1.1 Characteristic Bands in IR Spectra of Vesuvianite-Group Minerals

Vesuvianite-group minerals (VGM) are widespread and occur in different geological formations including regional metamorphic rocks, skarns, rodingites, etc. Specific crystal-chemical features of these minerals reflect conditions of their crystallization. As a rule, high-temperature VGM have high-symmetry structures (space group P4/nnc), whereas low-temperature samples are characterized by the symmetry P4/n or P4nc (Allen and Burnham 1992). The simplified crystal-chemical formula of VGM is X18(X´Y1)Y24Y38T0–5(SiO4)10(Si2O7)4O1-2W9 where X, X´ = Ca, Na, K, Fe2+, and REE (cations with coordination numbers from 7 to 9); Y1–Y3 = Al, Mg, Fe2+, Fe3+, Mn2+, Mn3+, Ti, Cr, Сu, Zn; T = B, Al, □; W = OH, F, O. The Y1 cations have tetragonal-pyramidal coordination, whereas the Y2 and Y3 cations occur in octahedra.

IR spectra of VGM are discussed in numerous publications (Paluszkiewicz and Żabiński 1992; Groat et al. 1995; Kurazhkovskaya and Borovikova 2003; Kurazhkovskaya et al. 2005; Borovikova and Kurazhkovskaya 2006); however, in most cases their interpretation is ambiguous. We have obtained IR spectra of 33 VGM samples from different kinds of localities which have been preliminarily investigated in detail using electron microprobe (including determination of boron), single-crystal X-ray structural analysis, DSC, 27Al NMR, ICP-MS, and Mössbauer spectroscopy. As a result, characteristic IR bands corresponding to different local situations in the structures of VGM have been revealed. Data on crystal structures, crystal chemistry, and IR spectra of these samples are published by Britvin et al. (2003), Panikorovskii et al. (2016a–d, 2017a–d), and Aksenov et al. (2016). The most important results of this investigation are listed below in comparison with data published elsewhere.

1.1.1 O–H-Stretching Vibrations

The following empirical correlations between O–H stretching frequencies in IR spectra of minerals and O···O and H···O distances (from structural data) were established by E. Libowitzky (1999):

$$ \nu\ \left({\mathrm{cm}}^{\hbox{--} 1}\right)=3592\hbox{--} 304\cdotp {10}^9\cdotp \exp \left[\hbox{--} d\left(\mathrm{O}\cdots \mathrm{O}\right)/0.1321\right] $$
(1.1)
$$ \nu\ \left({\mathrm{cm}}^{\hbox{--} 1}\right)=3632\hbox{--} 1.79\cdotp {10}^6\cdotp \exp \left[\hbox{--} d\left(\mathrm{H}\cdots \mathrm{O}\right)/0.2146\right] $$
(1.2)

Two decades ago this publication was of a great importance because it emphasized the existence of such correlations as a general trend. However, over time it became obvious that the Eqs. (1.1) and (1.2) are a very rough approximation and have a restricted applicability. First, it is to be noted that at high frequencies (above 3500 cm−1) substantial deviations from the correlations (1.1) and (1.2) are common because O–H stretching frequencies depend not only on O···O and H···O distances, but also on the nature of cations coordinating O–H groups and H2O molecules, as well as on the angle O–H···O, and the influence of these factors becomes most evident in case of weak hydrogen bonds. The Eqs. (1.1) and (1.2) predict that maximum possible values of O–H stretching frequencies for minerals are 3592 and 3632 cm−1 respectively. However, in many minerals including magnesium serpentines, brucite, kaolinite, amphiboles, etc. observed frequencies are much higher and even can exceed 3700 cm−1.

In the IR spectra of VGM some absorption bands of O–H stretching vibrations are poorly resolved. In such cases, band component analysis is the most important source of errors and artifacts during data processing because of low correctness of inverse mathematical problems: small errors in experimental data lead to strong uncertainty of the final result. Additional uncertainty is connected with arbitrary choice of the band shape (Gauss, Lorentz, Voigt, or Lorentz-Gauss cross-product function), the number of components, and the acceptable values of the correlation coefficient R (e.g., 0.99, 0.995, or 0.999). This matter is discussed in detail by Chukanov and Chervonnyi (2016) (the section 1.1 “Sources of Errors and Artifacts in IR Spectroscopy of Minerals”) where it is shown that different variants of band shape analysis may give a good and almost identical approximation accuracy (say, R2 ≈ 0.9995), but lead to totally different results.

For most VGM investigated by Chukanov et al. (2018) there are significant discrepancies between wavenumbers of observed O–H stretching bands and ν values calculated using correlations suggested by Libowitzky (1999). The above considerations explain why the attempts to apply Eqs. (1.1) and (1.2) to VGM failed.

Groat et al. (1995) distinguished 13 bands of O–H stretching vibrations in IR spectra of VGM, which have absorption maxima at the following wavenumbers (cm−1): 3670 (A), 3635 (B), 3596 (C), 3567 (D), 3524 (E), 3487 (F), 3430 (G), 3383 (H), 3240 (I), 3210 (J), 3156 (K), 3120 (L), and 3054 (M). The polarization of these bands with respect to the fourfold c axis is as follows: E∧c < 35° for the A–H bands, E⊥c for the I band, and E||c for the J–M bands (Groat et al. 1995; Bellatreccia et al. 2005). Consequently, the bands A–H and J–M can be assigned to the vibrations of differently coordinated O11–H1 and O10–H2 groups, respectively. The I band was tentatively assigned to O–H stretching vibrations of silanol group (Chukanov et al. 2018).

Our data show that actually significant deviations of the A–M band positions from the “ideal” values indicated by Groat et al. (1995) take place. In particular, IR spectra of many VGM samples contain a band in the range 3440–3470 cm−1, i.e., between F and G bands. Taking into account that in the group of 33 chemically and structurally investigated samples the intensity of this band shows distinct positive correlation with Ti content, it was assigned to vibrations of the O11–H1 group coordinated by Ti (Chukanov et al. 2018). Most Ti-rich samples are characterized by the space group P4/nnc. The only exception is a sample from the Ahkmatovskaya open pit, South Urals with 1.54 apfu Ti, space group P4/n showing bands at 3488 cm−1 (with a shoulder at 3460 cm−1) and 3424 cm−1 instead of a single band in the range 3440–3470 cm−1 (Fig. 1.1).

Fig. 1.1
figure 1

IR spectra of vesuvianite-group minerals with different contents of Ti in the region of O–H-stretching vibrations: a sample from Alchuri, Shigar Valley, Pakistan (Aksenov et al. 2016) with Ti2.21 (a); a sample from Hazlov, Karlovy Vary Region, Czech Republic with Ti0.48 (b); a sample from Myrseter area, Drammen, Buskerud, Norway with Ti0.00 (c); VGM from the Ahkmatovskaya open pit, South Urals, Russia with Ti1.54 (anomalous Ti-rich sample, space group P4/n) (d). Two vertical lines outline the region of the band corresponding to the Ti···O11–H1 group in P4/nnc VGM

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Vesuvianite from the Ahkmatovskaya open pit is the only Ti-bearing VGM having space group P4/n among 33 samples investigated by Chukanov et al. (2018). The observed splitting of the band of TiO–H stretching vibrations is the result of distribution of Ti between the sites Y3A and Y3B, whereas in high-temperature VGM having the space group P4/nnc Ti is accumulated in the single Y3 site.

IR band in the range 3375–3380 cm−1 which is close to the H band by Groat et al. (1995) was observed by us only for two samples with the symmetry P4/n and high contents of Cu and Mn3+. This band is more intense in the IR spectrum of the sample from the N’Chwaning III mine, Kuruman, South Africa with a relatively higher content of Mn3+ (1.83 apfu). Based on these data, the band in the range 3375–3380 cm−1 can be assigned to the Y3Mn3+···O11–H1 group.

The nominal position of the D band is 3567 cm−1 (Groat et al. 1995), but in IR spectra of some samples this band is shifted towards lower wavenumbers (up to 3560 cm−1). The D band is not observed in IR spectra of F-poor VGM and has the highest intensities in IR spectra of samples with most high contents of F (Britvin et al. 2003; Galuskin et al. 2003; Chukanov et al. 2018; see Fig. 1.2). Taking into account polarization E∧c < 35° (Groat et al. 1995), the D band is to be assigned to the group O11–H1 in the situation when F occupies neighboring O11 site.

Fig. 1.2
figure 2

IR spectra of vesuvianite-group minerals with different contents of F in the region of O–H-stretching vibrations: fluorvesuvianite holotype with 7.16 apfu F (a); VGM from Sakharyok massif, Keyvy Mts., Kola Peninsula with 3.06 apfu F (b); VGM from Gulshad, Kazakhstan with 0.24 apfu F (c); F-free alumovesuvianite holotype (d). Vertical line corresponds to the nominal position of the D band

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Galuskin et al. (2003) supposed that the J band corresponds to OH groups in the O10 site coordinating Fe in Y1 and forming hydrogen bond with F in the neighboring O10 site. However, this assumption was not confirmed by our investigations: IR spectra of most VGM, including F- and Fe-poor ones, contain distinct J band whose wavenumber varies from 3190 to 3225 cm−1. These values correspond to strong hydrogen bonds, which is hardly possible in cases when F is the H-bond acceptor.

The weak B band (in the range from 3628 to 3632 cm−1) is often observed in IR spectra of low-symmetry VGM. This band corresponds to very weak H-bonds formed by the groups O11–H1 with low values of the angle between O11–H1 and H1···O7 (see Lager et al. 1999).

1.1.2 B–O-Stretching Vibrations

In VGM boron can occupy sites with coordination numbers 3 or 4. IR spectra of most VGM samples contain shoulders in the range 1070–1170 cm−1 corresponding to stretching vibrations of [BO4] tetrahedra.

BO3 groups are connected with Y1O64O10 polyhedra via O10 oxygen atom to form the cluster T2Y1O7 (Fig. 1.3) where T2 = B and Y1 = Fe3+, Fe2+, Mn3+, Cu2+, Al, or Mg. As a result, four degrees of freedom corresponding to the bond lengths T2–O12 (×2), T2–O10, and Y1–O10 are involved in stretching vibrations of BO33−. This results in four nondegenerate modes and, consequently, the expected number of absorption bands in the region of stretching vibrations of BO3 groups (i.e., 1200–1570 cm−1) is equal to 4. However, in the IR spectrum of wiluite only three bands are observed in this region: the peaks at 1267 and 1373 cm−1 and the shoulder at 1415 cm−1 (Panikorovskii et al. 2017b; see Fig. 1.4). The fourth band corresponding to symmetric vibrations of BO33− is forbidden for a regular BO3 triangle and is weak in case of weak-distorted BO3 triangle. The latter case takes place in wiluite: the bond lengths T2–O10 and T2–O12 are 1.39–1.40 and 1.32 Å, respectively (Panikorovskii et al. 2017b). Weak absorption between 1267 and 1373 cm−1 may correspond to the symmetric stretching mode of BO33− groups (Fig. 1.4).

Fig. 1.3
figure 3

Local environment of the T2 and Y1 sites in vesuvianite-group minerals

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Fig. 1.4
figure 4

IR spectra of vesuvianite-group minerals in the region of B–O-stretching vibrations: wiluite from its type locality (a), B-rich VGM from Gulshad (b), a typical B-bearing vesuvianite from Somma-Vesuvius complex, Italy (c), and anomalous B-rich VGM from Titivskoe (d)

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Unlike wiluite, most boron-rich VGM contain significantly distorted BO3 triangle. For example, in a sample from Gulshad, Kazakhstan the bond lengths T2–O10 and T2–O12 are equal to 1.384 and 1.20 Å, respectively. As a result, four distinct IR bands (at 1557, 1467, 1419, and 1365 cm−1) are observed in the range 1200–1570 cm−1 (curve b in Fig. 1.4). As compared to wiluite, these bands are substantially shifted towards high frequencies because of shorter B–O bonds and shortened Y1–O bond (2.044 Å for the mineral from Gulshad and 2.15 Å for wiluite: Chukanov et al. 2018; Panikorovskii et al. 2017b). These differences may be due to different predominant cations in the Y1 site: Mg in wiluite and Fe3+ in the sample from Gulshad.

An anomalous IR spectrum with six absorption bands in the range 1200–1570 cm−1 shows boron-rich VGM from Titivskoe boron deposit, Yakutia, Russia (curve d in Fig. 1.4). Structural investigation of this sample (Panikorovskii et al. 2016a) showed the presence of domains with different symmetry (P4/nnc and P4/n).

1.1.3 Stretching and Bending Vibrations of SiO44− and Si2O76− Groups

Based on the available data on IR spectra of a restricted set of VGM Kurazhkovskaya and Borovikova (2003) concluded that for low-symmetry samples the band of Si–O-stretching vibrations in the range from 960 to 990 cm−1 is shifted on 10–15 cm−1 towards lower frequencies as compared to high-symmetry VGM. Our data confirm this conclusion only partly. Indeed, among nine samples with the space group P4/n, eight samples show strong IR bands in the range 962–968 cm−1, and in the IR spectrum of one more sample a band at 973 cm−1 is observed. Among 21 boron-poor samples with the space group P4/nnc, for 16 samples bands in the range 976–986 cm−1 are observed, but 5 samples show bands between 962 and 968 cm−1.

Another specific feature of low-symmetry VGM indicated by Kurazhkovskaya and Borovikova (2003), as well as by Borovikova and Kurazhkovskaya (2006) is the doublet ~575+615 cm−1 corresponding to O–Si–O bending vibrations. This regularity was confirmed by us as a general trend; however, among 21 boron-poor samples with the space group P4/nnc, 3 samples show doublets in the range ~575–615 cm−1 with components of approximately equal intensity.

1.2 Problem of Melanophlogite

Melanophlogite is a clathrate compound which contains guest molecules N2, CO2, and CH4 entrapped within the cages of the 3D framework built by SiO4 tetrahedra (Gies 1983; Nakagawa et al. 2001; Kolesov and Geiger 2003). The cubic unit cell of the structure of melanophlogite includes two [512] cages and six [51262] cages (Fig. 1.5). The former are considered to be occupied mainly by CH4 molecules and the latter by N2 and CO2 (Gies et al. 1982; Gies 1983). In melanophlogite from Mt. Hamilton, California, USA, the occupancy factor of the CH4 site in the [512] cage is about 90% (Gies 1983). The molecules of carbon dioxide located in the [51262] cages can rotate and are statistically distributed between 12 possible equivalent orientations. Most part of N2 and CO2 occurs in the [51262] cage, but minor part of these molecules can be present in the [512] cage (Gies 1983; Kolesov and Geiger 2003). Based on the available structural data, the general formula of melanophlogite can be written as follows: (CH4,N2,CO2)2–x(N2,CO2)6–y(Si46O92).

Fig. 1.5
figure 5

[512] cages (a) and [51262] cages (b) in the structure of melanophlogite

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The IR spectrum of melanophlogite from Fortullino, Livorno province, Tuscany, Italy (cubic, with a = 13.4051(13) Å, according to single-crystal X-ray diffraction data) contains a strong band at 2330–2336 cm−1 corresponding to antisymmetric vibrations of CO2 molecules (Chukanov and Chervonnyi 2016; see Fig. 1.6). A weaker peak at ~2375 cm−1 present it IR spectra of some melanophlogite samples from this locality (Chukanov 2014) may be due to rotational splitting or correspond to a minor amount of CO2 molecules in the [512] cages, which are predominantly occupied by CH4. Weak bands at 3700 and 3596 cm−1 (Fig. 1.7) are, respectively, due to asymmetric and symmetric stretching vibrations of H2O molecules that do not form hydrogen bonds. The band at 3316 cm−1 can be tentatively assigned to silanol groups or H-bonded H2O molecules.

Fig. 1.6
figure 6

Powder IR spectrum of melanophlogite from Fortullino, Italy

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Fig. 1.7
figure 7

IR spectrum of melanophlogite from Fortullino in the 1400–3800 cm−1 range. Very weak bands in the range from 2800 to 3000 cm−1 correspond to grease impurity

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Two CO2 modes at 1277 and 1378 cm−1 observed in the Raman spectrum of melanophlogite from Mt. Hamilton, California, USA correspond to the first overtone of the ν2-bending mode and the symmetric ν1-stretching mode, respectively, both bands being components of a vibrational system coupled via Fermi resonance (Kolesov and Geiger 2003).

Raman spectrum of melanophlogite from Mt. Hamilton has been investigated previously at 4 K (Kolesov and Geiger 2003). The bands at 2900 and 2909 cm−1 in the single-crystal Raman spectrum of melanophlogite from this locality have been assigned to asymmetric stretching modes of CH4 located in the [512] and [51262] cages, respectively. Along with the main band at 1378.5 cm−1 assigned to CO2 molecules in the [51262] cages, the shoulder at 1376 cm−1 was registered and was attributed to CO2 in the [512] cages. Kolesov and Geiger (2003) also reported the presence of the band of symmetric N≡N-stretching vibrations located at 2321 cm−1 which corresponds to N2 molecules. The IR forbidden band of N2 in binary mixtures with other molecules has been observed at about 2328 cm−1 (Bernstein and Sandford 1999). It is to be noted that this value is close to the wavenumber of antisymmetric vibrations of CO2 molecules, which is forbidden in Raman spectra. However, this band is not observed by us in Raman spectra of melanophlogite from different Italian localities (Figs. 1.8, 1.9 and 1.10).

Fig. 1.8
figure 8

Raman spectrum of melanophlogite from Fortullino

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Fig. 1.9
figure 9

Raman spectrum of melanophlogite from Racalmuto

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Fig. 1.10
figure 10

Raman spectrum of melanophlogite from Miniera Giona

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Raman spectrum of the sample from Fortullino (Fig. 1.8) exhibits strong bands of CO2 at 1277 and 1383 cm−1. Two weak bands at 1257 and 1398 cm−1 accompanying the components of the Fermi-doublet are the so-called hot bands arising from the transitions from an excited vibrational level to the ground vibrational level (Wang et al. 2011). In accordance with Frezzotti et al. (2012) and Wang et al. (2011), carbon dioxide in this mineral is in the high density fluid state with D ≈ 1 g/cm3. This is confirmed by the down shift of the Fermi doublet frequencies from 1388 and 1285 cm−1 (Δ = 103 cm−1) for CO2 at normal conditions to the values 1383 and 1277 cm−1 (Δ = 106 cm−1); the increased value of Δ in the spectrum of melanoflogite from Fortullino (Fig. 1.8) also corresponds to a high-density CO2 fluid. The very weak band at 2908 cm−1 which is attributed to CH4 and the band at 3611 cm−1 attributed to OH-groups are in a good agreement with the IR spectrum of this sample (Chukanov and Chervonnyi 2016). Thus, nitrogen, which is a substantial component in melanophlogite from Mt. Hamilton, is absent in the sample from Rio Fortullino.

Melanophlogite from Racalmuto, Sicily (Fig. 1.9) is characterized by a higher content of CH4 detected by the bands at 2901 and 3055 cm−1 and by a trace amount of C2H6 detected by the very weak bands at 998 and 2945 cm−1 (Kohlrausch 1943; Momma et al. 2011). The presence of H2S molecules is detected by the presence in the Raman spectrum of the band at 2595 cm−1. This sample does not contain N2 and CO2 molecules in detectable amounts. The sample of melanophlogite from Racalmuto contains a small amount of mineral impurities of calcite or aragonite (the band at 1087 cm−1: Edwards et al. 2005) and celestine (998 cm−1: Frezzotti et al. 2012). The weak band at 802 cm−1 and the bands at 363 and 276 cm−1 relate to the spectrum of the melanophlogite-type silicon-oxygen framework.

Melanophlogite from Miniera Giona, Sicily (Fig. 1.10) contains H2S molecules (the band of H–S-stretching vibrations at 2593 cm−1) and a low amount of CH4 molecules (2904 cm−1) whereas bands of N2 and CO2 are not observed in its Raman spectrum. This sample contains anhydrite admixture detected by the Raman bands at 1132 and 1004 cm−1 (Frezzotti et al. 2012). Broad peaks in Figs. 1.9 and 1.10 are due to fluorescence.

Powder IR spectra of melanophlogite from some other localities do not show any presence of methane molecules. In the frequency range from 2800 to 3000 cm−1, a sample from Chvaletice, Bohemia shows the presence of three overlapping, relatively broad bands indicative of the contamination by a polyatomic aliphatic hydrocarbon, most probably grease. Similar but much weaker bands are present in the IR spectrum of melanophlogite from Fortullino, Italy (Fig. 1.7), but no characteristic bands of methane are observed in this spectrum too.

The IR spectra of melanophlogite samples from Racalmuto and Miniera Giona (Fig. 1.11) show much lower contents of CO2 (the bands at 2337–2338 cm−1) and substantially higher contents of CH4 (the bands at 3005–3008 cm−1) as compared with the sample from Fortullino. Moreover, the bands in the range from 2850 to 2950 cm−1 in the IR spectrum of melanophlogite samples from Miniera Giona indicate the presence of hydrocarbons heavier than methane. IR spectrum of the sample from Racalmuto confirms the presence of H2S (Fig. 1.12).

Fig. 1.11
figure 11

Powder IR spectra of melanophlogite from Racalmuto (a) and Miniera Giona (b)

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Fig. 1.12
figure 12

IR spectrum of melanophlogite from Racalmuto in the region of S–H-stretching vibrations

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The band assignment and the distribution of different components between [512] and [51262] cages in melanophlogite samples from different localities are given in Table 1.1. These examples show that, in all probability, melanophlogite is not a single mineral species, but a mineral group including minerals with different combinations of small molecules (CO2, CH4, H2S, N2, H2O, C2H6) entrapped in the [512] and [51262] cages. In particular, the mineral from Fortullino may be the CO2-dominant analogue of melanophlogite, and in the samples from Racalmuto and Miniera Giona H2S may be a species-defining component.

Table 1.1 The assignment of IR and Raman bands and the distribution of different enclathrated components between [512] and [51262] cages for melanophlogite samples from different localities
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1.3 Problem of Nakauriite

Nakauriite was initially described as a new mineral with the general formula (Mn,Ni,Cu)8(SO4)4(CO3)(OH)6·48H2O (Suzuki et al. 1976). The mineral occurs in fissure-fillings in brucite-bearing serpentine at Nakauri, Aichi Prefecture, Japan, and is intimately intergrown with chrysotile. Most analytical data for nakauriite, including powder X-ray diffraction pattern, chemical composition and IR spectrum, have been obtained for a polymineral mixture, in which chrysotile is the main phase. The above formula does not conform to analytical data of Suzuki et al. (1976). Taking into account strong predominance of Cu over Mn and Ni, the simplified formula Cu8(SO4)4(CO3)(OH)6∙48H2O is given for nakauriite in the IMA list of minerals. Published IR spectrum of nakauriite from its type locality contains strong bands of admixed chrysotile at 1075, 950 and 613 cm−1 (Suzuki et al. 1976; see curve a in Fig. 1.13), but characteristic bands of sulfate anions are not observed.

Fig. 1.13
figure 13

IR spectra of nakauriite-bearing samples from Nakauri drawn using data from Suzuki et al. (1976) (a), and from Karkodin, South Urals, Russia (b) and IR spectrum of an almost pure nakauriite from Nakauri (c). Bands of admixed serpentine are marked by asterisk

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Later nakauriite was reported from several localities in Great Britain, USA, Austria, Italy, and Russia (Braithwaite and Pritchard 1983; Barnes 1986; Postl and Moser 1988; Palenzona and Martinelli 2007; Popov et al. 2016), as well as a secondary product in metallurgic slags. All available data indicate that nakauriite (1) does not contain sulfate anion and (2) usually forms fine intergrowths with magnesium serpentine-group minerals (see curve b in Fig. 1.13). In particular, electron microprobe analyses of nakauriite from Japan and from Nevada (Peacor et al. 1982) do not show substantial amounts of sulfur. Microchemical tests show only traces of SO42− (Braithwaite and Pritchard 1983).

We managed to obtain an almost pure fraction of nakauriite from its type locality. Its IR spectrum is given in Fig. 1.13 (curve c). Specific features of the mineral are unusually low frequency of the non-split band of C–O-stretching vibrations (1361 cm−1), as well as relatively low intensity and high width of the band of out-of-plane bending vibrations of CO32− groups at 864 cm−1. The bands at 1600 and 1650 cm−1 indicate the presence of H2O molecules. The same features are inherent in carbonate members of the hydrotalcite group (Chukanov 2014) whose structures contain brucite-like layers and interlayer CO32− anions and water molecules. Weak bands at 1066 and 1143 cm−1 may be due to the presence of trace amounts of SO42− anions.

Our electron microprobe analyses of nakauriite samples from Nakauri (Japan), Karkodin, (Russia) and Monte Ramazzo mine (Italy) show that this mineral is Mg-dominant. Only three metal cations have been found, namely Mg, Cu2+, and Ni2+. The atomic ratio Mg:Cu:Ni in a sample from Nakauri is 0.75:0.23:0.02. In another sample from Nakauri Mg:Cu:Ni = 0.80:0.19:0.01. In the sample from Karkodin Mg:Cu:Ni = 0.75:0.23:0.02. Nakauriite from Monte Ramazzo does not contain Ni in detectable amounts; Mg:Cu = 0.80:0.20. In all analyzed samples the content of sulfur is below its detection limit. The predominance of Mg over Cu in nakauriite from Karkodin was noted also by Popov et al. (2016).

Thermal data for nakauriite from Suzuki et al. (1976) are nearly consistent with the general simplified formula (Mg3Cu2+)(OH)6(CO3)·4H2O. Hypothetically, nakauriite can be an interstratification of brucite and copper carbonate modules. Powder X-ray diffraction data of nakauriite from South Urals kindly provided by I.V. Pekov are given in Table 1.2.

Table 1.2 Powder X-ray diffraction data of nakauriite from Karkodin, South Urals, Russia (MoKα radiation)
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Suzuki et al. (1976) reported the absence of the reflection near 7.8 Å in some samples from Nakauri. It was supposed that this reflection may be due to an impurity (Peacor et al. 1982; Braithwaite and Pritchard 1983). However, nakauriite from Karkodin shows a relatively strong reflection at 7.88 Å (Table 1.2). In our opinion, the reflections at 7.32 and 7.88 Å may correspond to nakauriite-type phases with different contents of interlayer water.

A further detailed investigation of nakauriite and revision of its chemical formula are required.

1.4 Relationship Between Nepskoeite and Shabynite

Nepskoeite was initially described as a new chloride-hydroxide mineral with the formula Mg4Cl(OH)7·6H2O (Apollonov 1998). However, IR spectrum of nepskoeite published by Apollonov (1998) contains very strong band at 1297 cm−1 corresponding to stretching vibrations of orthoborate groups, as well as distinct band at cm−1 that may correspond to bending vibrations of orthoborate groups (Fig. 1.14).

Fig. 1.14
figure 14

IR spectrum of nepskoeite drawn using data from Apollonov (1998)

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Our reinvestigation of nepskoeite from the type material confirmed the presence of boron in this mineral. In particular, IR spectrum contains strong bands at 1301 and 734 cm−1 corresponding to stretching and bending vibrations of BO33− anions, respectively (Chukanov 2014; see Fig. 1.15). Moreover, color reaction with quinalizarin shows a high content of boron in nepskoeite.

Fig. 1.15
figure 15

IR spectrum of nepskoeite drawn using data from Chukanov (2014)

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IR spectrum of nepskoeite shows similarity with that of shabynite Mg5(BO3)(Cl,OH)2(OH)5·4H2O (Fig. 1.16). In particular, the IR spectrum of shabynite contains strong bands at 1302 and 732 cm−1. However, substantial differences between these spectra are observed in the region of vibrations of H2O molecules above 1500 cm−1.

Fig. 1.16
figure 16

IR spectrum of shabynite drawn using data from Chukanov (2014)

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There is also close similarity between powder X-ray diffraction (PXRD) patterns of nepskoeite and shabynite (Table 1.3). However, nepskoeite shows strong reflections at 10.64 and 3.498 cm−1 that are absent in the PXRD pattern of shebynite. On the other hand, strongest reflections of different shabynite samples are observed at 9.62 and 3.191 cm−1. Refraction indices of nepskoeite (α = 1.532, β ≈ γ = 1.562) are somewhat lower than those of shabynite (α = 1.543, β = 1.571, γ = 1.577).

Table 1.3 Strongest lines (with I ≥ 10%) of the PXRD patterns of nepskoeite and shabynite
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Based on these data, nepskoeite can be tentatively considered as a hydrous orthoborate chloride, a high-hydrated analogue of shabynite. Both minerals need further investigations, first of all, determination of unit-cell dimensions and H2O content by means of direct methods.

1.5 Orthoborate Groups in Lead Carbonate Minerals

The CO32− and BO33− anions have the same trigonal planar configuration, but in most orthoborate and carbonate minerals these groups do not show appreciable isomorphous substitutions. The main causes of the absence of isomorphous substitutions between these groups are differences in their charges and sizes. In addition, BO33− groups are often significantly distorted (see, e.g., Kolitsch et al. 2012, as well as Sect. 1.1). However, some lead carbonate minerals are exceptions from this regularity. For example, IR spectra of some samples of molybdophyllite Pb8Mg9(Si10O28)(CO3)3(OH)8O2·H2O, which is nominally a boron-free mineral, show weak IR bands of B–O-stretching vibrations at 1170 and 1240 cm−1) (Fig. 1.17), whereas Raman spectrum of the structurally investigated molybdophyllite sample does not show bands of borate groups (Kolitsch et al. 2012).

Fig. 1.17
figure 17

IR spectrum of molybdophyllite from the Långban deposit, Bergslagen ore region, Filipstad district, Värmland, Sweden (Chukanov 2014, sample Sil247). The wavenumbers of BO33− groups are indicated

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The crystal structure of molybdophyllite (Kolitsch et al. 2012) does not contain boron-dominant sites and, consequently, BO33− anions occur in CO32−-dominant positions, unlike roymillerite Pb24Mg9(Si9AlO28)(SiO4)(BO3)(CO3)10(OH)14O4 and britvinite Pb15Mg9(Si10O28)(CO3)2(BO3)4(OH)12O2, in which orthoborate and carbonate groups are ordered in different sites (Yakubovich et al. 2008; Chukanov et al. 2017b; see Fig. 1.18).

Fig. 1.18
figure 18

IR spectra of (a) roymillerite and (b) britvinite from Långban, Värmland, Sweden. Bands of stretching vibrations of BO33− groups are indicated

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The IR spectrum of hydrocerussite Pb3(OH)2(CO3)2 from Långban (curve b in Fig. 1.19) contains weak bands at 1230 (shoulder), 742, and 470 cm−1. These bands may be assigned to stretching and bending vibrations of orthoborate anions partly substituting regular CO3 triangles in the hydrocerussite structure. Indeed, in the IR spectrum of fluoborite containing regular BO3 triangle strong bands of B–O-stretching and O–B–O bending vibrations are observed at 1241, 743, and 468 cm−1 (Chukanov 2014). It is important to note that bands of orthoborate groups are absent in the IR spectra of hydrocerussite from Merehead quarry, England (curve a in Fig. 1.19), some related minerals (curves c and d in Fig. 1.19), as well as synthetic analogues of hydrocerussite and plumbonacrite (Brooker et al. 1983). In all probability, the presence of borate groups in hydrocerussite from Långban is the result of high activity of boron that accompanied formation of this deposit where different borate minerals are common.

Fig. 1.19
figure 19

IR spectra of (a) hydrocerussite from Merehead quarry, (b) hydrocerussite from Långban, (c) hydrocerussite-related phase NaPb5(CO3)4(OH)3 from Lavrion, and (d) cerussite from Merehead quarry. Bands of BO33− groups are indicated

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The IR spectra of plumbonacrite Pb5O(OH)2(CO3)3 from Merehaad and Långban (Fig. 1.20) are similar and differ from the IR spectra of synthetic plumbonacrite analogue (Brooker et al. 1983) by additional bands of BO33− groups at 1270, 1221, 742, and 464–465 cm−1. Unlike hydrocerussite, a mineral with the only site of CO32− groups, in the crystal structure, plumbonacrite is characterized by five positions of carbonate groups (Krivovichev and Burns 2000). The IR spectrum of plumbonacrite contains two bands of asymmetric B–O-stretching vibrations (at 1270 and 1221 cm−1), which indicates the presence of BO33− groups in different sites.

Fig. 1.20
figure 20

IR spectra of (a) plumbonacrite from Merehead quarry, (b) plumbonacrite from Långban, (c) somersetite, and (d) mereheadite. Bands of stretching vibrations of BO33− groups are indicated

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The IR spectrum of somersetite (curve c in Fig. 1.20) is similar to those of plumbonacrite and hydrocerussite and contains bands of BO33− groups at 1217 and 738 cm−1. The crystal structure of somersetite consists of electroneutral [Pb3(OH)2(CO3)2] hydrocerussite block and electroneutral [Pb5O2(CO3)3] block with the structure derivative from plumbonacrite. The only band of B–O-stretching vibrations observed in the IR spectrum of somersetite indicates that all admixed BO33− groups belong to the hydrocerussite block.

The relatively weak bands at 480 and 1200 cm−1 in the IR spectrum of grootfonteinite Pb3O(CO3)2 indicate the presence of minor amounts of undistorted orthoborate groups, but these groups are absent in most samples of structurally related minerals and compounds (see Fig. 1.21).

Fig. 1.21
figure 21

IR spectra of (a) grootfonteinite holotype, (b) synthetic compound KPb2(CO3)2(OH) (Brooker et al. 1983), (c) synthetic compound NaPb2(CO3)2(OH) (Brooker et al. 1983), and (d) abellaite holotype (Ibáñez-Insa et al. 2017). Band of stretching vibrations of BO33− groups is indicated

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In hydrocerussite, plumbonacrite, somersetite, and grootfonteinite CO3/BO3 are undistorted. As a result, bands of admixed B–O-stretching vibrations of BO33− anions are observed in the narrow wavenumber range from 1200 to 1270 cm−1. In contrast to these minerals, in mereheadite Pb47O24(OH)13Cl25(BO3)2(CO3) BO3 triangles are significantly distorted with B–O distances varying from 1.23 to 1.32 Å (Krivovichev et al. 2009). This results in splitting of the band of B–O-stretching vibrations into several components (at 1141, 1183, and 1261 cm−1); in addition, broad band at 1324 cm−1 corresponding to a mixed mode involving B–O- and C–O-stretching vibrations appears (curve d in Fig. 1.20).