DOI: 10.31038/GEMS.2024663

Abstract

In this short contribution, we present Raman data for the main lines of the synthetic system NaHCO₃-NaDCO₃. Furthermore, we show that some CO₂– rich fluid inclusions in pegmatite quartz in the 1,400 Ma old Rønne granite from Bornholm Island contain D-rich nahcolite. Moreover, we also found ¹³C-rich CO₂ in some fluid inclusions, as well as coronene [C₂₄H₁₂], a highly condensed six-ring polycyclic aromatic hydrocarbon. The occurrence of ¹³C-rich diamonds in a granite-pegmatite system forces the acceptance of supercritical fluids coming fast from the old mantle region. Maybe supercritical fluids are generally responsible for pegmatite formation.

Keywords

Raman spectroscopy, NaHCO₃-NaDCO₃-rich CO₂ inclusions, ¹³CO₂-rich inclusions, 13C-rich diamond, Pegmatites, Bornholm Island

During the study of nahcolite-rich [NaHCO₃] inclusions in pegmatite quartz from Bornholm [1], we found carbonates that could not identified with Raman spectroscopy because of missing reference spectra. Other with Raman determined carbonates and bicarbonates are calcite, zabuyelite [Li₂CO₃], rare amounts of natrite [Na₂CO₃], gregoryite [K₂CO₃], kalicinite [KHCO₃], and dawsonite [NaAl(CO₃) (OH)₂]. Also, graphite is present. A list of carbonate species is in Table 1, given in Thomas et al. 2011 [1]. Because most inclusions are composed of solid carbonates in CO₂ only and there are a small couple of silicate melt inclusions, we can assume that the trapping temperature must be about 700°C or higher. After our studies [2,3] about supercritical fluids coming from mantle deeps, unusual mineral phases are possible. We think here on deuterium-bearing carbonates. No Raman spectra are available for deuterium-bearing nahcolite; therefore, we have synthesized such phases in the NaHCO₃ – NaDCO₃ system. Furthermore, we observed exceptional ¹³CO₂-rich fluid inclusions, which can traced back to the reaction of the supercritical fluid with ¹³C-rich diamond present in the pegmatite quartz from Bornholm Island.

Sample Material

The about 1,400 Ma old granite from the Klippelokke quarry, 3 km ENE of Rønne (Bornholm Island, Denmark), contain uncomplicated quartz-feldspar pegmatite veins (subhorizontal or vertical) with a conspicuous graphic texture and only minor amounts of mica. The potassium felspar is flesh-red (called “red admirals”), and the quartz glyphs are smoky-colored (see Thomas et al. 2011) [1]. The quartz contains mainly fluid inclusions of secondary origin. However, a small number of quartz grains contain a very high number of carbonate- CO₂ inclusions. Some inclusions also contain significant amounts of zabuyelite [Li₂CO₃] (see Thomas et al. 2011) [1]. In such grains, secondary fluid inclusions are rare. Figure 1 show typical nahcolite- bearing CO₂ inclusions.

Figure 1: Typical nahcolite+D and CO₂-bearing inclusion in pegmatite quartz from Bornholm Island.

Because there are no Raman spectra of deuterium-bearing nahcolite present as a reference in the literature, we have prepared such crystals by reaction of analytical poor NaHCO₃ and D₂O (heavy water).

The simple reaction is NaHCO₃ + D₂O → NaDCO₃ +HDO (D₂O in excess).                (1)

By the further reaction of NaDCO₃ with the produced HDO, we obtain, according to the following equation, the stable compound Na₂HD(CO₃)₂:

2 NaDCO₃ + HDO → Na₂HD(CO₃)₂ + D₂O ­                  (2)

The pure NaDCO₃ compound is rare after the reactions (1) because the DHO concentration increases steadily. The pure NaDCO₃ phase forms during fractionated crystallization under the microscope as tiny crystals (Figures 2 and 3). X-ray studies about the last compound must follow.

Figure 2: NaDCO₃ crystals grown from a concentrated NaDCO₃ solution under the microscope (in transmitted light).

Figure 3: NaDCO₃ crystals on silicon grown from a concentrated NaDCO₃ solution under the microscope (in reflected light). The arrows show the pure NaDCO₃ crystals.

Methodology

For our preliminary studies, we used only microscopic and Raman spectroscopic technics.

Raman Spectroscopy

We have performed all microscopic and Raman spectroscopic studies with a petrographic polarization microscope with a rotating stage coupled with the EnSpectr Raman spectrometer R532. The Raman spectra were recorded in the spectral range of 0–4000 cm^-1 using an up to 50 mW single-mode 532 nm laser, an entrance aperture of 20 µm, a holographic grating of 1800 g/mm, and a spectral resolution ranging from 4–6 cm^-1. Generally, we used an objective lens with a magnification of 100x – the Olympus long-distance LMPLFLN100x objective. The laser power on the sample is adjustable down to 0.02 mW. The Raman band positions were calibrated before and after each series of measurements using the Si band of a semiconductor-grade silicon single-crystal. The run-to-run repeatability of the line position (based on 20 measurements each) is ± 0.3 cm^-1 for Si (520.4 ± 0.3 cm^-1) and 0.5 cm^-1 for diamond (1332.7 ± 0.4 cm^-1 over the range of 80–2000 cm^-1). The FWHM = 4.26 ± 0.42 cm^-1. FWHM is the Full- Width at Half Maximum. We used a water-clear natural diamond crystal (Mining Academy Freiberg: 2453/37 from Brasil) as a diamond reference (for more information, see Thomas et al. 2022 [3].

class=”pdfsubheading”>Calibration Curve for the Determination of NaDCO₃

For the construction of a provisional calibration curve between NaHCO₃ and NaDCO₃, we solved a small amount of analytical pure NaHCO₃ in D₂O 99.9% from PelementSamples, Belchertown, MA/ USA. We converted it into a significant excess of pure heavy water [D₂O] into NaDCO₃ according to reaction (1). We gave a droplet of this solution on a microscope glass slide with a hollow or semiconductor- grade silicon wafer (Figure 3 and Figure 4).

Figure 4: Raman spectrum of NaDCO₃ on a Si wafer (520 cm^-1 reference).

As described above, we produced NaDCO₃-rich phases by reaction of NaHCO₃ and heavy water (D₂O) according to the equation (1) and (2). Richardson and Hood (1937) [4] wrote that the concentration of NaDCO₃ is directly proportional to the amount of D₂O. That means pure NaDCO₃ crystals are rare (Figure 5).

Figure 5: Raman spectra of NaHCO₃ (a) and very NaDCO₃-rich nahcolite (b) with D = 0.99.

The obtained Raman data are presented in Figure 6. For the first experiment, we used glass test tubes. The solution is strongly alkaline (pH ~ 11) and reacts readily with glass, forming K₂CO₃, KHCO₃, and other compounds. The formation of K₂CO₃ could proved by Raman spectroscopy (see also Conrad 2020) [5]. Therefore, for most experiments, we used later plastic vessels. In Table 1 are the results of the Raman determination listed.

Figure 6: Calibration curve for the determination of the deuterium in mol fractions determined from the Raman shift.

Table 1: Results of the Raman measurements on pure NaHCO₃ and NaDCO₃ and mixed Na(HD)CO₃.

Compound	Origin	Mean (cm-1)	FWHM (cm-1)	n
NaHCO3	RRUFF R070237	1045.3	4.82	1
NaHCO3	This work	1045.1 ± 0.9	5.41 ± 0.13	12
On Si
NaHCO3	This work	1044.1 ± 0.3	7.14 ± 0.23	6
Na(H0.18D0.82)CO3	This work	1064.4 ± 1.8	13.71 ± 4.39	12
NaDCO3	This work	1069.1 ± 0.2	5.91 ± 0.33	13
On glass
Na(H0.02D0.98CO3	This work	1068.3 ± 0.4	6.10 ± 1.17	10
NaDCO3	This work	1069.3 ± 0.6	5.45 ± 1.18	11

Table 2: Results of the Raman measurements on pure NaHCO₃ and mixed Na(H, D)CO₃ in CO₂ inclusion in pegmatite quartz from Bornholm Island.

Compound	Origin	Mean (cm-1)	FWHM (cm-1)	n
NaHCO3	RRUFF R070237	1045.3	4.82	1
NaHCO3	This work	1045.2 ± 0.4	5.58 ± 0.89	15
Na(H0.32D0.68CO3	This work	1061.3 ± 2.5	19.87 ± 6.58	21

Table 3: lists the main Raman lines of synthetic NaHCO₃, NaDCO₃, and mixed phases in inclusions in pegmatite quartz from Bornholm Island.

NaHCO3 (synthetic)	Rel. Intensity	NaDCO3 (synthetic)	Rel. Intensity	NaDCO3-rich Bornholm	Rel. Intensity
88.7	s	75.1	s	70.0	m
110.5	vs	110.2	s	99.0	vs
141.4	s	150.3	m	153.8	s
		164.6	m
203.8	w
224.5	w			225.6	w
684.7	w	672.4	vw	695.2	vw
		701.3	vw
1045.1	vs	1069.2	vs	1065.1	s
1266.7	m
1434.5	vw			1428.3	vw

Relative intensities: vs: Very Strong, s: Strong, m: Medium, w: Weak, vw: Very Weak. The mean for Bornholm is 1061.3 ± 2.5 cm^-1 (21 different inclusions) and corresponds to D = 0.68 ± 0.11 and for 1065.1 cm^-1 D = 0.84, the highest value.

From our observation under the microscope, we see for the reaction NaHCO₃ + D₂O an order of:

NaHCO₃ → NaH_nD_1-nCO₃ → NaDCO₃ according to the equations (1) and (2).

Results

NaDCO₃-rich CO₂ Inclusion

In a small number of quartz grains in the Bornholm pegmatite, we found a high concentration of NaHCO₃-rich CO₂ inclusions. The amount of nahcolite [NaHCO₃] in these inclusions is a high variable. Figures 7 and 8 show that variability, from about 0 to more than 40% (in rare cases up to 100%).

Figure 7: Complex NaHCO₃-Na(H, D)CO₃ inclusion in pegmatite quartz from Bornholm Island. V – CO₂-rich vapor phase.

Figure 8: NaHCO₃-rich CO₂ inclusion in pegmatite quartz. a) The NaHCO₃ inclusion is composed of pure nahcolite. b) a deuterium-rich nahcolite inclusion in quartz. The volume of this solid phase is about 40%.

¹³CO₂-rich Vapor Phase in the NaHCO₃-NaDCO₃-Rich Fluid Inclusions

Some NaHCO₃-NaDCO₃-rich CO₂ inclusions contain also ¹³CO₂– rich phases. According to Vitkin et al. 2021 [6], there is a significant difference between the Raman position of pure ¹²CO₂ and ¹³CO₂, with 1388 cm^-1 and 1370 cm^-1 (Raman mode ν1), respectively. From measurements at nine different inclusions, we obtained a mean of 1381.6 ± 1.44 cm^-1 and a FWHM = 14.8 ± 4.7 cm^-1 corresponding to 35.56 ± 8% ¹³CO₂. A natural reference with secondary CO₂ inclusion in quartz, taken at the same conditions, gave almost pure ¹²CO₂: 1387.94 ± 0.28 cm^-1 (n = 11 different inclusions). Using a mean value comparison at a 0.999 statistical certainty results in a significant difference. A different method is used by Remingi et al. 2023 [7]. The ¹³CO₂-rich fluid phase is the result of the interaction between supercritical fluid and ¹³C-rich diamond. Figure 9 shows an example of ¹³C rich diamond beside calcite in pegmatite quartz from Bornholm Island.

Figure 9: ¹³C-rich diamond in a near spherical calcite inclusion in pegmatite quartz from Bornholm Island. The figure shows the diamond grain (D) in calcite. In the calcite is a CO₂-rich fluid inclusion (Fl).

From 10 different measuring points on the diamond (see Figure 10), a mean of 1316.11 ± 2.5 cm^-1 and an FWHM = 60.54 ± 7.16 cm^-1, and according to Thomas et al. (2021) [6], this value corresponds to about 40% ¹³C, which is relatively high. In the ¹³CO₂-rich inclusions, we have often observed coronene [C₂₄H₁₂] (~1351 cm^-1, FWHM = 9.9). Coronene is a highly condensed six-ring polycyclic aromatic hydrocarbon. For the formation, high temperatures are necessary.

Figure 10: Raman spectrum of ¹³C-rich diamond in pegmatite quartz from Bornholm Island.

Discussion

The enrichment of D₂O can be explained by enrichment of D₂O by diffusion because the light water (H₂O) diffuses faster than the heavy water D₂O (Thomas and Davidson 2019) [8-11] in the supercritical fluid. The presence of ¹³C-rich diamond (Figure 10) shows clearly that a supercritical fluid has transported diamonds via supercritical fluid from mantle depths to the intrusion level of pegmatites. These findings force the idea that supercritical fluids are responsible for the formation of some pegmatites, as the author and coauthors have shown in many papers.

Acknowledgment

We dedicate this paper to Adolf Rericha from Falkensee/Germany for his insistent interest in supercritical fluids.

References

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