly trioctahedral species, form up to 20 vol.% of the pegmatitic variety of mariupolite (nepheline syenite) from the Oktiabrski massif (SE Ukraine). Mariupolite, as the host rock of the micas, commonly exhibits a subsolidus overprint due to the circulation of post-magmatic fluids and metasomatic alterations, e.g. fenitization [cf.10]. Hence, micas can be a sensitive monitor of its magmatic crystallization and later post-magmatic alteration. The objects of the study are dioctahedral and trioctahedral mica-group minerals from the mariupolite of the alkaline Oktiabrski massif. Special attention was paid to the main and subordinate elements present in the structure of these minerals to assess their petrologic significance and genetic condition. Moreover, the characterization of solid inclusions found in the dark micas with the SEM–EDS analyses supported by Raman microspectroscopy was done to discuss the sequence of their crystallization. The chemistry of mica-group minerals together with the assembly of the solid inclusions seem to have the potential for a better understanding of the final stage of the host rock’s consolidation.
represent a leucocratic, coarse-crystalline variety of mariupolite with pegmatitic and mainly chaotic texture, composed mainly of plagioclase and nepheline. The predominant mineral is plagioclase (Ab94–92An6–8), which occurs as fine, prismatic crystals with a few twin individuals, or larger crystals of an older generation. Nepheline forms large, dynamically deformed crystals with non-uniform light extinction. Members of the biotite series, K-feldspar, pyrochlore-group minerals, sodalite, natrolite, aegirine, cancrinite, muscovite and Fe-bearing chlorite all appear as subordinate components. Dark mica forms well preserved flakes or tiny crystals forming fan like textures. The latter variety of dark mica exhibits some traces of an alteration process. K-feldspar forms large tabular crystals with well-developed cleavages along (0 0 1) and (0 1 0). Some of them reveal a crosshatched pattern, typical of microcline. Aegirine forms stout prismatic crystals of varying size and shows strong pleochroism: a = green, b = pale green, c = pale yellowgreen. Sodalite is characterized by blue colour visible in thin section. Natrolite locally forms prismatic crystals, but it sometimes occurs as pseudomorphs after nepheline megacrysts [20]. Zircon is abundant in the form of euhedral crystals with a characteristic oscillatory zoning along the grain margins, and irregular patchy zoning in the interior [10]. Euhedral to occasionally subhedral crystals of the pyrochlore-group minerals occur in the interstices between albite and possess a characteristic reaction rim built of needle-shaped crystals of Fe-bearing chlorite [21]. The results of a bulk-rock analyses were presented in Duman´ska-Słowik et al. [22]. Analytical methods Scanning electron microscopy (SEM) Back-scattered electron (BSE) images of polished sections were obtained using a FEI Quanta 200 FEG scanning electron microscope equipped with a EDS detector. The system operated at 25 kV accelerating voltage in a high-vacuum mode. The SEM–EDS observations were applied to identify solid inclusions within a mica matrix. These analyses were then supported by micro Raman spectroscopy.
Geological background
Raman microspectroscopy (RS)
The Oktiabrski alkaline massif (Ukrainian Shield, SE Ukraine) has been known since Joseph Morozewicz described it in the late 19th century. The massif, which is 34 km2 in area [11], is a unique province of alkaline magmatism of Proterozoic age ca. 1.8 Ga [12,13] (Volkova 2000, 2001). The general geology and genesis of this massif were described in detail by Tichonienkova et al. [14], Donskoy [15], Solodov [16], Krivdik et al. [11] and Volkova [12,13]. According to these data, it is mainly composed of alkali syenites and foyaites with minor amounts of mariupolite (i.e. rare aegirine–nepheline–albite syenite), subalkaline gabbro, and their derivatives (peridotites, pyroxenites). A few dikes of aegirine foyaites and agpaitic phonolites outcrop mainly outside the massif among host granitoides. The vast majority of rocks belong to the agpaitic series, while mariupolite is intermediate between the miaskitic and agpaitic types. Mariupolite contains such accessory minerals as zircon, pyrochlore, britholite, which is more typical of the miaskitic nepheline syenites, though melanocratic varieties of mariupolites have an agpaitic index up to 1.5 (owing to the presence of aegirine) [17].
Raman spectra of inclusions in mica crystals were recorded using a Renishaw InVia Raman spectrometer, operating in a confocal mode and working in a backscatter geometry with a Leica microscope equipped with 100 and 50 magnification objectives. The samples were excited with a 785 nm line of high power nearIR (HP NIR) laser. The spectrum of albite was collected using the microscope accessory for Thermo Electrons Nicolet NXR FT-Raman spectrometer and excited with a 1064 nm laser line. Laser power was kept low enough to protect organic inclusions from possible laser-induced degradation. The laser focus diameter was ca. 1– 2 lm. It should be noted that almost no sample preparation was performed. The surface of the mica flakes was cleaned carefully with distilled water and acetone before measurements were made to ensure that it was not contaminated. Subsequently the flakes were inspected by an optical Leica microscope and Raman spectra were collected from the most spectacular inclusions.
Petrography of mariupolite
Electron-microprobe analyses were made at the Inter-Institute Analytical Complex for Minerals and Synthetic Substances of Warsaw University with a Cameca SX 100 operating in a wavelengthdispersive (WDS) mode under the following conditions: accelerated
Mariupolite, named by Morozewicz [18,19] covers only a small area (ca. 5–10%) of the Oktiabrski massif. The samples examined
Electron-microprobe analyses (EMPA)
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voltage of 15 kV, beam current of 10 nA, peak count-time of 20 s and background time of 10 s. The following standards, analytical lines, diffracting crystals and mean detection limits (wt.%) were used: orthoclase – K (Ka, PET, 0.03), Rb glass – Rb (La, TAP, 0.10), albite – Na (Ka, TAP, 0.04), wollastonite – Ca (Ka, PET, 0.05) and Si – (Ka, TAP, 0.03), hematite – Fe (La, LIF, 0.14), rutile – Ti (Ka, PET, 0.05), rhodonite – Mn (Ka, LIF, 0.13), diopside – Mg (Ka, TAP, 0.02), jadeite – Al (Ka, TAP, 0.03), phlogopite – F (Ka, TAP, 0.61). The raw data were corrected with the PAP procedure [23] using/ an original software./NOT CLEAR The crystal-chemical formulae of white mica were calculated on the basis of 24 (O, OH, F) with H2O iterated by stoichiometry, whereas those of dark mica on the basis of 24 (O, OH, F) with H2O and Fe3+/Fetotal ratio iterated in relation to (OH + F) = 4 per formula unit (p.f.u.), [4]Fe3+ = 8–Al–Si p.f.u., [6] Fe3+ = 6–Fe2+–Mn–Mg–2Ti p.f.u., and Fe3+ = [4]Fe3++[6]Fe3+ p.f.u. The XRD patterns were recorded with a Philips PW 3020 X’PertAPD Diffractometer system (with a Cu anode and graphite monochromator) at 35 kV voltage and 30 mA current, in the 2H range of 5–75° with a 0.05°(2H)/s step. The characterization of the weathering process of dark mica was carried out by XRD on a air-dried, glycol-saturated and heated sample at 560 °C for 2 h [e.g. 24,25].
Results Light mica The light mica forms very tiny crystals with sizes of up to 30 lm and having no inclusions. Within the host rock it mainly coexists with aegirine, albite, nepheline, dark mica and microcline. The compositions obtained for the mica (Table 1) are very homogenous, which gives the opportunity to define a mean structural formula as: (K1.948Na 0.058)(Al3.967 Fe2+ 0.049)]Si5.992Al2.007O20](OH)4, almost identical to the ideal end-member muscovite formula defined by Miller et al. [26] and Rieder et al. [1]. Tischendorf et al. [2] proposed a graphical presentation for potassium mica based on octahedrally coordinated cations, i.e. mgli (Mg–Li) vs feal (Fe + Mn + Ti–(VI)Al) in atoms per formula unit. The data presented in this diagram plot into muscovite Al2 field (Fig. 1). Relatively small amounts of Na+ substitute for K+ in the interlayer site; the octahedral sites are occupied mainly by Al with traces of Fe2+, whereas the tetrahedral positions are filled typically with Si and Al. Rubidium, Ca, Mn, Mg and Ti and F occur under the detection limits.
Dark mica The dark mica forms mainly euhedral to subhedral flakes, 0.05– 4.0 cm in length, with pleochroism changing the colour from light green to yellowish green along the z axis and yellowish brown to dark brown along the x axis. Some crystals are rich in pleochroic halos revealing the presence of zircon inclusions. The mica frequently occurs in paragenesis with albite, nepheline, zircon, aegirine, and natrolite, and its flakes usually are enriched in numerous inclusions of main magmatic minerals (mainly Na- and K-feldspars) what makes the poikilitic texture locally observed within the host rock. Feldspars forming anhedral crystals are randomly scattered within the mica matrix (Fig. 2). The other solid inclusions are represented by aegirine, pyrochlore, Fe compounds (oxide/ hydroxide), Ti–Mn compounds (oxide/hydroxide) and Ti–Mn–Fe oxides. Aegirine forms long prismatic crystals, whereas pyrochlore occurs as euhedral grains. The single and complex compounds of Fe, together with Ti–Mn oxides/hydroxides, occur as anhedral inclusions frequently coexisting with albite or aegirine crystals (Fig. 2). Hence, two- or three phase solid inclusions (e.g. albite–
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FeO/OH, albite–aegirine–Ti–MnO/OH) do not belong to the rarity found in the crystal of the trioctahedral mica. Coarse-crystalline, pegmatitic dark mica, (K1.896 Rb0.019 Na0.014) 3+ 3+ [(Fe2+ 3.557 Mg1.345 Mn0.028) (Ti 0.363Fe0.344)] [Si5.727 Al 2.100 Fe0.173 O20] (OH3.241 F0.759], shows more complex composition than muscovite (Table 1). The interlayer site of the biotite is mainly filled by K, while Na and Rb occur in much lower concentrations. The octahedral sites are occupied mainly by Fe2+, Mg, Ti, Fe3+ and Mn. The predominance of Fe2+ over Mg in these sites makes this mica closer to annite than 3+ phlogopite, i.e. Fe2+ occupy the 2 –Mg joints [1,27]. Silicon, Al and Fe tetrahedral sites. Using molecular proportions, the chemical index of alteration (CIA) (Nesbitt and Young 1982 fide [28]) was calculated for the biotite crystals to be approximately 54%. Hence, the trioctahedral mica is quite fresh. Clauer et al. [29] noted that fresh micas are enriched in K and impoverished in Na and Ca. The chemical composition of fresh dark mica indicates the total Al values of 1.963–2.239 a.p.f.u, pronounced variations of Mg contents, i.e. 0.842–2.135 a.p.f.u. and the average Fe2+/(Fe2+ + Mg) ratio of 0.725. These data presented in the idealised annite–phlogopite–siderophyllite–eastonite quadrilateral [30] plot the mica in the field of annite [vide 31] (Fig. 3). In the host rock, the aggregates of tiny flakes of a dark mica with visible signs of alteration and characteristic fan like texture coexist with nepheline, K-feldspar and cancrinite. The interlayer site of this trioctahedral mica, (K1.748 Rb0.032 Na0.027) [(Fe2+ 1.164 Mg0.264 Mn0.039) (Ti0.101Fe3+ 3.229)] [Si5.160 Al2.840 O20] (OH4.000], is filled mainly by K, also with traces of Na and Rb, and remains partly vacant (0.131–0.294) (Table 1). Similarly, its octahedral sites are occupied mainly by Fe3+, Fe2+, and subordinately Mg, Ti and Mn with vacancies ranging from 1.008 up to 1.526 a.p.f.u. The tetrahedral sites are filled by Si and Al, and the anion positions are occupied solely by O + OH and devoid of F. The CIA index of this dark mica variety is 64, corresponding to the fresh or slightly weathered biotites. The chemical composition of the mica indicates high and variable Al contents (2.656–3.264 a.p.f.u), considerable depletion in Mg (0.137– 0.364 a.p.f.u) and an average Fe2+/(Fe2+ + Mg) ratio of 0.760. In the annite–phlogopite–siderophyllite–eastonite diagram (Fig. 3), the mica clusters around the annite–siderophyllite border, whereas in the mgli vs feal diagram [2] it plots into the Fe2Mg biotite field (Fig. 1). An XRD analyses of the weathered biotite show the presence of smectite and vermiculite as secondary phases (Fig. 4). The Raman spectrum of fresh, pegmatitic dark mica is presented in Fig. 5 with characteristic bands at 406, 515, 646, 764 and 888 cm 1. The wide and strong peak at ca. 515 cm 1 and weaker at 406 cm 1 are associated with the complex set of translational modes of cations within the octahedral and interlayer sites relative to the SiO4 groups in the tetrahedral layer. The bands at 646 and 764 cm 1 are contributed by vibrational modes of Si–O–Si bonds. The band at 888 cm 1 arises from the stretching mode of the Si– O bond in SiO4 tetrahedra [32]. The slightly broaden Raman bands of the annite (Fig. 5) suggests some structural disorder in the crystalline mica [e.g. 33]. In Raman spectra of the mica, characteristic bands are shifting a bit across the local composition of the mineral. The presence of various types of solid inclusions within the biotite flakes, identified under SEM were confirmed by Raman microspectroscopy. Albite with characteristic bands at 290, 408, 478, 507, 762, 816, 1098 and 1113 cm 1, belongs to the most numerous inclusions found within the mica (Fig. 6). The bands at 478 and 762 cm 1 are due to Na–O stretching vibrations, whereas at 816 it is connected with Si–O bending and stretching vibrations, respectively. The bands at 1098 and 1113 cm 1 are associated with breathing modes of tetrahedra involving T–O stretching vibrations. A very distinctive band at 507 cm 1 is attributed to the compression of four-membered tetrahedral rings and/or and O–Na–O breathing [34,35].
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Table 1 Representative results of microprobe analyses of micas from mariupolite. Analyses No sample
1 No 17-muscovite
2 No 32 fresh ‘biotite’
3 No 3 fresh ‘biotite’
4 No 58 weathered ‘biotite’
5 No 71 weathered ‘biotite’
wt.% SiO2 TiO2 Al2O3 Fe2O3 FeO MnO MgO CaO Na2O K2O Rb2O H2O F Total
45.05 0.00 37.92 0.00 0.50 0.00 0.00 0.00 0.16 11.52 0.00 4.50 0.00 99.65
36.69 2.70 11.03 2.42 27.45 0.32 5.90 0.00 0.00 9.35 0.00 2.92 1.76 100.55
35.77 3.32 11.00 3.38 27.38 0.34 4.93 0.00 0.06 9.29 0.52 3.08 1.35 100.41
34.06 0.48 15.55 30.73 5.78 0.27 1.39 0.11 0.08 8.78 0.52 3.93 0.00 101.70
32.35 1.20 15.21 23.32 13.87 0.35 0.61 0.00 0.10 8.65 0.63 3.77 0.00 100.06
(IV)
Si Al Fe3+ T site (VI) Al Ti Fe3+ Fe2+ Mn Mg O site Ca Na K Rb A site O OH F (IV) (IV)
6.004 1.996 0.000 8.000 3.960 0.000 0.000 0.056 0.000 0.000 4.016 0.000 0.045 1.958 0.000 2.003 20 4.000 0.000
5.863 2.078 0.059 8.000 0.000 0.325 0.232 3.669 0.044 1.406 5.676 0.000 0.000 1.906 0.000 1.906 20 3.110 0.889
5.770 2.090 0.140 8.000 0.000 0.403 0.271 3.694 0.046 1.184 5.598 0.000 0.021 1.911 0.027 1.959 20 3.311 0.689
5.201 2.799 0.000 8.00 0.000 0.056 3.531 0.731 0.035 0.317 4.670 0.018 0.028 1.711 0.026 1.783 20 4.000 0.000
5.147 2.853 0.000 8.000 0.000 0.144 2.793 1.846 0.047 0.146 4.976 0.000 0.033 1.756 0.032 1.821 20 4.000 0.000
Note: 0.00 – under detection limit.
Fig. 1. The positions of mica species from mariupolite plotted in simplified mgli/ feal (Mg–Li vs Fe + Mn + Ti–(VI)Al) diagram of Tischendorf et al. (2007). Symbols: circle – muscovite, square – fresh biotite, triangle – slightly weathered biotite.
Another, quite common inclusion found is a pyrochlore-group mineral (Fig. 7) with marker bands at 464, 546, 782 and 1145 cm 1 (http://rruff.info/pyrochlore/display=default/R060288). In the pyrochlore, the bands have no constant position and intensities; they rather shift across the local defects in their structure. The possible shifts of the 515 cm 1 band is associated with the variability of the B–X bond length in the pyrochlore structure [vide 36]. This black inclusion is accompanied by a carbonaceous matter (Fig. 7), and its presence is clearly marked by bit broaden bands appearing in the region of 1311–1602 cm 1 [e.g. 37,38]. The band at 1322 cm 1 (called D-band) is attributed to a disorder-induced first order Raman mode, while the G-band at 1616 cm 1 is due to a first order E2g Raman mode [39].
The traces of nepheline found within the mica matrix (Fig. 8) are evidenced by bands at 404, 940 and 1065 cm 1. The marker band at 404 cm 1 is due to msT–O–T vibrations [33]. The other bands at Fig. 8, i.e. 671, 712, 759 and 1585 cm 1 are attributed to the mica matrix (vide Fig. 5) and carbonaceous matter (vide Fig. 7), respectively. The presence of natrolite is also clearly marked by 145, 393, 521, 730 cm 1 bands (Fig. 9). The bands at 145 cm 1 are attributed to a square–rhombic distortion of the Si2Al2O10 ring, while 393 cm 1 comes from the liberation of the TO4 tetrahedra. The bands at 521 and 730 cm 1 are connected with O–T–O bending vibrations [40]. In Fig. 9 the Raman signal from the mica matrix is clearly visible as the intensive band in the region of ca. 500 cm 1, whereas the presence of three bands at 486, 509 and 674 cm 1 could suggest the traces of iron compounds (maghemite c-Fe2O3?) [41–42]. Among other Fe-bearing minerals, hematite manifests its presence by showing Fe–O symmetric bending vibrations at 291 and 409 cm 1 [41]. The relatively wide and intensive band at 645 cm 1 and second order band at 1320 cm 1 can be attributed to the traces of magnetite (Fig. 10) [42]. Discussion In mariupolite, mica-group minerals are represented mainly by dark micas of the biotite series, whereas muscovite is rather an accessory component and restricted only locally to host rocks The calculated Na/(Na + K) ratio for muscovite ranges from 0.01 to 0.04, suggesting it formed during post-magmatic activity and was strongly affected by hydrothermal solutions [3]. The
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Fig. 2. Solid inclusions assembly in fresh dark mica, BSE image. Symbols: Ab – albite, K-Fs – K-feldspar, Aeg – aegirine, Pcl – pyrochlore.
Fig. 3. Nomenclature of fresh and weathered biotite from mariupolite expressed in the Fe2+/(Fe2+ + Mg) vs RAl (a.p.f.u) diagram of Deer et al. [31].
hydrothermal genesis of muscovite is also supported by low Fe concentration, since the Fe content is much higher in a typical magmatic muscovite [3]. Moreover, muscovite from the mariupolite is completely devoid of Ti, impoverished in Al and Na, what is also consistent with its post-magmatic, hydrothermal genesis [43–45]. The hypothetical post-magmatic origin of the mica is also consistent with its textural characteristics, as tiny, not welldeveloped flakes without characteristic twinning. Interestingly, this muscovite is completely devoid of F and Ca, elements which should have accumulated during hydrothermal activity and should have been present in the environment of its growth. It is very probable, that crystallization of the fluorite, a second-rank component of mariupolite, competed with muscovite for F and Ca [vide 23]. Coarse-crystalline dark mica, representing members of the biotite series, is relatively fresh or only slightly weathered. The presence of vermiculite in the slightly weathered biotite samples indicates a typical early stage of weathering [e.g. 46,28], leading in the next, more advanced stage to smectite. According to Nahon
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Fig. 4. XRD patterns of slightly weathered biotite recorded at following conditions: air-dried, glycol-saturated and at 560 °C heated preparation.
Fig. 5. The Raman spectrum of fresh biotite in the range 1700–120 cm excitation).
1
(785 nm
[47], transformation into vermiculite results from the exfoliation of biotite and layers opening during weathering, accompanying by the loss of K+ observed for slightly weathered biotite. Weathering of the biotites leads also to the depletion of octahedral cations and an increase in octahedral vacancies of up to1.526 a.p.f.u. The compositional relationships of the two generations of biotite, i.e. fresh and slightly weathered, are presented in the diagram based on Vacancy [at the interlayer site] vis Vacancy [at the octahedral sites] (Fig. 11). The positive correlation is observed only for slightly weathered (re-equilibrated) biotite (R = 0.43). This secondary biotite has significantly higher vacancies in both the interlayer and octahedral sites, exhibiting, along with a simultaneous increase of aluminium content, a late post-magmatic evolution of the primary biotite [48] due to the late subsolidus reactions associated with weathering and the recrystallization of dark mica to small flakes.
Fig. 6. The photomicrograph of albite inclusion and its Raman spectrum in the range 1800–200 cm 1 (1064 nm excitation).
To recognise between the primary, early magmatic, re-equilibrated, late-magmatic or secondary post-magmatic Fe-bearing trioctahedral micas, the generations of biotites was obtained using the (FeO + MnO)–10TiO2–MgO ternary diagram after Nachit [48]. In this diagram, the fresh, pegmatitic dark micas indicate composition of the primary biotites (Fig. 12), however, some of them cluster between the primary and re-equilibrated, primary biotite fields.
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Fig. 7. The photomicrograph of inclusion of the pyrochlore-group mineral and its Raman spectrum in the range 1800–200 cm 1 (785 nm excitation).
Fig. 9. The photomicrograph of the inclusion with the natrolite presence and its Raman spectrum in the range 1600–200 cm 1 (785 nm excitation).
Fig. 8. The photomicrograph of the inclusion with traces of nepheline and carbonaceous matter and their Raman spectrum in the range 2000–200 cm 1 (785 nm excitation).
Fig. 10. The photomicrograph of hematite and magnetite inclusion and its Raman spectrum in the range 2000–150 cm 1 (785 nm excitation).
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Fig. 11. Plot of Vacancy [interlayer sites] vis Vacancy [octahedral sites] of fresh and slightly weathered biotite.
Fig. 13. The positions of dark micas from mariupolite in Mg discrimination diagram after Nachit et al. [50].
RAl (a.p.f.u) biotite
The slightly weathered fine-crystalline dark mica displays compositions plotting within the fields of the primary re-equilibrated and secondary biotite. Moreover, the trioctahedral mica compositions can also describe the nature of the host magma [e.g. 7–8,49]. To distinguish biotite from various types of host magmas: i.e. alumino-potassic, calc-alkaline, subalkaline, alkaline and peralkaline, Nachit et al. [50] proposed a classification system based on total amounts of Al and Mg in its structure. In the total Al vs Mg diagram, the primary, unaltered biotite plots into the alkaline suite whereas the slightly weathered biotite falls mainly into the alkaline filled with only three analytical spots in the subalkaline fields (Fig. 13). It is also observed a negative correlation between total Al vs Mg for slightly weathered biotite (R = 0.675) and no correlation for fresh biotite (R = 0.04). According to Abdel-Rahman [7], the biotite composition depends on the nature of the magmas it crystallized from. The author observed that the mean FeO(total), MgO and Al2O3 contents in the biotite are 30.6, 4.4 and 11.2 wt.% in the alkaline suites; 22.1, 6.3 and 18.9 wt.% in the peraluminous suites and 19.7, 11.2 and 14.9 wt.% in the calc-alkaline suites. Both of the dark micas investigated, i.e. fresh biotite and slightly weathered biotite show characteristics of phases typical of alkaline suites with mean FeO(t), MgO and Al2O3 contents of 30.4, 5.6, 11.1 and 33.5, 1.1, 15.4 wt.%, respectively. Abdel-Rahman [7] suggested discrimination diagrams on the basis of these chemical components for biotite from various igneous rocks crystallized from three
Fig. 12. The positions of dark micas from mariupolite in (FeO⁄ + MnO)–10TiO2– MgO ternary classification diagram of biotite after Nachit [48]. FeO⁄ = total iron oxides (FeO + Fe2O3).
Fig. 14. The positions of dark micas from mariupolite in Al2O3–MgO biotite discrimination diagram (after Abdel-Rahman [7]). A – alkaline suites, P – peralkaline suites, C – calc-alkaline orogenic suites.
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magmatic intrusion or incorporated into the host rock during post-magmatic and hypergene weathering. The presence of organic components in other minerals seems to be quite common [e.g. 38,52]. Conclusions
Fig. 15. The positions of dark micas from mariupolite in MgO–FeO–Al2O3 biotite discrimination diagram (after Abdel-Rahman [7]). A – alkaline suites, P – peralkaline suites, C – calc-alkaline orogenic suites.
distinct magma types: (1) anorgenic alkaline suites (field A), (2) peraluminous suites (field P) and (3) calc-alkaline orogenic suites (field C). The compositions of fresh and slightly weathered biotite were plotted into Al2O3 vs MgO, Al2O3 vs FeO and MgO vs FeO diagrams (Fig. 14). On the diagrams with the Al2O3 wt.% content, slightly weathered biotite clusters in the peralkaline field or between the alkaline and peralkaline field are due to the fact that it is more enriched in Al than fresh biotite, which clusters in the all diagrams into alkaline field. Amongst the diagrams proposed by Abdel-Rahman [7] the FeO(t)–MgO–Al2O3 (wt.%) triangular diagram seems to be the best since it provides information on the variation in peraluminosity and the oxidation–reduction state of the rock-hosting mica. Both fresh and slightly weathered biotites fall in the alkaline suite (Fig. 15). It is noticed that iron-rich biotites (near annite) originate from the alkaline magmas [7]. Iron enrichment in biotite occurs at the expense of magnesium and aluminium via two substitution schemes: i.e. Mg M Fe and 2Al M 3Fe2+ [7]. The high Fe2+/(Fe2+ + Mg) ratio for fresh and slightly biotites, i.e. 0.726 and 0.760, respectively, reflects the Mg-poor character of the more felsic residual magma. Robert [51] noticed that an incorporation of Al into the structure of a biotite series depends both on Fe and Mg contents and temperature range. Hence, the initial composition of mica is more enriched in Fe and Al, and depleted in Mg when temperature is lower. The results obtained for slightly weathered biotite from mariupolite seem to be consistent with these observations. The inclusions found in biotite can be divided into two groups, i.e. primary inclusions, forming during or shortly after the crystallization of the biotite host and secondary inclusions, formed after mica crystallization, mainly due to the action of post-magmatic fluids during the stage of host rock cooling (metasomatic activity). The primary inclusions are represented by aegirine, pyrochlore, nepheline, feldspars and magnetite. It is very probable that aegirine, a mariupolite component, which surely crystallized in the first stage of aluminosilicate magma differentiation, was enclosed by faster growing mica crystal [10]. Natrolie, hematite and Ti–Mn oxides/hydroxides found between the cleavage planes of biotite belong to the secondary solid inclusions. Hematite could be formed as a result of the alteration of the primary magnetite or during the fenitization of the mariupolite. The origin of carbonaceous matter, accompanying some mineral inclusions found within mica flakes is much more complex. It could have been assimilated during
1. Biotite series composition clearly defines the nature of the host magma it originated from as Fe-rich and Mg-depleted. Annite from mariupolite is undoubtedly associated with alkaline magma. However, during the evolution of the magma with temperature decreasing, its physic-chemical conditions have evolved into more peraluminous. As a result, in later crystallization stages Al-rich and Mg-depleted re-equilibrated dark mica representing the biotite-series was formed. The alkaline character of the host rocks is proved by the composition of dark micas and the presence of feldspathoid minerals (nepheline). 2. Annite hosts numerous solid inclusions: aegirine, albite, nepheline, pyrochlore, magnetite, natrolite, hematite, Ti–Mn compounds together with carbonaceous matter, which were incorporated into the host mineral during magmatic or postmagmatic processes. 3. Muscovite, which is an accessory component of mariupolite exhibits post-magmatic and hydrothermal genesis. Interestingly, this light mica is completely devoid of fluorine, most probably due to the co-crystallization of fluorite, another second-rank mariupolite component.
Acknowledgements This work was financially supported by the AGH – University of Science and Technology (Kraków, Poland), Research Project No 11.11.140.319. We sincerely thank the anonymous reviewers for their careful reading and friendly comments. References [1] M. Rieder, G. Cavazzini, Y.S. D’Yakonov, V.A. Frank-Kamenetskii, G. Gottardi, S. Guggenheim, P.V. Koval, G. Müller, A.M.R. Neiva, E.W. Radoslovich, J.L. Robert, F.P. Sassi, H. Takeda, Z. Weiss, D.R. Wones, Can. Mineral. 36 (1998) 905–912. [2] G. Tischendorf, H.J. Förster, B. Gottesmann, M. Rieder, Mineral. Mag. 71 (3) (2007) 285–320. [3] S.W. Bailey, in: S.W. Bailey (Ed.), Reviews in Mineralogy, Mineralogical Society of America, Washington, 1984, pp. 1–12. ˇ erny, D.M. Burt, in: S.W. Bailey (Ed.), Reviews in Mineralogy, [4] D.C. C Mineralogical Society of America, Washington, 1984, pp. 257–297. [5] C.V. Guidotti, in: S.W. Bailey (Ed.), Reviews in Mineralogy, Mineralogical Society of America, Washington, 1984, pp. 357–456. [6] G. Tischendorf, H.J. Förster, B. Gottesmann, Mineral. Mag. 65 (2001) 249–276. [7] A.M. Abdel-Rahman, J. Petrol. 35 (1994) 525–541. [8] A.A.T. Shabani, A.E. Lalonde, J.B. Whalen, Can. Mineral. 41 (2003) 1381–1396. [9] F. Moazamy, Application of biotite composition in determination of tectonic setting of granitoids of Borujerd–Hamedan, M.Sc. thesis (in Farsi), Tarbiat Moalem University of Iran, 2006. [10] M. Duman´ska-Słowik, M. Sikorska, W. Heflik, Acta Geol. Pol. 61 (3) (2011) 277– 288. [11] S.G. Krivdik, V.A. Nivin, A.A. Kul’chitskaya, D.K. Voznak, A.M. Kalinichenko, V.N. Zagnitko, A.V. Dubyna, Geochem. Int. 45 (3) (2007) 270–294. [12] T.P. Volkova, Sb. Nauch. Trudov 4 (2000) 9–10 (in Ukrainian). [13] T.P. Volkova, Nauk. Praci DonDTU 36 (2001) 63–69 (in Ukrainian). [14] R.J. Tichonienkova, J.D. Osokin, A.A. Gonzjejev, Rare-metals metasomatites of alkaline massives Nauka (1967) (in Ukrainian). [15] A.N. Donskoy, The Nepheline Complex of Alkaline Oktiabrski Massif, The Ukrainian Academy of Science, Kiev, 1982 (in Ukrainian). [16] N.A. Solodov, The mineralo-genesis of rare metal formations, Niedra, 1985 (in Ukrainian). [17] V.V. Sharygin, S.G. Krivdik, L.N. Pospelova, A.V. Dubina, Dokl. Earth Sci. 425A (3) (2009) 499–504. [18] J. Morozewicz, Tscher. Mineral. Petrog. Mitt. 21 (1902) 238–246 (in German). [19] J. Morozewicz, Mariupolite and its relatives, Rep. Pol. Geol. Inst. 2 (3) (1929). [20] M. Duman´ska-Słowik, P. Baranov, W. Heflik, L. Natkaniec-Nowak, S. Shevchenko, L.I. Tsotsko, Z. Dt. Gemmol. Ges. 60 (2011) 37–48.
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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Micas from mariupolite of the Oktiabrski massif (SE Ukraine): An insight into the host rock evolution – Geochemical data supported by Raman microspectroscopy Magdalena Duman´ska-Słowik a,⇑, Aleksandra Wesełucha-Birczyn´ska b, Adam Pieczka a a Faculty of Geology, Geophysics and Environmental Protection, Department of Mineralogy, Petrography and Geochemistry, AGH – University of Science and Technology, 30 Mickiewicza Av., 30-059 Kraków, Poland b Faculty of Chemistry, Jagiellonian University, 3 Ingardena Str., 30-060 Kraków, Poland
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Fresh annite, slightly weathered
biotite (annite–siderophyllite) and muscovite are found in mariupolite. Annite contains a wealth of inclusions: aegirine, albite, nepheline, pyrochlore, magnetite, natrolite, hematite. Fresh annite is a primary magmatic phase. Slightly altered dark mica is reequilibrated biotite or secondary post magmatic component of mariupolite. Muscovite is typical post-magmatic, hydrothermal phase free of inclusions.
a r t i c l e
i n f o
Article history: Received 23 May 2014 Received in revised form 30 July 2014 Accepted 24 August 2014 Available online 8 September 2014 Keywords: Biotite Muscovite Solid inclusions Oktiabrski massif Ukraine
a b s t r a c t Muscovite and two dark mica varieties (the coarse-crystalline, pegmatitic, and fine-crystalline with signs of early weathering) representing members of the biotite series, originating from mariupolite of the Oktiabrski massif, (Ukraine), were investigated along with their solid inclusions using electron microprobe and Raman micro-spectroscopy to discuss their genesis and relationship to the parental magma. 3+ The coarse-crystalline, pegmatitic biotite, (K1.90Rb0.02Na0.01)(Fe2+ 3.56Mg1.34Ti0.36Fe0.34Mn0.03)[(Si5.73Al2.10Fe3+ 0.17)O20](OH3.24 F0.76) represents the primary, magmatic annite that crystallized from an alkaline, Fe-rich and Mg-depleted host magma, whereas the fine-crystalline biotite, partly altered to vermiculite, (K1.752+ Rb0.03Na0.03)(Fe3+ 3.23Fe1.16Mg0.26Mn0.04Ti0.10)[(Si5.16 Al2.84)O20](OH)4.00, devoid of F, represents a re-equilibrated or secondary, post-magmatic Fe3+-bearing mica crystallized from alkaline to the subalkaline host magma. Muscovite, (K1.96Na0.06)(Al3.97Fe2+ 0.06)[(Si5.99Al2.01)O20](OH)4, with low Na/(Na + K) ratio, low Fe and devoid of Ti and also F, forms only tiny, subhedral flakes formed in the post-magmatic, hydrothermal stage. The primary, unaltered biotite contains numerous solid inclusions of primary origin (albite, aegirine, zircon, K-feldspar, nepheline, pyrochlore, magnetite) and secondary origin (natrolite, hematite, Ti-Mn oxides/hydroxides); most of them are accompanied by a carbonaceous substance, all confirmed by scanning electron microscopy and Raman microspectroscopy. Ó 2014 Elsevier B.V. All rights reserved.
⇑ Corresponding author. E-mail address: [email protected] (M. Duman´ska-Słowik). http://dx.doi.org/10.1016/j.saa.2014.08.127 1386-1425/Ó 2014 Elsevier B.V. All rights reserved.
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Introduction Micas are one of the most common group of sheet silicates in the Earth. They are widespread in a variety of geological environments from igneous, sedimentary to metamorphic rocks. Their simplified formula, end-members and species for the true, brittle and interlayer-deficient micas are presented by Rieder et al. [1] and Tischendorf et al. [2]. The specific mica structure enables them to incorporate a large number of main and trace elements of different radii and charges [3], so the concentrations of some of the elements can be sensitive indicators of the physiochemical environment, in which a host rock crystallized [e.g. 4,5]. The compositions of the mica-group minerals are mainly dependent on geochemical and physical parameters as well as temperature, pressure, oxygen-, fluorine- and chlorine fugacities, whole rock composition and the coexisting mineral association [6]. Abdel-Rahman [7], Shabani et al. [8] and Moazamy [9] noticed that compositions of dark micas, representing the biotite-subgroup minerals (a series including phlogopite, siderophyllite, annite and eastonite), depend largely upon the nature of the magmas, from which they have crystallized. Hence, the compositions of mica-group minerals usually bear an important imprint on the petrogenetic processes. Micas, mainly trioctahedral species, form up to 20 vol.% of the pegmatitic variety of mariupolite (nepheline syenite) from the Oktiabrski massif (SE Ukraine). Mariupolite, as the host rock of the micas, commonly exhibits a subsolidus overprint due to the circulation of post-magmatic fluids and metasomatic alterations, e.g. fenitization [cf.10]. Hence, micas can be a sensitive monitor of its magmatic crystallization and later post-magmatic alteration. The objects of the study are dioctahedral and trioctahedral mica-group minerals from the mariupolite of the alkaline Oktiabrski massif. Special attention was paid to the main and subordinate elements present in the structure of these minerals to assess their petrologic significance and genetic condition. Moreover, the characterization of solid inclusions found in the dark micas with the SEM–EDS analyses supported by Raman microspectroscopy was done to discuss the sequence of their crystallization. The chemistry of mica-group minerals together with the assembly of the solid inclusions seem to have the potential for a better understanding of the final stage of the host rock’s consolidation.
represent a leucocratic, coarse-crystalline variety of mariupolite with pegmatitic and mainly chaotic texture, composed mainly of plagioclase and nepheline. The predominant mineral is plagioclase (Ab94–92An6–8), which occurs as fine, prismatic crystals with a few twin individuals, or larger crystals of an older generation. Nepheline forms large, dynamically deformed crystals with non-uniform light extinction. Members of the biotite series, K-feldspar, pyrochlore-group minerals, sodalite, natrolite, aegirine, cancrinite, muscovite and Fe-bearing chlorite all appear as subordinate components. Dark mica forms well preserved flakes or tiny crystals forming fan like textures. The latter variety of dark mica exhibits some traces of an alteration process. K-feldspar forms large tabular crystals with well-developed cleavages along (0 0 1) and (0 1 0). Some of them reveal a crosshatched pattern, typical of microcline. Aegirine forms stout prismatic crystals of varying size and shows strong pleochroism: a = green, b = pale green, c = pale yellowgreen. Sodalite is characterized by blue colour visible in thin section. Natrolite locally forms prismatic crystals, but it sometimes occurs as pseudomorphs after nepheline megacrysts [20]. Zircon is abundant in the form of euhedral crystals with a characteristic oscillatory zoning along the grain margins, and irregular patchy zoning in the interior [10]. Euhedral to occasionally subhedral crystals of the pyrochlore-group minerals occur in the interstices between albite and possess a characteristic reaction rim built of needle-shaped crystals of Fe-bearing chlorite [21]. The results of a bulk-rock analyses were presented in Duman´ska-Słowik et al. [22]. Analytical methods Scanning electron microscopy (SEM) Back-scattered electron (BSE) images of polished sections were obtained using a FEI Quanta 200 FEG scanning electron microscope equipped with a EDS detector. The system operated at 25 kV accelerating voltage in a high-vacuum mode. The SEM–EDS observations were applied to identify solid inclusions within a mica matrix. These analyses were then supported by micro Raman spectroscopy.
Geological background
Raman microspectroscopy (RS)
The Oktiabrski alkaline massif (Ukrainian Shield, SE Ukraine) has been known since Joseph Morozewicz described it in the late 19th century. The massif, which is 34 km2 in area [11], is a unique province of alkaline magmatism of Proterozoic age ca. 1.8 Ga [12,13] (Volkova 2000, 2001). The general geology and genesis of this massif were described in detail by Tichonienkova et al. [14], Donskoy [15], Solodov [16], Krivdik et al. [11] and Volkova [12,13]. According to these data, it is mainly composed of alkali syenites and foyaites with minor amounts of mariupolite (i.e. rare aegirine–nepheline–albite syenite), subalkaline gabbro, and their derivatives (peridotites, pyroxenites). A few dikes of aegirine foyaites and agpaitic phonolites outcrop mainly outside the massif among host granitoides. The vast majority of rocks belong to the agpaitic series, while mariupolite is intermediate between the miaskitic and agpaitic types. Mariupolite contains such accessory minerals as zircon, pyrochlore, britholite, which is more typical of the miaskitic nepheline syenites, though melanocratic varieties of mariupolites have an agpaitic index up to 1.5 (owing to the presence of aegirine) [17].
Raman spectra of inclusions in mica crystals were recorded using a Renishaw InVia Raman spectrometer, operating in a confocal mode and working in a backscatter geometry with a Leica microscope equipped with 100 and 50 magnification objectives. The samples were excited with a 785 nm line of high power nearIR (HP NIR) laser. The spectrum of albite was collected using the microscope accessory for Thermo Electrons Nicolet NXR FT-Raman spectrometer and excited with a 1064 nm laser line. Laser power was kept low enough to protect organic inclusions from possible laser-induced degradation. The laser focus diameter was ca. 1– 2 lm. It should be noted that almost no sample preparation was performed. The surface of the mica flakes was cleaned carefully with distilled water and acetone before measurements were made to ensure that it was not contaminated. Subsequently the flakes were inspected by an optical Leica microscope and Raman spectra were collected from the most spectacular inclusions.
Petrography of mariupolite
Electron-microprobe analyses were made at the Inter-Institute Analytical Complex for Minerals and Synthetic Substances of Warsaw University with a Cameca SX 100 operating in a wavelengthdispersive (WDS) mode under the following conditions: accelerated
Mariupolite, named by Morozewicz [18,19] covers only a small area (ca. 5–10%) of the Oktiabrski massif. The samples examined
Electron-microprobe analyses (EMPA)
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voltage of 15 kV, beam current of 10 nA, peak count-time of 20 s and background time of 10 s. The following standards, analytical lines, diffracting crystals and mean detection limits (wt.%) were used: orthoclase – K (Ka, PET, 0.03), Rb glass – Rb (La, TAP, 0.10), albite – Na (Ka, TAP, 0.04), wollastonite – Ca (Ka, PET, 0.05) and Si – (Ka, TAP, 0.03), hematite – Fe (La, LIF, 0.14), rutile – Ti (Ka, PET, 0.05), rhodonite – Mn (Ka, LIF, 0.13), diopside – Mg (Ka, TAP, 0.02), jadeite – Al (Ka, TAP, 0.03), phlogopite – F (Ka, TAP, 0.61). The raw data were corrected with the PAP procedure [23] using/ an original software./NOT CLEAR The crystal-chemical formulae of white mica were calculated on the basis of 24 (O, OH, F) with H2O iterated by stoichiometry, whereas those of dark mica on the basis of 24 (O, OH, F) with H2O and Fe3+/Fetotal ratio iterated in relation to (OH + F) = 4 per formula unit (p.f.u.), [4]Fe3+ = 8–Al–Si p.f.u., [6] Fe3+ = 6–Fe2+–Mn–Mg–2Ti p.f.u., and Fe3+ = [4]Fe3++[6]Fe3+ p.f.u. The XRD patterns were recorded with a Philips PW 3020 X’PertAPD Diffractometer system (with a Cu anode and graphite monochromator) at 35 kV voltage and 30 mA current, in the 2H range of 5–75° with a 0.05°(2H)/s step. The characterization of the weathering process of dark mica was carried out by XRD on a air-dried, glycol-saturated and heated sample at 560 °C for 2 h [e.g. 24,25].
Results Light mica The light mica forms very tiny crystals with sizes of up to 30 lm and having no inclusions. Within the host rock it mainly coexists with aegirine, albite, nepheline, dark mica and microcline. The compositions obtained for the mica (Table 1) are very homogenous, which gives the opportunity to define a mean structural formula as: (K1.948Na 0.058)(Al3.967 Fe2+ 0.049)]Si5.992Al2.007O20](OH)4, almost identical to the ideal end-member muscovite formula defined by Miller et al. [26] and Rieder et al. [1]. Tischendorf et al. [2] proposed a graphical presentation for potassium mica based on octahedrally coordinated cations, i.e. mgli (Mg–Li) vs feal (Fe + Mn + Ti–(VI)Al) in atoms per formula unit. The data presented in this diagram plot into muscovite Al2 field (Fig. 1). Relatively small amounts of Na+ substitute for K+ in the interlayer site; the octahedral sites are occupied mainly by Al with traces of Fe2+, whereas the tetrahedral positions are filled typically with Si and Al. Rubidium, Ca, Mn, Mg and Ti and F occur under the detection limits.
Dark mica The dark mica forms mainly euhedral to subhedral flakes, 0.05– 4.0 cm in length, with pleochroism changing the colour from light green to yellowish green along the z axis and yellowish brown to dark brown along the x axis. Some crystals are rich in pleochroic halos revealing the presence of zircon inclusions. The mica frequently occurs in paragenesis with albite, nepheline, zircon, aegirine, and natrolite, and its flakes usually are enriched in numerous inclusions of main magmatic minerals (mainly Na- and K-feldspars) what makes the poikilitic texture locally observed within the host rock. Feldspars forming anhedral crystals are randomly scattered within the mica matrix (Fig. 2). The other solid inclusions are represented by aegirine, pyrochlore, Fe compounds (oxide/ hydroxide), Ti–Mn compounds (oxide/hydroxide) and Ti–Mn–Fe oxides. Aegirine forms long prismatic crystals, whereas pyrochlore occurs as euhedral grains. The single and complex compounds of Fe, together with Ti–Mn oxides/hydroxides, occur as anhedral inclusions frequently coexisting with albite or aegirine crystals (Fig. 2). Hence, two- or three phase solid inclusions (e.g. albite–
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FeO/OH, albite–aegirine–Ti–MnO/OH) do not belong to the rarity found in the crystal of the trioctahedral mica. Coarse-crystalline, pegmatitic dark mica, (K1.896 Rb0.019 Na0.014) 3+ 3+ [(Fe2+ 3.557 Mg1.345 Mn0.028) (Ti 0.363Fe0.344)] [Si5.727 Al 2.100 Fe0.173 O20] (OH3.241 F0.759], shows more complex composition than muscovite (Table 1). The interlayer site of the biotite is mainly filled by K, while Na and Rb occur in much lower concentrations. The octahedral sites are occupied mainly by Fe2+, Mg, Ti, Fe3+ and Mn. The predominance of Fe2+ over Mg in these sites makes this mica closer to annite than 3+ phlogopite, i.e. Fe2+ occupy the 2 –Mg joints [1,27]. Silicon, Al and Fe tetrahedral sites. Using molecular proportions, the chemical index of alteration (CIA) (Nesbitt and Young 1982 fide [28]) was calculated for the biotite crystals to be approximately 54%. Hence, the trioctahedral mica is quite fresh. Clauer et al. [29] noted that fresh micas are enriched in K and impoverished in Na and Ca. The chemical composition of fresh dark mica indicates the total Al values of 1.963–2.239 a.p.f.u, pronounced variations of Mg contents, i.e. 0.842–2.135 a.p.f.u. and the average Fe2+/(Fe2+ + Mg) ratio of 0.725. These data presented in the idealised annite–phlogopite–siderophyllite–eastonite quadrilateral [30] plot the mica in the field of annite [vide 31] (Fig. 3). In the host rock, the aggregates of tiny flakes of a dark mica with visible signs of alteration and characteristic fan like texture coexist with nepheline, K-feldspar and cancrinite. The interlayer site of this trioctahedral mica, (K1.748 Rb0.032 Na0.027) [(Fe2+ 1.164 Mg0.264 Mn0.039) (Ti0.101Fe3+ 3.229)] [Si5.160 Al2.840 O20] (OH4.000], is filled mainly by K, also with traces of Na and Rb, and remains partly vacant (0.131–0.294) (Table 1). Similarly, its octahedral sites are occupied mainly by Fe3+, Fe2+, and subordinately Mg, Ti and Mn with vacancies ranging from 1.008 up to 1.526 a.p.f.u. The tetrahedral sites are filled by Si and Al, and the anion positions are occupied solely by O + OH and devoid of F. The CIA index of this dark mica variety is 64, corresponding to the fresh or slightly weathered biotites. The chemical composition of the mica indicates high and variable Al contents (2.656–3.264 a.p.f.u), considerable depletion in Mg (0.137– 0.364 a.p.f.u) and an average Fe2+/(Fe2+ + Mg) ratio of 0.760. In the annite–phlogopite–siderophyllite–eastonite diagram (Fig. 3), the mica clusters around the annite–siderophyllite border, whereas in the mgli vs feal diagram [2] it plots into the Fe2Mg biotite field (Fig. 1). An XRD analyses of the weathered biotite show the presence of smectite and vermiculite as secondary phases (Fig. 4). The Raman spectrum of fresh, pegmatitic dark mica is presented in Fig. 5 with characteristic bands at 406, 515, 646, 764 and 888 cm 1. The wide and strong peak at ca. 515 cm 1 and weaker at 406 cm 1 are associated with the complex set of translational modes of cations within the octahedral and interlayer sites relative to the SiO4 groups in the tetrahedral layer. The bands at 646 and 764 cm 1 are contributed by vibrational modes of Si–O–Si bonds. The band at 888 cm 1 arises from the stretching mode of the Si– O bond in SiO4 tetrahedra [32]. The slightly broaden Raman bands of the annite (Fig. 5) suggests some structural disorder in the crystalline mica [e.g. 33]. In Raman spectra of the mica, characteristic bands are shifting a bit across the local composition of the mineral. The presence of various types of solid inclusions within the biotite flakes, identified under SEM were confirmed by Raman microspectroscopy. Albite with characteristic bands at 290, 408, 478, 507, 762, 816, 1098 and 1113 cm 1, belongs to the most numerous inclusions found within the mica (Fig. 6). The bands at 478 and 762 cm 1 are due to Na–O stretching vibrations, whereas at 816 it is connected with Si–O bending and stretching vibrations, respectively. The bands at 1098 and 1113 cm 1 are associated with breathing modes of tetrahedra involving T–O stretching vibrations. A very distinctive band at 507 cm 1 is attributed to the compression of four-membered tetrahedral rings and/or and O–Na–O breathing [34,35].
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Table 1 Representative results of microprobe analyses of micas from mariupolite. Analyses No sample
1 No 17-muscovite
2 No 32 fresh ‘biotite’
3 No 3 fresh ‘biotite’
4 No 58 weathered ‘biotite’
5 No 71 weathered ‘biotite’
wt.% SiO2 TiO2 Al2O3 Fe2O3 FeO MnO MgO CaO Na2O K2O Rb2O H2O F Total
45.05 0.00 37.92 0.00 0.50 0.00 0.00 0.00 0.16 11.52 0.00 4.50 0.00 99.65
36.69 2.70 11.03 2.42 27.45 0.32 5.90 0.00 0.00 9.35 0.00 2.92 1.76 100.55
35.77 3.32 11.00 3.38 27.38 0.34 4.93 0.00 0.06 9.29 0.52 3.08 1.35 100.41
34.06 0.48 15.55 30.73 5.78 0.27 1.39 0.11 0.08 8.78 0.52 3.93 0.00 101.70
32.35 1.20 15.21 23.32 13.87 0.35 0.61 0.00 0.10 8.65 0.63 3.77 0.00 100.06
(IV)
Si Al Fe3+ T site (VI) Al Ti Fe3+ Fe2+ Mn Mg O site Ca Na K Rb A site O OH F (IV) (IV)
6.004 1.996 0.000 8.000 3.960 0.000 0.000 0.056 0.000 0.000 4.016 0.000 0.045 1.958 0.000 2.003 20 4.000 0.000
5.863 2.078 0.059 8.000 0.000 0.325 0.232 3.669 0.044 1.406 5.676 0.000 0.000 1.906 0.000 1.906 20 3.110 0.889
5.770 2.090 0.140 8.000 0.000 0.403 0.271 3.694 0.046 1.184 5.598 0.000 0.021 1.911 0.027 1.959 20 3.311 0.689
5.201 2.799 0.000 8.00 0.000 0.056 3.531 0.731 0.035 0.317 4.670 0.018 0.028 1.711 0.026 1.783 20 4.000 0.000
5.147 2.853 0.000 8.000 0.000 0.144 2.793 1.846 0.047 0.146 4.976 0.000 0.033 1.756 0.032 1.821 20 4.000 0.000
Note: 0.00 – under detection limit.
Fig. 1. The positions of mica species from mariupolite plotted in simplified mgli/ feal (Mg–Li vs Fe + Mn + Ti–(VI)Al) diagram of Tischendorf et al. (2007). Symbols: circle – muscovite, square – fresh biotite, triangle – slightly weathered biotite.
Another, quite common inclusion found is a pyrochlore-group mineral (Fig. 7) with marker bands at 464, 546, 782 and 1145 cm 1 (http://rruff.info/pyrochlore/display=default/R060288). In the pyrochlore, the bands have no constant position and intensities; they rather shift across the local defects in their structure. The possible shifts of the 515 cm 1 band is associated with the variability of the B–X bond length in the pyrochlore structure [vide 36]. This black inclusion is accompanied by a carbonaceous matter (Fig. 7), and its presence is clearly marked by bit broaden bands appearing in the region of 1311–1602 cm 1 [e.g. 37,38]. The band at 1322 cm 1 (called D-band) is attributed to a disorder-induced first order Raman mode, while the G-band at 1616 cm 1 is due to a first order E2g Raman mode [39].
The traces of nepheline found within the mica matrix (Fig. 8) are evidenced by bands at 404, 940 and 1065 cm 1. The marker band at 404 cm 1 is due to msT–O–T vibrations [33]. The other bands at Fig. 8, i.e. 671, 712, 759 and 1585 cm 1 are attributed to the mica matrix (vide Fig. 5) and carbonaceous matter (vide Fig. 7), respectively. The presence of natrolite is also clearly marked by 145, 393, 521, 730 cm 1 bands (Fig. 9). The bands at 145 cm 1 are attributed to a square–rhombic distortion of the Si2Al2O10 ring, while 393 cm 1 comes from the liberation of the TO4 tetrahedra. The bands at 521 and 730 cm 1 are connected with O–T–O bending vibrations [40]. In Fig. 9 the Raman signal from the mica matrix is clearly visible as the intensive band in the region of ca. 500 cm 1, whereas the presence of three bands at 486, 509 and 674 cm 1 could suggest the traces of iron compounds (maghemite c-Fe2O3?) [41–42]. Among other Fe-bearing minerals, hematite manifests its presence by showing Fe–O symmetric bending vibrations at 291 and 409 cm 1 [41]. The relatively wide and intensive band at 645 cm 1 and second order band at 1320 cm 1 can be attributed to the traces of magnetite (Fig. 10) [42]. Discussion In mariupolite, mica-group minerals are represented mainly by dark micas of the biotite series, whereas muscovite is rather an accessory component and restricted only locally to host rocks The calculated Na/(Na + K) ratio for muscovite ranges from 0.01 to 0.04, suggesting it formed during post-magmatic activity and was strongly affected by hydrothermal solutions [3]. The
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Fig. 2. Solid inclusions assembly in fresh dark mica, BSE image. Symbols: Ab – albite, K-Fs – K-feldspar, Aeg – aegirine, Pcl – pyrochlore.
Fig. 3. Nomenclature of fresh and weathered biotite from mariupolite expressed in the Fe2+/(Fe2+ + Mg) vs RAl (a.p.f.u) diagram of Deer et al. [31].
hydrothermal genesis of muscovite is also supported by low Fe concentration, since the Fe content is much higher in a typical magmatic muscovite [3]. Moreover, muscovite from the mariupolite is completely devoid of Ti, impoverished in Al and Na, what is also consistent with its post-magmatic, hydrothermal genesis [43–45]. The hypothetical post-magmatic origin of the mica is also consistent with its textural characteristics, as tiny, not welldeveloped flakes without characteristic twinning. Interestingly, this muscovite is completely devoid of F and Ca, elements which should have accumulated during hydrothermal activity and should have been present in the environment of its growth. It is very probable, that crystallization of the fluorite, a second-rank component of mariupolite, competed with muscovite for F and Ca [vide 23]. Coarse-crystalline dark mica, representing members of the biotite series, is relatively fresh or only slightly weathered. The presence of vermiculite in the slightly weathered biotite samples indicates a typical early stage of weathering [e.g. 46,28], leading in the next, more advanced stage to smectite. According to Nahon
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Fig. 4. XRD patterns of slightly weathered biotite recorded at following conditions: air-dried, glycol-saturated and at 560 °C heated preparation.
Fig. 5. The Raman spectrum of fresh biotite in the range 1700–120 cm excitation).
1
(785 nm
[47], transformation into vermiculite results from the exfoliation of biotite and layers opening during weathering, accompanying by the loss of K+ observed for slightly weathered biotite. Weathering of the biotites leads also to the depletion of octahedral cations and an increase in octahedral vacancies of up to1.526 a.p.f.u. The compositional relationships of the two generations of biotite, i.e. fresh and slightly weathered, are presented in the diagram based on Vacancy [at the interlayer site] vis Vacancy [at the octahedral sites] (Fig. 11). The positive correlation is observed only for slightly weathered (re-equilibrated) biotite (R = 0.43). This secondary biotite has significantly higher vacancies in both the interlayer and octahedral sites, exhibiting, along with a simultaneous increase of aluminium content, a late post-magmatic evolution of the primary biotite [48] due to the late subsolidus reactions associated with weathering and the recrystallization of dark mica to small flakes.
Fig. 6. The photomicrograph of albite inclusion and its Raman spectrum in the range 1800–200 cm 1 (1064 nm excitation).
To recognise between the primary, early magmatic, re-equilibrated, late-magmatic or secondary post-magmatic Fe-bearing trioctahedral micas, the generations of biotites was obtained using the (FeO + MnO)–10TiO2–MgO ternary diagram after Nachit [48]. In this diagram, the fresh, pegmatitic dark micas indicate composition of the primary biotites (Fig. 12), however, some of them cluster between the primary and re-equilibrated, primary biotite fields.
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Fig. 7. The photomicrograph of inclusion of the pyrochlore-group mineral and its Raman spectrum in the range 1800–200 cm 1 (785 nm excitation).
Fig. 9. The photomicrograph of the inclusion with the natrolite presence and its Raman spectrum in the range 1600–200 cm 1 (785 nm excitation).
Fig. 8. The photomicrograph of the inclusion with traces of nepheline and carbonaceous matter and their Raman spectrum in the range 2000–200 cm 1 (785 nm excitation).
Fig. 10. The photomicrograph of hematite and magnetite inclusion and its Raman spectrum in the range 2000–150 cm 1 (785 nm excitation).
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Fig. 11. Plot of Vacancy [interlayer sites] vis Vacancy [octahedral sites] of fresh and slightly weathered biotite.
Fig. 13. The positions of dark micas from mariupolite in Mg discrimination diagram after Nachit et al. [50].
RAl (a.p.f.u) biotite
The slightly weathered fine-crystalline dark mica displays compositions plotting within the fields of the primary re-equilibrated and secondary biotite. Moreover, the trioctahedral mica compositions can also describe the nature of the host magma [e.g. 7–8,49]. To distinguish biotite from various types of host magmas: i.e. alumino-potassic, calc-alkaline, subalkaline, alkaline and peralkaline, Nachit et al. [50] proposed a classification system based on total amounts of Al and Mg in its structure. In the total Al vs Mg diagram, the primary, unaltered biotite plots into the alkaline suite whereas the slightly weathered biotite falls mainly into the alkaline filled with only three analytical spots in the subalkaline fields (Fig. 13). It is also observed a negative correlation between total Al vs Mg for slightly weathered biotite (R = 0.675) and no correlation for fresh biotite (R = 0.04). According to Abdel-Rahman [7], the biotite composition depends on the nature of the magmas it crystallized from. The author observed that the mean FeO(total), MgO and Al2O3 contents in the biotite are 30.6, 4.4 and 11.2 wt.% in the alkaline suites; 22.1, 6.3 and 18.9 wt.% in the peraluminous suites and 19.7, 11.2 and 14.9 wt.% in the calc-alkaline suites. Both of the dark micas investigated, i.e. fresh biotite and slightly weathered biotite show characteristics of phases typical of alkaline suites with mean FeO(t), MgO and Al2O3 contents of 30.4, 5.6, 11.1 and 33.5, 1.1, 15.4 wt.%, respectively. Abdel-Rahman [7] suggested discrimination diagrams on the basis of these chemical components for biotite from various igneous rocks crystallized from three
Fig. 12. The positions of dark micas from mariupolite in (FeO⁄ + MnO)–10TiO2– MgO ternary classification diagram of biotite after Nachit [48]. FeO⁄ = total iron oxides (FeO + Fe2O3).
Fig. 14. The positions of dark micas from mariupolite in Al2O3–MgO biotite discrimination diagram (after Abdel-Rahman [7]). A – alkaline suites, P – peralkaline suites, C – calc-alkaline orogenic suites.
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magmatic intrusion or incorporated into the host rock during post-magmatic and hypergene weathering. The presence of organic components in other minerals seems to be quite common [e.g. 38,52]. Conclusions
Fig. 15. The positions of dark micas from mariupolite in MgO–FeO–Al2O3 biotite discrimination diagram (after Abdel-Rahman [7]). A – alkaline suites, P – peralkaline suites, C – calc-alkaline orogenic suites.
distinct magma types: (1) anorgenic alkaline suites (field A), (2) peraluminous suites (field P) and (3) calc-alkaline orogenic suites (field C). The compositions of fresh and slightly weathered biotite were plotted into Al2O3 vs MgO, Al2O3 vs FeO and MgO vs FeO diagrams (Fig. 14). On the diagrams with the Al2O3 wt.% content, slightly weathered biotite clusters in the peralkaline field or between the alkaline and peralkaline field are due to the fact that it is more enriched in Al than fresh biotite, which clusters in the all diagrams into alkaline field. Amongst the diagrams proposed by Abdel-Rahman [7] the FeO(t)–MgO–Al2O3 (wt.%) triangular diagram seems to be the best since it provides information on the variation in peraluminosity and the oxidation–reduction state of the rock-hosting mica. Both fresh and slightly weathered biotites fall in the alkaline suite (Fig. 15). It is noticed that iron-rich biotites (near annite) originate from the alkaline magmas [7]. Iron enrichment in biotite occurs at the expense of magnesium and aluminium via two substitution schemes: i.e. Mg M Fe and 2Al M 3Fe2+ [7]. The high Fe2+/(Fe2+ + Mg) ratio for fresh and slightly biotites, i.e. 0.726 and 0.760, respectively, reflects the Mg-poor character of the more felsic residual magma. Robert [51] noticed that an incorporation of Al into the structure of a biotite series depends both on Fe and Mg contents and temperature range. Hence, the initial composition of mica is more enriched in Fe and Al, and depleted in Mg when temperature is lower. The results obtained for slightly weathered biotite from mariupolite seem to be consistent with these observations. The inclusions found in biotite can be divided into two groups, i.e. primary inclusions, forming during or shortly after the crystallization of the biotite host and secondary inclusions, formed after mica crystallization, mainly due to the action of post-magmatic fluids during the stage of host rock cooling (metasomatic activity). The primary inclusions are represented by aegirine, pyrochlore, nepheline, feldspars and magnetite. It is very probable that aegirine, a mariupolite component, which surely crystallized in the first stage of aluminosilicate magma differentiation, was enclosed by faster growing mica crystal [10]. Natrolie, hematite and Ti–Mn oxides/hydroxides found between the cleavage planes of biotite belong to the secondary solid inclusions. Hematite could be formed as a result of the alteration of the primary magnetite or during the fenitization of the mariupolite. The origin of carbonaceous matter, accompanying some mineral inclusions found within mica flakes is much more complex. It could have been assimilated during
1. Biotite series composition clearly defines the nature of the host magma it originated from as Fe-rich and Mg-depleted. Annite from mariupolite is undoubtedly associated with alkaline magma. However, during the evolution of the magma with temperature decreasing, its physic-chemical conditions have evolved into more peraluminous. As a result, in later crystallization stages Al-rich and Mg-depleted re-equilibrated dark mica representing the biotite-series was formed. The alkaline character of the host rocks is proved by the composition of dark micas and the presence of feldspathoid minerals (nepheline). 2. Annite hosts numerous solid inclusions: aegirine, albite, nepheline, pyrochlore, magnetite, natrolite, hematite, Ti–Mn compounds together with carbonaceous matter, which were incorporated into the host mineral during magmatic or postmagmatic processes. 3. Muscovite, which is an accessory component of mariupolite exhibits post-magmatic and hydrothermal genesis. Interestingly, this light mica is completely devoid of fluorine, most probably due to the co-crystallization of fluorite, another second-rank mariupolite component.
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