First Occurrence of Titanian Hydroxylclinohumite in Marble-Hosting Gem Spinel Deposits, Luc Yen, Vietnam

Abstract

In this paper, we report the very first occurrence of titanian hydroxylclinohumite in the marble-hosted gem spinel deposits of the Luc Yen district, northern Vietnam. Hydroxylclinohumite is anhedral and associated with forsterite, tremolite, pargasite, diopside, spinel, dolomite and calcite. Hydroxylclinohumite from the Luc Yen deposit was characterized via electron microprobe analysis, single-crystal X-ray diffraction study, and Raman spectrometry. The average composition is (Mg_0.69Ti_0.29Fe_0.02)_Σ1.00Mg_7.91(SiO₄)_4.08[(OH)_1.10F_0.53O_0.37]_Σ2.00. (ideally (Mg_0.7Ti_0.3)_Σ1Mg_8.0(SiO₄)₄[(OH)_1.2F_0.5O_0.3]₂). The compositions of the analyzed hydroxylclinohumites have a narrow range of Mg/(Mg+Fe+Ti) values (0.96–0.97) and a defined hydroxylclinohumite solid-solution series. Compared with other occurrences, the Luc Yen hydroxylclinohumite has an average titanium content, which attains 0.31 atoms per formula unit (3.93 wt.% TiO₂) and a low iron content of 0.04 atoms per formula unit (0.42 wt.% FeO). The formation of hydroxylclinohumite is favored by the proportion of Mg, and Si in the precursor rocks and the increased activity of H₂O in the fluid phase.

1. Introduction

In the recent literature, considerable attention has been paid to hydroxylclinohumite (ideally ${\mathrm{M}\mathrm{g}}_9{(\mathrm{S}\mathrm{i}\mathrm{O}}_4{)}_4\left[{\left(\mathrm{O}\mathrm{H}\right)}_2\right]$, the OH-dominant equivalent of clinohumite) as a possible site of water in the upper mantle of the Earth [1,2,3,4,5,6,7,8]. Before its discovery in the Zelentsovskaya mine near Magnitka (Zlatoust district, Southwestern Urals) and approval by the CNMNC MMA as an independent mineral species [9], hydroxylclinohumite has been reported for many decades as an accessory mineral in the rocks of ultrabasic composition including carbonatites, kimberlites, peridodites and serpentinites [1,2,10,11,12,13,14,15]. It should be noted that magnesian humite group of minerals found in carbonate rocks are reported as typically titanium-poor and fluorine-rich, whereas those in ultrabasic rocks are enriched in titanium and generally contain little or no fluorine [15]. In general, the composition of titanian hydroxylclinohumite can be represented by the formula ${{(\mathrm{M}}_{1-x}{\mathrm{T}\mathrm{i}}_x{)}_{\mathsf{\Sigma }1.0}\mathrm{M}}_8{(\mathrm{S}\mathrm{i}\mathrm{O}}_4{)}_4[{\left(\mathrm{O}\mathrm{H}\right)}_{2-2x}{\mathrm{O}}_{2x}{]}_{\mathsf{\Sigma }2.0},$ where, M is Mg, Fe²⁺, Mn and Ni, and x has values up to 0.5 [16,17,18].

Against the background of a renewed interest in magnesian minerals of the humite group, we present the results of a comprehensive study (chemical composition, Raman spectroscopy and X-ray data) of the first discovery of titanian hydroxylclinohumite in marbles from the Luc Yen gem spinel deposit, which is located in northern Vietnam in the Yen Bai province. Discovered at the end of the last century [19] and then singled out as an important source of spinel and corundum of jewelry quality [20,21,22], this deposit immediately attracted the close attention of researchers. The main issues discussed were related to the peculiarities of the chemical composition of spinel [21,23,24,25], specifically in order to identify the diagnostic signs that make it possible to distinguish Luc Yen spinel entering the jewelry market from the spinels of other deposits. Special studies concern the nature of the spinel coloration [19,26], assessment of the age of the host marbles [27], and the conditions of their formation [28,29,30,31,32,33]. Most attention has been paid to the geology and genesis of the spinel-containing marbles [24].

Clinohumites have long since been described as being associated with gem spinel in the Cong Troi deposit of Luc Yen [34]. Later, the major element of clinohumite and its stable isotope composition were described briefly in [35] and some petrological constrains were placed on the minimal temperature and fluid composition of a clinohumite-bearing assemblage. Afterwards, Hurai et al. [36] claimed to present the first detailed study of Luc Yen hydroxylclinohumite, but, unfortunately, it appeared not to be hydroxylclinohumite; this was because as a result of refining its crystal structure the formula corresponded to clinohumite ([36]: (Mg_8.95Ti_0.05)(SiO₄)₄[F_1.03(OH)_0.97]_Σ2.00, or, taking into account the presence of titanium (and seeking to preserve electroneutrality), we have: (Mg_8.95Ti_0.05)(SiO₄)₄[F_1.03(OH)_0.87O_0.10)]_Σ2.00. Consequently, the refined predominance of fluorine over hydroxide (F_1.03OH_0.87) in comparison with the results of electron microprobe analysis (≈ (OH)_1.05F_0.95) cannot be as a result of measurement inaccuracy, and the matrix effect of EPMA analysis of fluorine [37].

The aim of the current work is to present detailed information on the first occurrence of titanian hydroxylclinohumite in the Luc Yen (Vietnam) marble-hosted deposit of ruby and gem spinel (the first discovery of titanian hydroxylclinohumite in this genetic type of deposits).

2. Geological Setting and Petrography

The primary Luc Yen gem spinel deposit is situated in the Yen Bai province of northern Vietnam. This region comprises two structural zones separated by a fault: the Lo Gam metamorphic zone and the Day Nui Con Voi range [20]. The Lo Gam zone consists predominantly of marbles in which multicolored spinel is disseminated as lenses and “pockets” forming the famous Luc Yen gem deposit [20,38]. In addition to carbonates (calcite, and dolomite) and spinel, the marble units in the Luc Yen deposit contain phlogopite, pargasite, forsterite, clinohumite, preiswerkite [39], pyrite, corundum, dravite, pyrrhotite, and graphite.

According to our previous investigation, only some blue, lavender and purplish spinel is associated with humite minerals [33]. Type 4 blue spinel from Phan Thanh could be found in grey calcitic marbles with abundant graphite and straw-colored chondrodite–norbergite [22]. At the same time, type 1 lavender and purplish spinel from the Cong Troi deposit is often associated with ginger or orange-brown clinohumite, pargasite, and pyrite in dolomite–calcite marble (Figure 1).

Clinohumite is usually presented as orange-brown blebs or subhedral grains of 1–3 mm in size embedded in the dolomite–calcite matrix. Sometimes, it forms aggregates up to 1–3 cm, overgrowing spinel grains, filling its cavities and fissures (Figure 1). Parts of these clinohumites belong to hydroxylclinohumite. Clinohumite often contains the following minerals: calcite, forsterite, pargasite, pyrite, serpentine and rutile (Figure 2).

3. Materials and Methods

3.1. Microprobe Analysis

Major element compositions of hydroxylclinohumite from thin sections of samples were determined at the Electron Microprobe Laboratory in the Geomodel Center of St. Petersburg University using a JEOL-8200 Electron Microprobe. Natural standards were used for calibration. An operating voltage of 15 kV and a beam current of 20 nA were used with a beam diameter of 1 μm (5 µm spot size). Representative results of the electron microprobe analyses of the Luc Yen hydroxylclinohumite are listed in

Table 1. Results of electron microprobe analyses of hydroxylclinohumite from the Luc Yen deposit.

	V-11-2d	V-11-2a	V-12-03	$\overline{\mathit{X}}$	σ
SiO2 wt%	38.58	38.64	38.91	38.71	0.17
TiO2	3.33	3.90	3.93	3.72	0.34
MgO	54.79	54.57	54.63	54.66	0.11
* FeO	0.35	0.42	0.00	0.26	0.22
F	1.85	1.50	1.37	1.57	0.25
** H2O	1.47	1.60	1.61	1.56	0.08
Total	100.41	100.63	100.45	100.50	0.12
-O=F2	0.76	0.64	0.58	0.66	0.09
Total	99.65	99.99	99.87	99.84	0.17
*** Si apfu	4.07	4.07	4.10	4.08	0.02
Ti	0.27	0.30	0.31	0.29	0.02
Mg	8.63	8.59	8.59	8.60	0.02
Fe	0.03	0.04	0.00	0.02	0.02
OH	1.04	1.13	1.13	1.10	0.05
F	0.62	0.50	0.46	0.53	0.08
O	0.34	0.37	0.41	0.37	0.03
Mg#	0.97	0.96	0.96	0.96	0.01

* Total Fe as FeO; ** calculated from stoichiometry; *** atomic proportions based on 13 cations; Mg#—Mg/(Mg+Fe+Ti); $\overline{X}$—arithmetic mean; σ—standard deviation.

.

3.2. Single-Crystal X-ray Diffraction

Single-crystal X-ray diffraction study for samples V-11-2d, V-11-2a and V-12-03 was conducted via a XtaLAB Synergy-S diffractometer (Rigaku corp., Tokyo, Japan) equipped with a hybrid photon counting detector HyPix-6000HE using monochromatic radiation MoKα (λ = 0.71069Å) at the Centre of the Collective Use of Equipment, Kola Science Centre. More than half of the diffraction sphere was collected with scanning step 1°, and exposure time 0.5–1 s. The data were integrated and corrected via the CrysAlisPro [40] program package, which was also used to apply an empirical absorption correction using spherical harmonics, as implemented in the SCALE3 ABSPACK scaling algorithm. The SHELXL program [41] was used for the crystal structures’ refinement. The crystal structures were drawn using the VESTA 3 program [42]. The structures of clinohumite were refined in the traditional (non-standard) setting P2₁/b in order to emphasize the structural relationship to olivine [43]; R₁ = 0.027, 0.023 and 0.025 for the V-11-2d, V-11-2a and V-12-03 samples, respectively. Position of the hydrogen atoms was determined using difference Fourier maps. The SCXRD data are deposited in CCDC (https://www.ccdc.cam.ac.uk/, accessed on 23 April 2023) under entries No. 2257900- 2257902. Crystal data, data collection information and structure refinement details are given in

Table 2. Crystal data and structure refinement of hydroxylclinohumite from the Luc Yen deposit.

Identification Code	V-11-2d	V-11-2a	V-12-03
Temperature/K	293(2)
Crystal system	monoclinic
Space group	P21/b11
a/Å	4.7381(2)	4.74060(10)	4.7386(2)
b/Å	10.2423(4)	10.2431(3)	10.2416(3)
c/Å	13.6531(6)	13.6551(4)	13.6597(5)
α/°	100.917(4)	100.935(3)	100.905(3)
Volume/Å3	650.58(5)	651.03(3)	650.95(4)
Z	2
ρcalcg/cm3	3.199	3.197	3.200
μ/mm−1	1.118	1.122	1.135
F(000)	623.0	623.0	624.0
Crystal size/mm3	0.15 × 0.13 × 0.1	0.22 × 0.15 × 0.14	0.17 × 0.12 × 0.1
Radiation	Mo Kα (λ = 0.71073)
2Θ range for data collection/°	8.1 to 67.142	8.098 to 66.374	8.1 to 66.498
Index ranges	−7 ≤ h ≤ 6, −15 ≤ k ≤ 14, −19 ≤ l ≤ 18	−5 ≤ h ≤ 7, −14 ≤ k ≤ 12, −18 ≤ l ≤ 20	−7 ≤ h ≤ 7, −14 ≤ k ≤ 15, −15 ≤ l ≤ 19
Reflections collected	7228	7179	7001
Independent reflections	2188 [Rint = 0.0308, Rsigma = 0.0301]	2158 [Rint = 0.0230, Rsigma = 0.0236]	2145 [Rint = 0.0247, Rsigma = 0.0257]
Data/restraints/parameters	2188/3/147	2158/3/147	2145/3/147
Goodness-of-fit on F2	1.096	1.060	1.101
Final R indexes [I >= 2σ (I)]	R1 = 0.0266, wR2 = 0.0689	R1 = 0.0230, wR2 = 0.0570	R1 = 0.0249, wR2 = 0.0721
Final R indexes [all data]	R1 = 0.0313, wR2 = 0.0719	R1 = 0.0269, wR2 = 0.0594	R1 = 0.0296, wR2 = 0.0752
Largest diff. peak/hole/e Å−3	0.51/−0.49	0.54/−0.45	0.48/−0.45

; atom coordinates and selected interatomic distances are in Supplementary Tables S1–S6.

3.3. Raman Spectrometry

The Raman spectrum of hydroxylclinohumite was recorded via Horiba Jobin–Yvon LabRam HR800 spectrometer using solid-state laser with λ = 532 nm (power on the sample 15 mW) and 100x objective. The sample was oriented randomly and measured at room temperature. The data were obtained in the range of 70–4000 cm⁻¹ and 2 cm⁻¹ spectral resolution. The calibration was carried out using Si standard (520.7 cm⁻¹).

4. Results

4.1. Major Element Mineral Chemistry

The SiO₂, TiO₂, MgO, FeO and F contents of the Luc Yen hydroxylclinohumite are similar to those reported in previous studies. The microprobe analysis yielded the averaged structural formula (Mg_0.69Ti_0.29Fe_0.02)_Σ1.00Mg_7.90(SiO₄)_Σ4.09[(OH)_1.10F_0.53O_0.37]_Σ2.00, which is close to the ideal formula, (Mg_0.69Ti_0.29Fe_0.02)_Σ1.00Mg_8.00(SiO₄)_Σ4.00[(OH)_0.92F_0.50O_0.58]_Σ2.00. The Mg# is lower in the Luc Yen hydroxylclinohumite than in others reported to date. The characteristic geochemical features of our hydroxylclinohumite are the high content of titanium, which attains 0.31 atoms per formula unit (3.94 wt.% TiO₂) and a low iron of 0.04 atoms per formula unit (0.41 wt.% FeO). The contents of Cl, Ca, and Mn are below the detection limits. The increased silicon content may be associated with microinclusions of humites with a ratio of forsterite and brucite layers greater than four [44,45].

4.2. Structure Description and Refinement

Both composition and crystal structure of humite group minerals are close to olivine. Their crystal structure is based on isolated silica-oxygen tetrahedrons [SiO₄], connected with each other through Mg atoms. Forsterite layers (Mg₂SiO₄) are interspersed with Mg(OH,F)₂ layers; their ratio progressively changes from norbergite (1/1) to clinohumite (4/1). In general, the crystal structure of clinohumite is based on a hexagonal close-packed array (2H) of anions similar to that of forsterite (Figure 3a) [46]. As in the olivine structure type, in the structures of all humite minerals, 1/2 of the octahedral voids are occupied by the M cations while the occupancy of the available tetrahedral voids decreases from 1/8 in olivine over 1/9 in clinohumite (n = 4) to 1/12 in norbergite (n = 1) [47]. This structure can be also described as a heteropolyhedral framework consisting of stacking of identical sheets parallel to the (001) plane [48]. In turn, the sheets based on chains of edge-sharing Mg1-5 octahedra are connected by vertex-shared Si1-2 tetrahedra. The F ions and OH groups are located in the small cavities on the “missing” (compared with forsterite structure) SiO₄ tetrahedra (Figure 3b).

The crystal structure of hydroxylclinohumite contains five independent Mg sites. During refinement, the Mg5 site in all samples has negative values of the tensors and the corresponding atom described as non-positively defined (NPD) with numerous peaks of excess electron density 1 e^– within 0.5 Å distance around the Mg5 site. The final refinements demonstrate an admixture of Ti at Mg5 site and the refined occupancies were (Mg_0.82Ti_0.18), (Mg_0.80Ti_0.20) and (Mg_0.78Ti_0.22) for samples V-11-2d, V-11-2a and V-12-03, respectively. Other Mg1-4 sites were refined as fully populated by Mg atoms.

The Si1-2 atoms has mean <Si–O> distances in the range 1.625–1.640 Å and populated by Si atoms only. The O9 atom in all structures has numerous peaks of electron density (~0.50 e^–) within 0.5 Å distance around site. According to previous data, the O9 site is populated by F in clinohumite [43], so the O9 site was refined with mixed occupancy of F. The final refinement occupancies for O9 site are (O_0.63F_0.37), (O_0.665F_0.335) and (O_0.68F_0.32) for samples V-11-2d, V-11-2a and V-12-03, respectively. Other O sites populated by O atoms only.

Position of the H atoms were found using Fourier difference synthesis (F_o-F_calc) around O9(F) site (Figure 4). Initially, two residual density ~0.30 (H10) and 0.50 e^– (H9) peaks within 0.90–1.00 Å distance from O9 site for the samples V-11-2d and V-11-2a, respectively, were found. Both sites were refined as H atoms; however Mg-H10 distance was 1.85–1.95 Å, anomalously short to be real atom and excluded from the next refinement steps. For the V-12-O3 sample only one peak of residual density near O9 site was observed. This peak corresponds to the position of H9 atoms. The occupancy of the H9 sites was fixed according the charge balance requirements.

The refined crystal chemical formula for the hydroxylclinohumite samples can be written as (Mg_8.82Ti_0.18)_9.00(SiO₄)₄[(OH)_0.90F_0.74O_0.36]_2.00 for the V-11-2d sample, (Mg_8.80Ti_0.20)_9.00(SiO₄)₄[(OH)_0.93F_0.67O_0.40]_2.00 for the V-11-2a sample and (Mg_8.78Ti_0.22)_9.00(SiO₄)₄[OH_0.92F_0.64O_0.44]_2.00 for the V-12-03 sample, which agrees well with the empirical chemical data, considering close proximity between site-scattering powers of Ti and Fe. The discrepancy between refined predominance of hydroxide over fluorine compared to the electron microprobe analyses could result from measurement inaccuracy, matrix effect on EPMA analyses of fluorine [37], and/or minor chemical zonation. Considering all of these factors, the agreement between the structural and EPMA data is good. Additionally, the discrepancy between refined titanium content compared to the empirical formulae could result from the possibility of small Ti content (and Fe) could be distributed among all five MgO6 octahedra, but this cannot be supported by the structure refinement.

According to the SC XRD and powder neutron diffraction data, hydroxylclinohumite may contain two independent hydrogen sites [49,50]. In nature, hydroxylclinohumite which does not contain F and cations in Mg position is rare; more often Ti or Fe³⁺-containing species are found [51]. Incorporation of Ti⁴⁺ changes the positive cationic charge together with F admixture realized in the crystal structure of hydroxylclinohumite from the Luc Yen deposit, where OH do not exceed 1 apfu. According to our scheme of hydrogen bonds in hydroxylclinohumite (Figure 5), the $d_{\mathrm{O}\dots \mathrm{H}}$ distances is in the range 2.007–2.034 Å. The estimated peaks according to Libowitzky correlation should be in the range 3510–3530 cm⁻¹ [52].

4.3. Raman Spectroscopy

The Raman spectrum of hydroxylclinohumite from the Luc Yen deposit has the closest resemblance to the spectra of pure-Mg synthetic hydroxylclinohumite [53,54] and spectrum from the Luc Yen [36]. Spectra from [36] are presented with single crystal in two orientations. They illustrate strong polarization effect on the bands intensity. Our sample was randomly oriented and our spectrum is something average in orientation. It is also very similar to a natural hydroxylclinohumite from a marble-paragneiss unit of the Central Dabie medium-T/UHP eclogite-facies zone from the Ganjialing area, China [55] except some additional bands at 600–700 which Luc Yen hydroxylclinohumite lack probably due to very low Fe content.

Peaks positions (Figure 6a) are assigned according to Raman spectra reported in previous studies [53,54,55]): (1) the most intense bands 805, 827, 843, and 859 cm⁻¹ are corresponding to the stretching vibrations (ν₁) of the SiO₄ tetrahedra; (2) weak bands at 910, 929 and 964 cm⁻¹ are attributed to asymmetric stretching vibrations (ν₃) of the SiO₄ tetrahedra; (3) the bands of 744, 782 cm⁻¹ are ascribed to MgOH and TiOH deformations; (4) the bands at 550, 587, and 610 are corresponding to the out of plane (ν₄) bending modes of the SiO₄; (5) band at 430 cm⁻¹ is in plane (ν₂) bending mode of the SiO₄, and (6) the bands at 180, 230, 265, and 330 are due to the vibrations of the MgO₆ octahedra and the lattice vibrations. The most important peaks for hydroxylclinohumite in our spectrum are the ones which occur as two doublets at about 3389, 3406 cm⁻¹, 3556, and 3567 cm⁻¹ (Figure 6b) These bands in the OH-stretching region for the hydroxylclinohumite sample agree with those reported by Hurai et al. [36] who presented doublet 3339, and 3410 cm⁻¹ and triplet 3560, 3570, and 3578 cm⁻¹ and Liu et al. [53], who observed three strong bands at 3397, 3529 and 3564 cm⁻¹. The most intense band of stretching vibrations O–H at 3406 cm⁻¹ corresponds to strong hydrogen bonds OH···O, which are formed between OH groups being at a relatively short distance from each other. The faint band at 3556 cm⁻¹ corresponds to the local situation, when the O(5) position, being an acceptor of the hydrogen bond, is occupied by F [56].

5. Discussion

The rarity of hydroxylclinohumite in nature is mainly due to the behavior of CO₂, H₂O and F in geochemical processes, the chemical composition of rocks and the physicochemical parameters of the mineral-forming medium.

The main minerals that characterize the most common associations with hydroxylclinohumite are dolomite, calcite, pargasite, forsterite, spinel, and corundum. As it was noted [57,58,59], formation of humite group minerals is facilitated by the presence of an aqueous fluid in the system. Therefore, petrogenesis of hydroxylclinohumite-bearing marble is considered further from the point of view of a metasomatic process, even if it was implemented locally.

As the model, we choose the Na–Mg–Al–Si–H₂O–CO₂ system, in terms of which, the most common original rocks of the protolith (evaporites, and terrigenous sediments) and the secondary minerals developed after them can be described satisfactorily. As it is known [58,59,60], the analysis of phase equilibria in the presence of a mixed H₂O–CO₂ fluid contributes to understanding the conditions of metamorphic transformations of siliceous dolomites. Let us consider the connection of chemical potentials of CO₂ and H₂O in a binary fluid (CO₂ + H₂O) at ${p_{{\mathrm{CO}}_2}+p}_{{\mathrm{H}}_2\mathrm{O}}=p_{\mathrm{t}\mathrm{o}\mathrm{t}\mathrm{a}\mathrm{l}}=Const$ and $T=Const$. Under these conditions, the values of the chemical potential of H₂O in the fluid (${\mu }_{{\mathrm{H}}_2\mathrm{O}}$) are determined, as is known by the expression [39]:

$${\mu }_{{\mathrm{H}}_2\mathrm{O}}={\mu }_{{\mathrm{H}}_2\mathrm{O}}^0+RT\ln f_{{\mathrm{H}}_2\mathrm{O}}={\mu }_{{\mathrm{H}}_2\mathrm{O}}^0+RT\ln {\gamma }_{{\mathrm{H}}_2\mathrm{O}}{p}_{{\mathrm{H}}_2\mathrm{O}}={\mu }_{{\mathrm{H}}_2\mathrm{O}}^0+RT\ln {\gamma }_{{\mathrm{H}}_2\mathrm{O}}{(1-x}_{{\mathrm{CO}}_2})p_{\mathrm{t}\mathrm{o}\mathrm{t}\mathrm{a}\mathrm{l}}$$

where, ${\mu }_{{\mathrm{H}}_2\mathrm{O}}^0$ is the standard chemical potential of water at a given temperature and pressure; $f_{{\mathrm{H}}_2\mathrm{O}},{\gamma }_{{\mathrm{H}}_2\mathrm{O}},p_{{\mathrm{H}}_2\mathrm{O}}$ is fugacity, fugacity coefficient and partial pressure of H₂O in the fluid, respectively; and $x_{{\mathrm{CO}}_2}$ is the molar fraction of CO₂ in the fluid.

To ascertain the main parameters of the mineral-forming medium that determine the stability of minerals in marble, consider the topology of diagrams constructed in the coordinates of chemical potentials [61]. The analysis of the model system is based on the fundamental principles of Korzhinsky [61,62]: there is a local equilibrium among minerals, all components have differential mobility, and incoming fluids react with protolith. This approach has been successfully used to model the chemical transformation of metasomatic rocks [61].

The specific version of mineral associations with hydroxylclinohumite which can be formed in the system Na–Ca–Mg–Al–Si–H₂O–CO₂ depends on the chemical and mineral composition of the protolith.

In plotting the qualitative diagram (${\mu }_{{\mathrm{CO}}_2}-{\mu }_{{\mathrm{H}}_2\mathrm{O}}$) of the Na–Mg–Al–Si–CO₂–H₂O system for the transformations of forsterite and/or pargasite into hydroxylclinohumite, it is assumed that the pargasite of the Luc Yen deposit determines the main features of the chemical composition of amphibole in aluminum-rich pods in marble. Pargasite was found here in associations with dolomite, spinel, corundum, and hydroxylclinohumite. The mineral composition of the rocks was determined by the proportions of the Mg, Al and Si oxides, and the availability of sodium in the system determined by the presence of evaporites in the protolith [31,63] which leads to the formation of pargasite. Titanium oxide, which is present in the rocks in the form of ilmenite, titanite and rutile, can be considered a separate component. Lumping FeO and MgO together, we obtain three virtually inert components: (Mg, Fe)O, Al₂O₃, and SiO₂. CO₂ and H₂O are the main components producing the metamorphic transformation of the original rocks.

Thus, the number of phases in the model system is assumed to be six, and the number of inert components is three, leaving two completely mobile components. The temperature and pressure were taken as constant external equilibrium factors. The mineral equilibrium in such a system can be graphically depicted in a two-dimensional space. Therefore, we analyze in detail the behavior of the monovariant equilibrium reaction lines on the diagram in the coordinates of two fully mobile components (${\mu }_{{\mathrm{CO}}_2},{\mu }_{{\mathrm{H}}_2\mathrm{O}}$).

The equations of the chemical reactions were calculated for the normative compositions of the minerals, the chemical composition of which is close to that of their natural analogs (

Table 3. Crystallochemical formulas of minerals (end members of solid-solution) adopted for calculating chemical reaction equations.

Mineral	Symbol	Formula	Mg	Al	Si
Hydroxylclinohumite	Hchu	Mg9(SiO4)4(OH)2	9	0	4
Pargasite	Prg	NaCa2(Mg4Al)Σ5(Al2Si6O22)(OH)2	4	3	6
Forsterite	Fo	Mg2SiO4	2	0	1
Dolomite	Dol	CaMg(CO3)2	1	0	0
Spinel	Spl	MgAl2O4	1	2	0
Corundum	Crn	Al2O3	0	2	0

). The equations are given in

Table 4. Equations of chemical reactions occurring on lines of monovariant equilibrium.

N	Symbols *	Chemical Reactions
1	[PrgDol]	4Fo + Spl + H2O = Hchu + Crn
2	[PrgHchu]	Dol + Crn = Spl + (Cal) + CO2
3	[PrgFo]	Dol + Crn = Spl + (Cal) + CO2
4	[PrgSpl]	Dol + 4Fo + H2O = Hchu + (Cal) + CO2
5	[PrgCrn]	Dol + 4Fo + H2O = Hchu + (Cal) + CO2
6	[FoDol]	3Hchu + 22Crn + 4(Cal) + 2Na+ = 2Prg + 19Spl + 4CO2 + 2H+
7	[FoHchU]	Dol + Crn = Spl + (Cal) + CO2
8	[FoSpl]	19Dol + 2Prg + 2H+ = 3Hchu + 3Crn + 23(Cal) + 15CO2 + 2Na+
9	[FoCrn]	22Dol + 2Prg + 2H+ = 3Hchu + 3Spl + 26(Cal) + 18CO2 + 2Na+
10	[HchuDol]	2Prg + 16Spl + 4CO2 + 2H+= 12Fo + 19Crn + 4(Cal) + 3H2O + 2Na+
11	[HchuSpl]	2Prg + 16Dol + 2H+ = 12Fo + 3Crn + 20(Cal) + 3H2O + 12CO2 + 2Na+
12	[HchuCrn]	2Prg + 19Dol + 2H+= 12Fo + 3Spl + 23(Cal) + 2H2O + 15CO2 + 2Na+
13	[SplDol]	76Fo + 3Crn + 4(Cal) + 19H2O + 2Na+ = 16Hchu + 2Prg + 2H+ + 4CO2
14	[SplCrn]	Hchu + (Cal) + CO2 = 4Fo + Dol + 2H2O
15	[DolCrn]	88Fo + 3Spl + 4(Cal) + 22H2O + 2Na+ = 19Hchu + 2Prg + 4CO2 + 2H+

* Reactions are indicated by symbols of minerals not involved in the reaction: ([PrgDol] = ”Prg and Dol”- absent in the reaction).

. Figure 7 presents this diagram in its entirety. It should be noted that three reactions 7, 9, and 12 (Table 4) are omitted in Figure 7 because nonvariant points [Hchu], [Crn] and [Fo] are metastable.

The formation of hydroxylclinohumite in this system is controlled by three reactions (5, 4, and 14, blue solid line on Figure 7), by which it is completely destroyed when ${\mu }_{{\mathrm{H}}_2\mathrm{O}}$ decreases. Monovariant singular equilibria of the reactions 1–3 (Table 4) are controlled only by the chemical potential of CO₂ in the fluid and do not depend on the activity of water in the medium. Thus, in case of increasing ${\mu }_{{\mathrm{CO}}_2}$ (and $x_{{\mathrm{CO}}_2})$, spinel is replaced by an association of corundum+dolomite according to reactions 2 and 3 (red solid line on Figure 7). This monovariant line divides the diagram into two parts (Figure 7). In the upper part, only corundum is stable in association with forsterite, pargasite and dolomite (fields I, and II), and with increasing water activity, corundum associated with hydroxylclinohumite which replace forsterite and pargasite (fields III, VII, and VIII).

It follows from the diagram that at low activities of CO₂ and H₂O (below red solid line), spinel is stable with forsterite in excess of dolomite (field IV) and with an increase in aluminum content corundum+ spinel associations are possible. With increasing ${\mu }_{{\mathrm{H}}_2\mathrm{O}}$, hydroxylclinohumite is stable in association with spinel and corundum (fields V, VI, IX, X, and XI) at low activities of CO₂ (below red solid line).

Thus, the two-dimensional diagram clearly illustrates the stability of the main minerals in marble (dolomite±calcite in all parageneses) depending on the chemical potentials of H₂O and CO₂ in the mineral-forming medium.

The rarity of hydroxylclinohumite in marbles seems to be due to unusual chemical compositions of protolith and fluid: clinogumite–fluid partition coefficient’s of fluorine are always greater than unity, with average coefficients of $D_{\mathrm{F}}^{\mathrm{c}\mathrm{h}\mathrm{u}/\mathrm{f}\mathrm{l}\mathrm{u}\mathrm{i}\mathrm{d}}\approx 3$ [8].

The fact that hydroxylclinohumite overgrows partially broken and fractured spinel grains indicates its crystallization after the gem spinel formation probably due to dehydration of serpentine or breakdown of forsterite [15].

Experiments have proven clinohumite’s stability ranging between ~700° and 1100 °C and P between 29 and 77 kb [64], however, the presence of Ti and F makes it stable under certain P-T condition in the mantle. These temperatures are consistent with previous estimates of about 700 °C minimum temperature for the formation of the Luc Yen spinel [65].

6. Conclusions

Hydroxylclinohumite is known as an accessory phase in a wide variety of ultrabasic and carbonate rocks of high-pressure origin. The first occurrence of titanian hydroxylclinohumite in pods of gem spinel hosted marbles of the Luc Yen deposit, northern Vietnam is reported. In marbles, besides calcite and dolomite, it also associates with spinel, forsterite and pargasite. The article presents the complete chemical, structural and spectroscopic characteristics of hydroxylclinohumite.

The average composition of hydroxylclinohumite of the Luc Yen deposit is (Mg_0.69Ti_0.29Fe_0.02)_Σ1.00Mg_7.91(SiO₄)_4.08[(OH)_1.10F_0.53O_0.37]_Σ2.00. A characteristic geochemical feature of the studied hydroxylclinohumite is a reduced iron content, which reaches 0.04 atoms per formula unit (0.42 wt.% FeO).

In addition to data on element substitutions and the crystal structure, we describe the conditions for the formation of hydroxylclinohumite in the deposit. It follows from the clinogumite–fluid partition coefficient’s of fluorine ($D_{\mathrm{F}}^{\mathrm{c}\mathrm{h}\mathrm{u}/\mathrm{f}\mathrm{l}\mathrm{u}\mathrm{i}\mathrm{d}}\approx 3$) that the formation of hydroxylclinohumite requires that the activity (concentration) of H₂O in solution is approximately three times higher than the activity of fluorine.

Additionally, we incline to the conclusion that such middle-T/HP metamorphism favors crystallization of hydroxylclinohumite in marbles together or after with gem spinel formation in a subducting slab.