Magnesian Calcite and Dolomite in the Krečana Marble (Bukulja–Venčac Area, Central Serbia): A Possible Modification for Geothermometry Application Purposes?

Abstract

The chemical compositions and formation temperatures of magnesian calcite and dolomite were estimated by using the combination of chemical analysis, crystallographic parameters, and a plethora of various diagrams and mathematical calculations. This study presents an example of the calculated crystallo-chemical formula (Ca_0.960Mg_0.039Fe_0.001)CO₃, obtained from chemical analysis on a representative marble sample from the Bukulja–Venčac area in central Serbia. Substituting CaCO₃ with MgCO₃ and FeCO₃ in dolomite adds approximately 3–5 mol. %, enhancing the classification and indicating that it is more accurately identified as magnesium-excess dolomite. The estimated formation temperature of magnesian calcite (1) is approximately 528 °C, whereas magnesian calcite (2) forms at about 341 °C. The ~187 °C difference corresponds to ~3.28 mol. % MgCO₃ (~7.18% dolomite), reflecting the distinction between magnesian calcite (1) and magnesian calcite (2). Considering the presence of the submicroscopic intergrowth and exsolution of dolomite within magnesian calcite (1), which are further subdivided in magnesian calcite (2), the estimated formation temperature of ~341 °C appears to be more realistic. The synthesis of the results suggests that this combined method could be helpful in the geothermometry of marble samples after the treatment with acetic acid. However, despite the promising results, additional experiments are necessary to validate the proposed modified geothermometry approach.

1. Introduction

Marble is low- to ultra-high-temperature metamorphic raw material that is often used for decorative purposes and building in the construction industry (see, for example, [1,2,3,4,5,6,7,8,9] and references therein). Besides its application in various construction projects, marble can provide insights into geodynamical frameworks by the reconstruction of paleotemperatures affecting limestone protoliths. Coupled with the age of ancient depositional environment-producing calcareous protoliths, marbles can reveal past subsurface conditions that contributed to metamorphic imprinting (e.g., [10,11]). An example from the central Serbia is tested for paleotemperatures by sampling the Bukulja–Venčac marble-bearing area (Figure 1a,b). This geologically adverse region has a long history of Paleotethyan and Neotethyan geodynamic interferences, including younger Paleogene–Neogene core complex exhumation episodes that eventually contributed to the shallow exposure of Bukulja–Venčac marble-bearing area [10,12,13,14,15,16,17]. The Bukulja–Venčac area records traditional marble extraction spanning over a century. Importantly, the high-quality Venčac marble is a globally famous stone, used widely across many global locations and monuments (e.g., for decorative purposes within the White House in Washington, D.C. (USA) [18]).

The investigated Krečana site exposes a (pure) white marble deposit located at the Venčac Mountain (Serbia), positioned roughly 85 km to the south of the capital city Belgrade (Figure 1a,b). The geological structure of the wider (Bukulja)Venčac area includes several tectono-paleogeographic units distributed at a complex Paleozoic–Mesozoic tectonic junction referred to as the Vardar Zone (Figure 1b). The Vardar Zone is a composite ophiolite-bearing suture zone carrying the (overlapping) segments of Paleotethyan and Neotethyan Vardar paleo-oceanic sutures. The Vardar Zone is further connecting Drina–Ivanjica Block (Adria; Figure 1a,b, #3, 4) with Jadar Block both embedded within the latter suture zone (Figure 1a,b, #2). The eastern interface with Serbo-Macedonian gneiss, forms the European foreland in Jurassic-Cretaceous reference, formerly overriding the Vardar Neotethyan plates ([10,11,12,13,14,15,16,17,19,20,21,22,23,24,25,26,27,28,29]; Figure 1a,b, #5). Because the geodynamic origin of Venčac marble remains debated, questioning whether the marble can be linked to Paleozoic or Mesozoic Alpine episode, another unresolved issue is whether the marble parental rocks were exposed to contact- [11,29] or regional-type [10] metamorphism. Given the importance of these debated issues, this paper aims to enhance understanding of the area’s geological evolution by providing the subsurface formation temperatures using key minerals embedded into the marble.

Initially, we analyzed the crystallographic and mineralogical features of magnesian calcite and dolomite from the Krečana–Venčac marbles [30]. The provided new constraints on formation temperatures and pressure allowed for a better insight into the deposit’s origin supporting a more efficient marble exploitation [31,32,33,34,35]. Building on earlier research, the second part of this composite study (which extends from the first report of Tančić [30]) aims to correlate the chemical analysis results with the calculated mineral compositions and their formation temperatures. The study further compares these chemical data with the findings from the earlier X-ray powder diffraction (XRPD) study [30]. The results obtained by introducing a combination of several methods, allowed further consideration about their applicability for geothermometry purposes by refining a degree of usefulness of the treatment of marble with acetic acid. As the results will show, the lower temperature estimate (~341 °C) has been of geological relevance because of the removal of dolomite intergrowth and exsolution in magnesian calcite.

Although in the published first part of the study a colloquial term “Mg-calcite” was used as a shortcut for magnesium-bearing calcite (see [30]), in this second paper we will use the term “magnesian calcite”. The term is in accordance to the International Mineralogical Association’s (IMA) nomenclature rules describing the adjectival modifiers [36].

2. Approach, Materials and Methods

2.1. Approach

Mathematics, statistics, and computer science are well known for their close links to fields like crystallography, mineralogy, solid solutions, chemistry, geology, geosciences, geothermometry, geobarometry, Earth sciences, etc. ([37,38,39]; see also references cited therein). The multidisciplinary approach is used as in the previous research [40,41,42,43,44,45,46,47,48,49], including our use of geothermometric methods in this study (e.g., [35]). In particular, we used the crystallographic features of examined magnesian calcites to recalculate their MgCO₃ contents through various mathematical techniques, then compared these results with earlier findings [30] obtained via different methods [50,51,52,53,54]. The crystallographic and mineralogical properties of dolomite were also analyzed, and its chemical composition—especially the substitutions among Ca²⁺, Mg²⁺, and Fe²⁺ ions—was estimated using multiple diagrams, calculations, and combined approaches. Moreover, the MgCO₃ content in magnesian calcite (which coexists with dolomite) was evaluated, and the formation temperature was calculated using various diagrams, equations, and calculations. The diagrams and equations primarily pertain to the binary CaCO₃-MgCO₃ and ternary CaCO₃-MgCO₃-FeCO₃ systems.

The investigated Venčac site is accommodated at its eastern flanks (for the configurations see [10,11]; Figure 1b). The Oligocene–Early Miocene granitoid was intruded into the Bukulja crystalline [10], likely inducing to some extent the contact metamorphism of the surrounding, mainly Paleozoic rocks. The granite intrusion was followed by volcanic rocks, prevailingly phenoandesites, latites, and their pyroclastics [10] emplaced into mainly crystalline rocks. The age of intrusion is ca. 20–15 Ma (see [10,13], and references cited therein). However, the investigated Krečana deposit is not at the position near the exposed magmatic body (Figure 1b, small red rectangle).

2.2. Materials

Sample-1 is a natural bulk rock marble sample selected and picked up from the surface of the studied Krečana white marble deposit which is located at the Venčac Mountain area. The investigated eastern flank of the Venčac Mt. area contains the following rock assemblages: (i) Paleozoic rocks including phyllites, meta-sandstones, muscovite–biotite schists, small-grain gneisses, slightly metamorphosed limestones, siltstones, and marbles; (ii) Mesozoic rocks such as serpentinites, massive limestones, siltstones, sandstones, flysch, layers of sheet and brecciated limestones, and a pile of red marly limestones and siltstones; (iii) Tertiary formations consisting of Bukulja granitoid, granites, pegmatites, aplites, lamprophyres, hornfels, and skarns; (iv) Neogene deposits including conglomerates, marls, marly limestones, sandstones, and clays; and (v) Quaternary formations represented by deluvial and alluvial deposits [11,26,27,28]. Investigations carried out on the Venčac marbles [55] showed a mineral composition that includes chlorite, quartz, muscovite, and albite, which are typical of regional metamorphism, and an absence of garnet, tremolite, diopside, periclase, and wollastonite, which are typical of contact metamorphism.

2.3. Methods

The analysis of sample-1’s chemical composition was performed by applying the following processes: Si content was measured gravimetrically using a method adapted from [56]. The quantities of Al, Fe, Ti, Ca, Mg, Mn, Na, and K were determined after converting them into a soluble form, using atomic absorption spectrophotometry (AAS) with a Perkin Elmer 4000 instrument (PerkinElmer, Singapore), employing the flame technique. The contents of P and S were measured colorimetrically and turbidimetrically, respectively, with a Perkin Elmer Lambda 15 UV/VIS spectrophotometer. The CO₂ content was calculated after treating a precise sample amount (±0.2 mg) with diluted HCl (HCl: H₂O; 1:3), then measuring the volume of released CO₂. The determination of iron was performed after the accurately weighed sample (with an accuracy of ±0.2 mg) in a Pt vessel was treated with HClO₄ and HF-concentrated acids, and heated on hot plate. Sample decomposition took place in an inert CO₂ atmosphere. After the sample decomposition, the Pt vessel was put in Erlenmeyer flask with distilled water. Ferrous iron was determined by titration with standard KMnO₄ solution. Ferric iron was determined from the difference between total and ferrous iron. The H₂O⁻ (adsorbed water) and H₂O⁺ (structural or bound water) contents were determined by weighing the sample before and after treatments at 105 °C and 1000 °C for one hour, respectively. All element contents are expressed as oxides.

Sample-2 was obtained by treating sample-1 with diluted acetic acid (CH₃COOH: H₂O; 1:3), which was heated on hot plate at 50 °C for 48 h at the normal atmospheric pressure (ca. 1 bar). After that, this sample was rinsed with distilled water, and later dried in air. This procedure was mainly accomplished with the intention to eliminate some part of magnesian calcite, but to avoid possible destruction of the other mineral phases.

3. Results and Discussion

3.1. Re-Calculated Compositions of the Magnesian Calcite

First, the results for magnesian calcite (1) and (2), shown in [30], were re-checked by using the following quadratic least-squares regression Equations (1)–(4) from [54]:

a = 4.9906 − 0.50 × X_Mg + 0.56 × X_Mg²,

(1)

c = 17.069 − 2.27 × X_Mg + 2.1 × X_Mg²,

(2)

V = 368.1 − 122 × X_Mg + 131 × X_Mg², and

(3)

c/a = 3.420 − 0.118 × X_Mg + 0.05 × X_Mg²,

(4)

where X_Mg = mol. % MgCO₃.

Based on the calculated values (

Table 1. Calculated contents of MgCO₃ (in mol. %) in magnesian calcite (1) and (2) from Equations (1)–(4). Used unit cell parameters (a, c, V, and c/a) for the X_Mg calculations are taken from [30].

Reference			Magnesian Calcite (1): [30]	Magnesian Calcite (1): This Paper			Magnesian Calcite (2): [30]	Magnesian Calcite (2): This Paper
Equation (1) [54]	a	4.966 (2) *	5.21	5.22	a	4.9791 (9)	2.18	2.36
Equation (2) [54]	c	16.96 (1)	5.23	5.04	c	17.025 (5)	2.36	1.97
Equation (3) [54]	V	362.1 (4)	5.04	5.21	V	365.5 (1)	1.97	2.18
Equation (4) [54]	c/a	3.415	4.32	4.32	c/a	3.419	0.85	0.85

* Numbers in parentheses are the estimated standard deviations (ESDs) and refer to the last presented number (it is also valid throughout the text and other Tables).

), it is evident that these are nearly identical to those reported in the previous paper [30]. However, it is also clear that the current calculations correct minor discrepancies found in the previously published data, particularly with regard to the calculated values from Equations (1)–(3).

Because accurately assessing the Mg content in natural calcites is crucial for many research areas, we recalculated the compositions of the studied magnesian calcite (1) and magnesian calcite (2) from Krečana–Venčac [30] by using equations derived from the XRPD measurements analyzed by applying Rietveld refinement, which were published after the initial study [57,58]. The specific Equations (5)–(12) from [57] and (13)–(16) from [58] are employed for the recalculation:

X_Mg = −254.268064 × a + 1269.083982,

(5)

X_Mg = −55.423796 × c + 945.901576,

(6)

X_Mg = −997.807429 × c/a + 3412.426500,

(7)

X_Mg = −1.047482 × V + 385.628601,

(8)

X_Mg = −266.555258 × a + 1329.120352,

(9)

X_Mg = −51.725661 × c + 882.793319,

(10)

X_Mg = −694.720293 × c/a + 2378.806942,

(11)

X_Mg = −1.072219 × V + 393.759147,

(12)

a = −0.4178 × X_Mg + 4.9896 ⇒ X_Mg = (a − 4.9896)/(−0.4178),

(13)

c = −1.8603 × X_Mg + 17.061 ⇒ X_Mg = (c − 17.061)/(−1.8603),

(14)

c/a = −0.0877 × X_Mg + 3.4193 ⇒ X_Mg = (c/a − 3.4193)/(−0.0877), and

(15)

V = −100.97 × X_Mg + 367.93 ⇒ X_Mg = (V − 367.93)/(−100.97),

(16)

where X_Mg = mol. % MgCO₃. The values of the used unit cell parameters (a, c, c/a, and V) for the X_Mg calculations are the same as for Equations (1)–(4), shown in Table 1.

According to the resulting values (

Table 2. Calculated contents of MgCO₃ (in mol. %) in magnesian calcite (1) and (2) by using Equations (5)–(16).

Reference		Magnesian Calcite (1)	Magnesian Calcite (2)
Equation (5) [57]	a	6.39 ± 0.51	3.06 ± 0.23
Equation (6) [57]	c	5.91 ± 0.55	2.31 ± 0.28
Equation (7) [57]	c/a *	4.91	0.92
Equation (8) [57]	V	6.34 ± 0.42	2.77 ± 0.10
Average value (I)		6.21 ± 0.49	2.71 ± 0.21
Equation (9) [57]	a	5.41 ± 0.54	1.92 ± 0.24
Equation (10) [57]	c	5.53 ± 0.52	2.16 ± 0.26
Equation (11) [57]	c/a *	6.34	3.56
Equation (12) [57]	V	5.51 ± 0.43	1.86 ± 0.10
Average value (II)		5.48 ± 0.49	1.98 ± 0.20
Equation (13) [58]	a	5.65 ± 0.48	2.51 ± 0.21
Equation (14) [58]	c	5.43 ± 0.54	1.94 ± 0.27
Equation (15) [58]	c/a *	4.90	0.34
Equation (16) [58]	V	5.77 ± 0.39	2.41 ± 0.10
Average value (III)		5.62 ± 0.47	2.29 ± 0.19

* Rejected from the calculations, because of the largest deviations observed by [57,58], including this study.

), the most significant deviations from the previous magnesian calcite compositions [30] are obtained by using Equations (5)–(8), particularly their average value (I). Such an outcome is not very surprising, as these calibrations for Mg quantifications were obtained from biogenic calcites (used in that particular study [57]). It is a well-known fact that biogenic calcites show some significant differences compared to other types of calcites. For example, echinoid skeletal parts may vary by up to 5 mol. % MgCO₃, while algae have the neighboring domains reaching up to 10 mol. % MgCO₃ [54]. Biogenic magnesian calcites are often also characterized with the structural anisotropic contraction and octahedra distortion (see [30] and references therein).

In contrast, and as expected, a much better agreement with the previous magnesian calcite compositions [30] is achieved by Equations (9)–(12), which can provide the current calibrations [57]. Specifically, 5.14 mol. % of MgCO₃, calculated for magnesian calcite (1) [30], falls within the estimated standard deviation (ESD) of the average value 5.48 ± 0.49 (II). Meanwhile, 1.86 mol. % of MgCO₃, obtained for magnesian calcite (2) [30], falls within the estimated ESD of the average value 1.98 ± 0.20 (II) (Table 2).

The average values (III) were slightly higher and somewhat similar to the previously described average values (II), measured as 5.62 ± 0.47 and 2.29 ± 0.19, respectively (taking in consideration Equations (13)–(16)). Among these, the best agreement is shown by the c-axis (5.43 ± 0.54 and 1.94 ± 0.27, respectively; Equation (14); Table 2). This feature is also expected, because this equation was derived from the quantitative phase analyses of carbonate rocks that contain Mg-rich calcite and non-stoichiometric dolomite [58]. In other words, the resulting value is expected because the mineral composition is very similar to that of the investigated sample [30].

Furthermore, Merlini et al. [59] investigated how the unit cell parameters in the ternary CaCO₃-MgCO₃-FeCO₃ carbonate system can vary. The authors show that the unit cell axes and volume change nearly linearly across the four binary systems: calcite–magnesite, calcite–Fe-magnesite, dolomite–ankerite, and magnesite–siderite. These changes are independent of the sample’s symmetry. To recalculate the magnesian calcites (1) and (2) [30], we additionally use Equations (17)–(19) (taken from Merlini et al. [59]):

a = 4.985 × X_CaCO3 + 4.636 × X_MgCO3 + 4.696 × X_FeCO3,

(17)

c = 17.064 × X_CaCO3 + 15.033 × X_MgCO3 + 15.414 × X_FeCO3, and

(18)

V = 366.2 × X_CaCO3 + 279.2 × X_MgCO3 + 294.1 × X_FeCO3,

(19)

where X_CaCO3, X_MgCO3, and X_FeCO3 are, respectively, the molar fraction of the calcitic, magnesitic, and sideritic components in the considered carbonate.

The resulting values shown in

Table 3. Calculated unit cell parameters from Equations (17)–(19) of magnesian calcite (1) and (2) using contents of determined CaCO₃ and MgCO₃ from [30]^*.

Reference		Magnesian Calcite (1): [30]	Magnesian Calcite (1): This Paper	Magnesian Calcite (2): [30]	Magnesian Calcite (2): This Paper
	XCaCO3	0.9486	0.9486	0.9814	0.9814
	XMgCO3	0.0514	0.0514	0.0186	0.0186
Equation (17) [59]	a	4.966 (2) Å	4.967 Å	4.9791 (9) Å	4.9785 Å
Equation (18) [59]	c	16.96(1) Å	16.960 Å	17.025 (5) Å	17.026 Å
Equation (19) [59]	V	362.1 (4) Å3	361.7 Å3	365.5 (1) Å3	364.6 Å3

* According to the used methods and derived data from [30], the X_FeCO3 contents in this case are exclusively considered as “not detected”; therefore, these are not presented in this table.

indicate that the calculated a and c unit cell axes by using Equations (17) and (18) closely match those from the previous study [30]. The matching is a good indicator showing that these are nearly identical. The calculated volume V from Equation (19) for magnesian calcite (1) falls within the ESD range. However, the calculated volume for magnesian calcite (2) is slightly lower than the values reported by Tančić [30].

Based on the results presented in this sub-chapter, the composition of magnesian calcites (1) and (2) determined with the XRPD method [30] are obviously in alignment with a few more recent studies [57,58,59]. Accordingly, the results are highly accurate, being helpful for further research shown in this paper.

3.2. Chemical Investigations

The chemical composition of sample-1 is shown in

Table 4. Chemical composition of sample-1, recalculated chemical composition and calculated number of ions at the basis of three oxygen apfu.

Oxides	wt. %	Recalculated to 100%	Calculated at Three O apfu
SiO2	0.67	n.c.	n.c.
Al2O3	0.09	n.c.	n.c.
Fe2O3	0.00	n.c.	n.c.
TiO2	0.14	n.c.	n.c.
CaO	53.15	54.12	0.960 Ca
MgO	1.55	1.58	0.039 Mg
MnO	0.00	0.00	0.000 Mn
FeO	0.08	0.08	0.001 Fe
Na2O	0.05	n.c.	n.c.
K2O	0.04	n.c.	n.c.
P2O5	0.00	n.c.	n.c.
SO3	0.10	n.c.	n.c.
CO2	43.42	44.22	1.000 C
H2O−	0.42	n.c.	n.c.
H2O+	0.19	n.c.	n.c.
Σ	99.90	100.00	3.000 O

n.c.—not calculated.

. Oxides, which may theoretically be part of the calcite group of minerals (such as calcite–CaCO₃, magnesite–MgCO₃, rhodochrosite–MnCO₃, and/or siderite–FeCO₃), were initially recalculated to 100% with a purpose to excluding the presence of minor minerals (such as quartz, feldspars and clay minerals–micas [30]; see later in the text). Subsequently, by using the corrected results, the number of ions per formula unit (apfu) containing three oxygen atoms was calculated.

Accordingly, the crystallo-chemical formula of sample-1 was calculated as (Ca_0.960Mg_0.039Fe_0.001)CO₃, showing an ideal stoichiometry for this group of carbonates (i.e., cations:carbon:oxygen ratio is 1:1:3 within three decimal places; Table 4). By using this formula, the preliminary data show 4 mol. % of CaCO₃ substituted with 3.9 mol. % of MgCO₃ and 0.1 mol. % of FeCO₃. On the other hand, if this sample can hypothetically contain exclusively pure calcite (with no substituted Ca²⁺ by other cations) and dolomite [CaMg(CO₃)₂], then by using the magnesium content, the sample would amount to 7.28% dolomite. Nevertheless, it is well known that in calcite some content of Ca²⁺ has usually been substituted with other cations (in this case mostly with Mg²⁺ and very little Fe²⁺). Therefore, such substitution implies that the dolomite content should be lower, which is confirmed by the XRPD study [30], indicating the following content: magnesian calcite (1) (~95%), dolomite (~4%), and quartz (~1%). Sample-2 consists of magnesian calcite (2) (~79%), quartz, feldspars and clay minerals–micas (each ~6–7%), with insignificant quantities of dolomite (~1.5%).

By applying the calculated unit cell parameters of calcite and dolomite [30], Ca²⁺ was partly substituted for Mg²⁺ and/or Fe²⁺, both of which have a smaller ionic radii than Ca²⁺ (i.e., Mg²⁺ = 0.72 Å; Fe²⁺ = 0.78 Å; and Ca²⁺ = 1.00 Å; in coordination VI [60]). As mentioned earlier, the crystallographic analysis shows that in magnesian calcite (1), 5.14 mol. % of CaCO₃ was substituted with MgCO₃ whereas in magnesian calcite (2), such substitution was 1.86 mol. % [30]. Another argument is that the difference of 3.28 mol. % of CaCO₃ between these two is in connection with submicroscopic intergrowth and exsolution of dolomite in magnesian calcite (1). However, latter were most likely destroyed with acetic acid in magnesian calcite (2) [30].

To clarify, exsolution is a process in which a former fairly homogeneous phase separates into the two or more solid solution phases under subsolidus conditions. Exsolution occurs only in minerals with two or more pure end-member compositions that cannot mix together in normal conditions or in metastable minerals with complicated solid solutions by ion substitutions occurring at high temperature or pressure [61]. Additionally, dolomite and/or calcite can form from retrograde reactions, which can be challenging because of the differentiation from exsolved or preexisting grains [62], making this situation even more complex.

Nevertheless, the discrepancy that exists between chemical and powder diffraction results for samples containing magnesian calcite and dolomite occurs mainly due to the intergrowth and exsolution of dolomite within calcite. The discrepancy is well established in the earlier mid-XX century reports [63,64]. More specifically, if the submicroscopic intergrowth and exsolution of dolomite exist in magnesian calcite, then the coherency strain may shift the d-values sufficiently to yield incorrect MgCO₃ content in magnesian calcites ([30], and references therein). Consequently, the 3.90 mol. % MgCO₃ in sample-1 (obtained from chemical analysis; Table 4), clearly does not correspond to the ca. 7.14 mol. % MgCO₃ obtained from X-ray analysis. The latter sums to (i) 5.14 mol. % from magnesian calcite (1) and (ii) approximately 2 mol. % from dolomite [30]. In contrast, 1.86 mol. % MgCO₃, replacing CaCO₃ in magnesian calcite (2) reflects the true value. As a result, from sample-1, the dolomite content is approximately 4%, equivalent to about 2 mol. % MgCO₃. From sample-2, the MgCO₃ in magnesian calcite (2) is 1.86 mol. %. Overall, the total MgCO₃ content obtained by the X-ray analysis reaches ca. 3.86 mol. %. The latter resulting value obviously aligns with the chemical analysis and the calculated crystallo-chemical formula of the 3.90 mol. % MgCO₃ (Table 4).

The exact mechanism behind the influence of the diluted acetic acid to the submicroscopic intergrowth and exsolution of dolomite in magnesian calcite (1) is beyond the scope of this paper (therefore, it was not constrained in more detail). Nevertheless, the following is evident:

(i)

ca. 16% of magnesian calcite (1) and ca. 2.5% of dolomite within sample-1 were destroyed by this treatment;

(ii)

Such a destruction process undoubtedly resulted in the afore-described significant crystallographic differences between magnesian calcite (1) and magnesian calcite (2);

(iii)

The calculated MgCO₃ content of 1.86 mol. % in magnesian calcite (2) represents the afore-explained true value. Consequently, the presumed coherency strain between magnesian calcite (2) and dolomite in sample-2 sufficiently decreased to the level capable of preventing the shift in the crystallographic inter-planar (d) values (which was obviously occurred between magnesian calcite (1) and dolomite in sample-1).

3.3. Calculated Dolomite Composition

The following section introduces multiple methods for calculating dolomite composition. Prior to the computations, a few important prerequisites must be noted. It is essential to highlight that the studied dolomite from the Krečana–Venčac marble deposits [30] exhibits clearly visible super-lattice XRPD reflections with Miller’s indices (101), (015), and (021). The visibility of super-lattice XRPD reflections unmistakably allows it to be distinguished from calcite and high-Mg calcite [65], as well as from other Mg–carbonate phases [66]. Another key point is that the unit cell parameters and selected inter-planar spacings of this dolomite (a = 4.791 (3) Å; c = 15.95 (2) Å; V = 317.1 (4) ų; c/a = 3.329; and d₍₁₀₄₎ = 2.8702 Å [30]) are significantly smaller compared to the reference crystallographic standard [67]. This situation is most likely due to a partial substitution of Ca²⁺ with Mg²⁺ and/or Fe²⁺, since both have smaller ionic radii than Ca²⁺ [60]. Thirdly, to examine the mutually related correlations between the calculated unit cell parameters (i.e., a and c axes) and the measured d₍₁₀₄₎ inter-planar spacing of dolomite, the following Equations (20)–(22) [68] can be used:

c = 7.3097 × a − 19.135,

(20)

a = 1.0306 × d₍₁₀₄₎ + 1.8337, and

(21)

c = 7.6284 × d₍₁₀₄₎ − 6.0059.

(22)

Equations (20)–(22) demonstrate a considerably strong correlativity, with the values closely matching those computed within their ESDs, thus enabling further analysis. Specifically, Equation (20) yields c = 15.89 (3) Å; Equation (21) gives a = 4.792 Å; and, again, from Equation (21), c is confirmed as 15.89 Å. To further accurately identify the composition of this dolomite, various diagrams, equations, and calculations have been used. We believe that such a composite approach is necessary allowing accurate constraints on the (non-?)stoichiometric composition of the dolomite, and additionally could be helpful for similar cases in the future research.

(1) The variation diagrams include the following: (i) the unit cell axes a and c, and inter-planar spacing d₍₁₀₄₎, as shown by the composition of Ca-Mg carbonates [51]; (ii) the unit cell axes a and c across the full composition yield the range from CaCO₃ to CaMg(CO₃)₂ [52]; (iii) the unit cell volume V versus composition of both ordered and disordered Ca-Mg carbonates [53]; and (iv) parameters a, c, and V of the unit cell, together with c/a ratio changes as the functions of CaCO₃ content in low-iron sedimentary dolomites [69]. The CaCO₃ content (mol. %) in the Krečana–Venčac dolomite, calculated from these variation diagrams based on a, c, V, c/a, and d₍₁₀₄₎ parameters [51,52,53,69], are listed in

Table 5. Contents of CaCO₃ (in mol. %) in dolomite obtained from variation diagrams by a, c, V, c/a and d₍₁₀₄₎ parameters [51,52,53,69].

Reference	a	c	V	c/a	d(104)
[51]	45.0	47.0	/	/	45.0
[52]	45.5	45.5	/	/	/
[53]	/	/	47.0	/	/
[69]	45.5	47.5	46.5	50.2	/

.

Table 5 clearly shows that the CaCO₃ content in the studied dolomite is mostly under 50 mol. % in most of the diagrams. These values generally range from 45 to 47 mol. %, with an average of 46.5 mol. % CaCO₃. Such a difference indicates that the dolomite is non-stoichiometric, and that smaller Mg²⁺ and/or Fe²⁺ ions replaced larger Ca²⁺ ions; similarly to the magnesian calcites studied.

To support this statement, we use the variation diagram of CaFe(CO₃)₂ by Δ2θ°(2θ_D(211) − 2θ_CdF2) for dolomites which were synthesized from composition CaMg(CO₃)₂-CaFe(CO₃)₂ at 450 °C (as shown by Rosenberg [70]). Comparing this diagram with the studied dolomite (Δ2θ° = 2.473) and the standard conventional value (Δ2θ° = 2.435; [71]) shows that smaller Mg²⁺ was not replaced by larger Fe²⁺. Another diagram for dolomites which were synthesized from the composition CaMg(CO₃)₂-CaMn(CO₃)₂ at 450 °C [72], depicting CaMn(CO₃)₂ by Δ2θ°(2θ_D(211) − 2θ_CdF2), has also been used for support. For this diagram and the studied dolomite, which also has Δ2θ° = 2.435, it indicates that smaller Mg²⁺ was not substituted by larger Mn²⁺ (Mn²⁺ = 0.83 Å in coordination VI [60]).

(2) Since it was established that some Ca²⁺ ions were replaced by Mg²⁺ and Fe²⁺, Equations (23) and (24), presented by Goldsmith et al. [73] are utilized to calculate the unit cell axes in the CaCO₃-MgCO₃-FeCO₃ system:

a = 4.92954 × X_(CaCO3) + 4.69290 × X_(MgCO3) + 4.73269 × X_(FeCO3) + 4.71879 × X_(MnCO3), and

(23)

c = 16.5368 × X_(CaCO3) + 15.5004 × X_(MgCO3) + 15.8589 × X_(FeCO3) + 16.0111 × X_(MnCO3),

(24)

where X(MCO₃) is mole fraction of the respective carbonate component and M = Ca, Mg, Fe, and Mn.

By assuming that the dolomite sample contains various solid solution compositions (as shown in columns 1–9 of

Table 6. Calculated unit cell axes (a and c; in Å) of dolomite with Equations (23) and (24), at basis of its different presumed compositions (CaCO₃, MgCO₃ and FeCO₃; in mol. %).

	1	2	3	4	5	6	7	8	9
CaCO3	50.00	50.00	47.50	45.00	44.22	44.00	42.50	42.00	41.50
MgCO3	50.00	47.50	50.00	52.50	53.28	53.50	55.00	55.50	56.00
FeCO3	0.00	2.50	2.50	2.50	2.50	2.50	2.50	2.50	2.50
a	4.8112	4.8122	4.8063	4.8004	4.7985	4.7980	4.7944	4.7933	4.7921
c	16.0186	16.0276	16.0016	15.9757	15.9677	15.9654	15.9498	15.9446	15.9395

), which are used in Equations (23) and (24), the resulting unit cell axes are listed also in Table 6. For example, adding 0.1 mol. % of FeCO₃ (Table 4) to the dolomite structure, which represents about 4%, gives roughly 2.5 mol. %. Given the smaller unit cell parameters of the studied dolomite [30], such a size suggests that MgCO₃ partially substitutes CaCO₃ fraction, because dolomite—serving as a standard—has the unit cell axes of a = 4.809 Å and c = 16.02 Å [67]. Consequently, it appears that the presumed dolomite composition in this instance [73] is closest to the ideal stoichiometry of 50 mol. % CaCO₃ and 50 mol. % MgCO₃ (as the calculated unit cell axes most closely resemble those in column 1; Table 6).

Conversely, the substitution of 2.5 mol. % of FeCO₃ with MgCO₃ results in larger unit cell parameters than the reference standard (Table 6, column 2). The resulting larger unit cell parameters occur because Fe²⁺ has larger ionic radii than Mg²⁺ [60]. Such a relationship confirms that FeCO₃ is substituting CaCO₃ because this type of replacement leads to smaller unit cell axes (Table 6, column 3). Additionally, as the substitution of CaCO₃ with MgCO₃ increases, the calculated unit cell axes continue to have with a decrease (Table 6, columns 4–9), as expected. For instance, column 5 displays the dolomite composition with 3.28 mol. % MgCO₃, representing the exact difference between magnesian calcite (1) and magnesian calcite (2). Lastly, the unit cell axes of dolomite from Krečana–Venčac [30] align closely with the values derived from the compositions (I) shown in columns 7 and 8 (Table 6):

(Dolomite composition I): CaCO₃ = 42.00–42.50 mol. %; MgCO₃ = 55.00–55.50 mol. %; and FeCO₃ = 2.50 mol. %.

However, dolomite composition (II) adjusts the values from composition (I) to match the theoretical dolomite stoichiometric ratio of 54.35 mol. % CaCO₃: 45.65 mol. % MgCO₃ [74]. According to this ratio, the previous composition (I) should be corrected with the following more appropriate values:

(Dolomite composition II): CaCO₃ = 46.35–46.85 mol. %; MgCO₃ = 50.65–51.15 mol. %; and FeCO₃ = 2.50 mol. %,

which are in a very good agreement with the results in Table 5.

(3) On the other hand, we believe that by using the combination of the results from the chemical analysis (Table 4) with those from the X-ray investigations [30], compositional calculations should be at least equally reliable. Namely, in sample-1, 95% of magnesian calcite-1 and 4% of dolomite, after recalculations to the total of 100% (with the purpose to avoid minor quartz with quantity of 1% [30]), give 96% of magnesian calcite and 4% of dolomite. Since it was established that magnesian calcite contains 98.14 mol. % CaCO₃ and 1.86 mol. % MgCO₃ [30], the total CaCO₃ and MgCO₃ content in magnesian calcite and dolomite (at the basis ratio of CaCO₃:MgCO₃ = 54.35:45.65) was initially calculated as follows:

(i)

For CaCO₃: 0.96 × 98.14 = 94.21 mol. %; 0.04 × 54.35 = 2.17 mol. %; Σ₁ = 96.38 mol. %;

(ii)

For MgCO₃: 0.96 × 1.86 = 1.79 mol. %; 0.04 × 45.65 = 1.83 mol. %; Σ₂ = 3.62 mol. %.

The content of CaCO₃ (Σ₁) is 0.38 mol.% higher, while MgCO₃, along with FeCO₃ (Σ₂), has a 0.38 mol.% lower value than the number obtained from the chemical analysis (i.e., 4.00 mol. %; Table 4). The relationship indicates that 0.38 mol. % of CaCO₃ in dolomite was replaced by 0.38 mol. % of MgCO₃ and FeCO₃, corresponding to 9.5 mol. % of CaCO₃ being substituted in 4% dolomite. Based on these calculations, dolomite has the following composition (III):

(Dolomite composition III): CaCO₃ = 44.85 mol. %, MgCO₃ = 52.65 mol. %, and FeCO₃ = 2.50 mol. %.

Secondly, the same calculations for dolomite with ratio of CaCO₃:MgCO₃ = 50:50 are as follows:

(iii)

For CaCO₃: 0.96 × 98.14 = 94.21 mol. %; 0.04 × 50.00 = 2.00 mol. %; Σ3 = 96.21 mol. %;

(iv)

For MgCO₃: 0.96 × 1.86 = 1.79 mol. %; 0.04 × 50.00 = 2.00 mol. %; Σ4 = 3.79 mol. %.

The content of CaCO₃ (Σ₃) is 0.21 mol. % higher in this case, while the content of MgCO₃ (Σ₄) (together with FeCO₃) is 0.21 mol. % lower than the values obtained from chemical analysis (i.e., 4.00 mol. %; Table 4). The relationship suggests that 0.21 mol. % of CaCO₃ in dolomite was replaced with 0.21 mol. % of MgCO₃ and FeCO₃. This difference corresponds to 5.25 mol. % of CaCO₃ being substituted in 4% dolomite. From these calculations, it results that dolomite has the following composition (IV):

(Dolomite composition IV): CaCO₃ = 44.75 mol. %, MgCO₃ = 52.75 mol. %, and FeCO₃ = 2.50 mol. %.

As shown, the latter two calculations yielded nearly identical results for the dolomite (III) and (IV) compositions. The nearly identical results further indicate that, as a starting points for such analyses, both CaCO₃:MgCO₃ ratios (i.e., 54.35:45.65; and 50:50) could be used.

(4) Other calculations of the dolomite unit cell parameters could also be achieved with the already presented Equations (17)–(19), further applying its various afore-calculated compositions (I-IV).

The results presented at

Table 7. Calculated unit cell parameters (a, c, and V) of dolomites from Equations (17)–(19) using their various calculated compositions (I-IV), i.e., CaCO₃, MgCO₃, and FeCO₃ contents.

Reference		Dolomite Composition (I)	Dolomite Composition (II)	Dolomite Composition (III)	Dolomite Composition (IV)
	XCaCO3	0.4225	0.4660	0.4485	0.4475
	XMgCO3	0.5525	0.5090	0.5265	0.5275
	XFeCO3	0.0250	0.0250	0.0250	0.0250
Equation (17) [59]	a	4.785 Å	4.800 Å	4.794 Å	4.794 Å
Equation (18) [59]	c	15.901 Å	15.989 Å	15.953 Å	15.951 Å
Equation (19) [59]	V	316.3 Å3	320.1 Å3	318.6 Å3	318.5 Å3

show that the calculated unit cell axes (a and c) of dolomites from Equations (17)–(19) closely match those in Tančić [30], whereas the volumes (V) are slightly elevated relative to the dolomite compositions (III) and (IV). In contrast, the compositions (I) and (II) exhibit significantly larger deviations of all of these parameters. The presence of deviations suggests that calculations for the compositions (III) and (IV) are more reliable than those for (I) and (II). This relationship also further approves the proposed combination of the results from the chemical analysis (Table 4) coupled with those from the earlier XRPD study.

(5) Lastly, the Ca content in dolomite (n_Ca) is calculated from the unit cell parameters (i.e., a and c axes; and c/a ratio) and the measured d₍₁₀₄₎ inter-planar spacing of dolomite, using the following Equations (25)–(28) proposed by McCarty et al. [68]:

c = 0.8632 × n_Ca + 15.14 ⇒ n_Ca = (c − 15.14)/0.8632,

(25)

a = 0.1168 × n_Ca + 4.6903 ⇒ n_Ca = (a − 4.6903)/0.1168,

(26)

c/a = 0.0981 × n_Ca + 3.2309 ⇒ n_Ca = (c/a − 3.2309)/0.0981, and

(27)

d₍₁₀₄₎ = 0.119 × n_Ca + 2.7658 ⇒ n_Ca = (d₍₁₀₄₎ − 2.7658)/0.119.

(28)

In addition, the excess of Ca (Δn_Ca) in its B site could be calculated from the c axis by using the following Equation (29) [58]:

c = 16.0032 + 0.8632 × Δn_Ca ⇒ Δn_Ca = (c − 16.0032)/0.8632.

(29)

Based on Equations (25)–(28), the calculated nCa values are as follows: 93.84 ± 2.31% along the c axis; 86.22 ± 2.56% along the a axis; 100% along the c/a ratio; and 87.73% along the d₍₁₀₄₎ inter-planar spacing. These results show a notable deviation from earlier computations, except for the c axis. Conversely, using Equation (29), the excess Ca (Δn_Ca) appears to be −6.16 ± 2.32 apfu, indicating Mg-rich/Ca-deficient dolomite, consistent with the determined negative value, which closely matches previously reported data.

In conclusion, the smaller d-values and unit cell parameters observed in dolomite from Krečana–Venčac [30] are caused by the substitution of CaCO₃ with MgCO₃ and FeCO₃ of about 3–5 mol. %. This is in agreement with the available literature data, which show that such a substitution can reach up to 6 mol. % [75], and with documented composition ranging from Ca_1.16Mg_0.84(CO₃)₂ to Ca_0.96Mg_1.04(CO₃)₂, covering calcian (i.e., calcium-excess) to magnesian (i.e., magnesium-excess) dolomites [76]. Therefore, the studied dolomite sample should be classified as magnesian.

Although the results in this Chapter appear promising, some preliminary caution is, however, necessary because (i) potential errors may arise in the inter-planar spacing measurement obtained from XRPD analysis—and the subsequent unit cell parameters calculations—due to a small dolomite content (about 4%; [30]), and (ii) the smaller unit cell parameters observed in dolomite, along with the mentioned chemical composition, could also stem from increased pressure conditions [59,77,78,79]. However, (i) the measured inter-planar spacing of the two strongest peaks of quartz (i.e., d₍₁₀₁₎ = 3.3453 Å and d₍₁₀₀₎ = 4.2658 Å; [30]), which could be treated as the internal standard, indicate that the XRPD measurement was adequate and very reliable, whilst (ii) pressure conditions could be described as “normal” for this kind of facies (see later in the text).

3.4. Calculated Formation Temperatures

According to Essene’s [80] report, “calcite–dolomite thermometry is a useful method for determining metamorphic conditions because: (1) calcite and dolomite in marbles are close to the CaCO₃-CaMg(CO₃)₂ binary system in metamorphic terranes; (2) it is minimally affected by pressure and only requires rough pressure estimates for adjustments; (3) unlike most silicate isograds, it does not depend on fluid composition. Its main limitation is rapid re-equilibration, which restricts it to providing minimum temperature estimates in many rocks.”

By using the MgCO₃ content (in mol. %) in magnesian calcite (1) and (2), which occur alongside dolomite, the formation temperatures of these minerals were estimated by using several reliable reference datasets presented in

Table 8. Estimated temperatures for magnesian calcite (1) and (2) on the basis of the reference data [63,64,81,82,83,84,85,86,87], including the derived differences (Δ) between them.

	Magnesian Calcite (1)	Magnesian Calcite (2)	Δ
MgCO3 (mol. %)	5.14	1.86	3.28
Reference	temperature (°C)
[81]	≈480	≈320	≈160
[63]	≈500	≈300	≈200
[82]	≈530	≈350	≈180
[83]; their Figure 6	≈535	≈385	≈150
[83]; their Figure 8	≈530	≈360	≈170
[84]	≈560	≈340	≈220
Average value (I)	≈522.50	≈342.50	≈180
Equation (30) [64]	540.80	285.18	255.62
Equation (31) [85]; modified to Equation (32)	537.78	396.03	141.75
Equation (34) [86]	542.83	334.67	208.16
Equation (36) [86]	543.63	337.10	206.53
Average value (II)	541.26	338.24	203.02
[87] FeCO3(calcite) = 0.1 mol. % (Figure 2)	≈518	≈342	≈176
[87] FeCO3(dolomite) = 2.5 mol. % (Figure 3)	≈522	≈344	≈178
Average value (III)	≈520.00	≈343.00	≈177
Average value (IV)	≈528	≈341	≈187

. Specifically, the diagrams created by [63,81,82,83,84] served as the primary tool for these estimations. The average value (I) was then derived from these data. The average value (II) was calculated using different equations reported in [64,85,86]. In contrast, the average value (III) was obtained from the two figures reported by Bickle and Powell [87]. Finally, the average value (IV) was calculated from all of the results presented in Table 8.

The following Equation (30) was used by [64] and (31) by [85]:

Log_MgCO3 = [1.727 × 10⁻³ × t (°C)] − 0.223 ⇒ t (°C) = [(Log_MgCO3 + 0.223)/1.727] × 10³, and

(30)

Log₁₀X_MgCO3 = [−1690/t (°C)] + 0.795.

(31)

It is important to highlight that one minor (printing?) error was found in Equation (31), which can be verified by the temperatures listed in the report of Rice [85], and our own calculations which are presented in Table S1. Namely, from the presented calculations it is obvious that these temperatures should be expressed in Kelvin (K) rather than degrees Celsius (°C). Based on our research, the corrected version of the equation should be as follows:

T (K) = (−1690)/(Log₁₀X_MgCO3 − 0.795) ⇒ t (°C) = [(−1690)/(Log₁₀X_MgCO3 − 0.795)] − 273.15.

(32)

Additionally, we used the quite complex Equations (33) and (35) presented by Anovitz and Essene [86]:

T^Mg (K) = (−2360) × (X_Cal^MgCO3) + (−0.01345)/(X_Cal^MgCO3)² + 2620 × (X_Cal^MgCO3)² + 2608 × (X_Cal^MgCO3)^0.5 + 334,

(33)

i.e., t (°C) = T^Mg (K) − 273.15,

(34)

where X_Cal^MgCO3 is the mole fraction of MgCO₃ in calcite; and

T^FeMg (K) = T^Mg + 1718 × (X_Cal^FeCO3) + (−20610) × (X_Cal^FeCO3)² + 22.49 × (X_Cal^FeCO3/X_Cal^MgCO3) + (−26260) × (X_Cal^FeCO3 × X_Cal^MgCO3) + 1.333 × (X_Cal^FeCO3/X_Cal^MgCO3)² + 0.32837 × 10⁷ × (X_Cal^FeCO3 × X_Cal^MgCO3)²,

(35)

i.e., t (°C) = T^FeMg (K) − 273.15,

(36)

where X_Cal^FeCO3 and X_Cal^MgCO3 are the mole fractions of FeCO₃ and MgCO₃ in calcite, respectively; and T^Mg is the temperature for the pure CaCO₃-MgCO₃ system calculated from Equation (33).

In the following assessment of paleotemperatures, Equations (30), (32), (34), and (36) were used, and from these, the average value (II) was calculated. Furthermore, the effect of the additional Fe on the calcite–dolomite solvus was investigated by [87]. Within that study, Bickle and Powell [87] created temperature diagrams illustrating the variation of MgCO₃ contents in calcite with FeCO₃ in calcite and the variation of MgCO₃ contents in calcite with FeCO₃ in dolomite. As previously discussed, an estimated FeCO₃ content of about 0.1 mol. % was used for the investigated calcite (Figure 2) and in investigated dolomite of about 2.5 mol. % (Figure 3). From these values, the average value (III) was calculated.

Table 8 shows that the temperature values obtained more or less vary because they were derived from different diagrams and equations used, including the diagrams with additional Fe. The notably average values (I) and (III) are more similar to each other than to the average value (II). This difference can be partly explained by considering that Equation (30), given by Sheppard and Schwarcz [64], criticized by Rice [85], relies on data reported by Graf and Goldsmith, and Goldsmith and Newton [81,83] regarding the linear vs. non-linear data dependence. As a result, it tends to underestimate temperatures at low ranges (i.e., <500 °C) and overestimate them at a higher range. Indeed, in the samples studied (Table 8), the magnesian calcite (1) temperatures are nearly identical because they exceed 500 °C. However, for magnesian calcite (2), there is a significant difference of about 110 °C compared to Equation (31) [85], which gives higher temperature estimate at lower ranges. Finally, the average values (IV) are calculated from all of the results presented. These are about 528 °C for Mg calcite (1), and ~341 °C for magnesian calcite (2), while calculated difference between them is approximately 187 °C.

As previously discussed, earlier studies [30] and the chemical analysis in this paper (Table 4) clearly clarified the compositional differences between magnesian calcite (1) and magnesian calcite (2). These differences account for the variation in calculated formation temperatures, which is 187 °C (see Table 8). It was identified that 2% dolomite impurities in calcite increase the solvus temperature by 50 °C [64]. This 2% dolomite equates to 0.913 mol. % MgCO₃, indicating that 3.28 mol. % MgCO₃—representing the difference between magnesian calcite (1) and magnesian calcite (2)—corresponds to 7.18% dolomite. This highlights the difference which could be calculated as follows:

2%:50 °C = 7.18%:X ⇒ X = 179.5 °C.

(37)

Based on this evidence, it can be concluded that the difference (Δ₁ = 187 °C) between the average estimated formation temperatures for magnesian calcite (1) and (2) are very consistent with the difference (Δ₂ = 179.5 °C) calculated from dolomite impurities in calcite (i.e., Δ₁ − Δ₂ = 7.5 °C). This difference is even smaller if we take into account average values (III) (Δ₃ = 177 °C; Table 8; Figure 2 and Figure 3) of only Δ₁ − Δ₃ = 2.5 °C.

Therefore, in magnesian calcite (1), the presence of dolomite submicroscopic intergrowth and exsolution led to a much higher MgCO₃ content than the actual amount, in concordance with previous discussion. Consequently, it also approved that treatment with diluted acetic acid was very useful, because it successfully removed the high-temperature exsolution phases present in magnesian calcite (1) and that the formation temperature of 341 °C estimated from the magnesian calcite (2) composition should be treated as the “true” one. The constraints on temperature for magnesian calcite (1) of ~528 °C could likely be a suitable illustration showing the peak metamorphic conditions.

Furthermore, according to the calcite–aragonite–dolomite curve constructed by Goldsmith and Newton [83], it seems that the formation pressure should be less than 7–8 kbars (as a maximum possible value which is adequate to the ~341 °C), because of the absence of aragonite [55].

For the initial discussion about the origin of the Venčac marble metamorphic deposit, specifically whether it results from the already mentioned contact or regional metamorphism [10,11,29], it is essential to consider its mineral composition, listed in Chapter 2.2. The presence or absence of certain key minerals, which are essential and relevant, could be very helpful. Therefore, the metamorphic processes in this area were driven solely by temperature, without direct contact with magma.

The study shows that the preliminary evidence corroborates earlier suggestions that the marbles originate from regional metamorphism, formed at temperatures up to 410 °C and pressures near 3 kbar, within the quartz–albite–muscovite–chlorite subfacies of the greenschist facies [10,88]. The suggested subsurface conditions are confirmed and align well with the estimated formation temperature of ~341 °C for the magnesian calcite–dolomite association at the Krečana–Venčac marbles. These findings are also quite similar to those of marble sequences studied in the Biharia Nappe System (Apuseni Mountains, Romania; 316–370 °C) [89], as well as much closer parental carbonate unique sequence referred to as the “Ropočevo breccia” (slightly recrystallized breccia cropping out within the (northern) East Vardar Zone Upper Cretaceous turbidites of former European foreland, vicinity of Belgrade; Figure 1b, “RObre”) [19,90]. Nonetheless, the possibility remains that marbles in contact zones with magmatic formations could be of contact metamorphic origin. Consequently, further detailed research is needed to determine whether the metamorphism in this area was mainly regional or contact in nature.

4. Conclusions

The chemical analysis of marble (sample-1) revealed the crystallo-chemical formula (Ca_0.960Mg_0.039Fe_0.001)CO₃. The formula indicates that 4 mol. % of CaCO₃ is replaced by 3.9 mol. % MgCO₃ and 0.1 mol. % FeCO₃, which is not in agreement with the XRPD results for determined magnesian calcite (1) in this sample, because of the dolomite intergrowth and exsolution. On the contrary, the chemical analysis aligns closely with the XRPD results of 3.86 mol. % for magnesian calcite (2) [30] within sample-2.

Using various diagrams, equations, and calculations, an estimated magnesium dolomite composition is provided, indicating substitutions of approximately 3–5 mol. % of CaCO₃ by MgCO₃ and FeCO₃. Accordingly, it is more accurately identified as magnesium-excess dolomite.

The reference data and MgCO₃ contents in magnesian calcite associated with dolomite show that the calculated average formation temperature for magnesian calcite (1) is ~528 °C, whereas for magnesian calcite (2) it is ~341 °C. The ~187 °C difference between these temperatures is attributed to the submicroscopic intergrowth and exsolution of dolomite within magnesian calcite (1), which obviously elevate the temperature of equilibration. Consequently, the estimated formation temperature of this mineral is approximately 341 °C, and it should be treated as the “true” one. The temperature range fits to the proposed greenschist facies level of subsurface pressure–temperature conditions further outlining shallow depths during geodynamic interaction with other lithospheric scale entities.

Ultimately, the study demonstrates that integration of XRPD and chemical methods with suitable mathematical calculations can provide highly accurate and reliable mineral definitions, including past geothermo(baro)metric conditions, quite similarly to the recently introduced celestine geothermometry method presented by Tančić et al. [35]. We believe that the presented methodology could be applicable for geothermometry computation purposes on marble specimens (but only after their treatment with diluted acetic acid described in this paper). Despite quite a promising outcome, additional experiments are necessary to further examine the proposed modified geothermometry computation approach.