1 Thermogravimetric Analysis of the Copper Silicate Mineral Dioptase

Thermogravimetric analysis of the copper silicate mineral dioptase Cu6[Si6O18]·6H2O

Ray L. Frost, Yunfei Xi

Chemistry Discipline, Faculty of Science and Technology, Queensland University of Technology, GPO Box 2434, Brisbane Queensland 4001, Australia.

Abstract

There have been a few studies on the thermal decomposition of dioptase Cu6[Si6O18]·6H2O. The results of these analyses are somewhat conflicting and the conclusions vary among these thermo-analytical studies. The objective of this research is to report the thermal analysis of dioptase from different origins and to show the mechanism of decomposition. Thermal decomposition occurs over a very wide temperature range from around 400°C to 730°C with the loss of water. Two additional mass loss steps are observed at around 793 and 835°C with loss of oxygen.

The infrared spectra of dioptase in the hydroxyl stretching region enables the hydrogen bond distances of water molecules in the dioptase structure to be calculated. The large variation in the hydrogen bond distances offers an explanation as to why the decomposition of dioptase with loss of water occurs over such a wide temperature range.

Key words: dioptase, silicate minerals, thermogravimetric analysis, infrared spectroscopy

 Author to whom correspondence should be addressed ([email protected]) P +61 7 3138 2407 F: +61 7 3138 1804 1

Introduction

Dioptase [1, 2] is a cyclosilicate written with the formula Cu6[Si6O18]·6H2O emphasising the cyclosilicate structure. The formula of the mineral is also written as CuSiO3·H2O. However the first formula is the accepted IMA formula. The mineral is hexagonal with point group 3bar . The cell data shows the mineral to be space group R3bar with a =14.566, c = 7.778 and z= 18. According to Ribbe et al. [2] who state that the structure consists of puckered trigonal rings of 6 water molecules with an ice-like configuration sandwiched between similarly puckered trigonal rings of 6 silicate tetrahedra bonded laterally and vertically by Cu atoms. The Cu atom is coordinated by 4 O at 1.95-1.98 Å in nearly square-planar array, with 2 water molecules at 2.65 and 2.50 Å forming a tetragonally-distorted octahedron [3]. Dioptase is one of the few silicates to crystallize in the same symmetry class as dolomite and forms crystals that can have a typical carbonate rhombohedral shape.

There have been a few studies on the thermal decomposition of dioptase [4, 5]. The results of these analyses are somewhat conflicting and the conclusions vary among these thermoanalytical studies. Toussaint [5] determined the DTA of several copper silicate minerals including dioptase and suggested the mineral contained water only and not OH units. Rozinova and Muratov [6] determined the TG and DTA of a wide range of copper containing minerals and concluded that the product of the thermal decomposition was tenorite. Kiseleva et al. [4] determined the thermodynamic properties of dioptase and investigated the relationship with carbonate formation. Structural and thermoanalytical studies of dioptase have been progressed [7, 8]. Breuer and Eysel [7] analysed the structural and chemical varieties of dioptase. These authors found that the natural green dioptase transforms into a blue polymorph and is then dehydrated to a black compound of formula Cu6Si6O18. Goryainov [9] studied the changes in the infrared spectrum of dioptase as a function of temperature. The state of hydration of the mineral and changes in this state were determined.

Experimental

Minerals

The dioptase minerals were supplied by the Mineralogical Research Company. Three mineral samples were selected for study: (a) Tsumeb Mine, Tsumeb, Namibia (b) North West extension, Morenci Pit, Morenci, Greenlee County, Arizona (c) Okatumba, Kaokovelt Plateau, Namibia. Each of the mineral samples was used for the thermogravimetric analysis.

Thermogravimetric analysis

Thermal decomposition of chrysocolla was carried out in a TA® Instruments incorporated high-resolution thermogravimetric analyser (series Q500) in a flowing nitrogen atmosphere (80 cm3/min). Approximately 25 mg of sample was heated in an open platinum crucible at a rate of 5.0 °C/min up to 1000°C at high resolution. The TGA instrument was coupled to a Balzers (Pfeiffer) mass spectrometer for gas analysis. Only selected gases such as water and carbon dioxide were analysed.

Infrared spectroscopy

Infrared spectra were obtained using a Nicolet Nexus 870 FTIR spectrometer with a smart endurance single bounce diamond ATR cell. Spectra over the 4000525 cm-1 range were obtained by the co-addition of 128 scans with a resolution of 4 cm-1 and a mirror velocity of 0.6329 cm/s. Spectra were co-added to improve the signal to noise ratio.

Spectral manipulation such as baseline correction/adjustment and smoothing were performed using the Spectracalc software package GRAMS (Galactic Industries Corporation, NH, USA). Band component analysis was undertaken using the Jandel ‘Peakfit’ software package that enabled the type of fitting function to be selected and allows specific parameters to be fixed or varied accordingly. Band fitting was done using a Lorentzian-Gaussian cross-product function with the minimum number of component bands used for the fitting process. The Gaussian-Lorentzian ratio was maintained at values greater than 0.7 and fitting was undertaken until reproducible results were obtained with squared correlations of r2 greater

than 0.995.

Results and Discussion

The thermogravimetric analysis and derivative thermogravimetric analysis of the three dioptase minerals from (a) Tsumeb Mine, Namibia (b) North West extension, Morenci Pit, Arizona (c) Okatumba, Kaokovelt Plateau, Namibia, are displayed in Figs. 1 to 3. The thermogravimetric analyses of the three minerals are similar. In Fig. 1, there is a broad mass loss step over the 400 to 730°C with DTG peaks at 472.2 and 626.1°C. Two additional mass loss steps are observed at 793.7 and 835.7°C. It is proposed that the following reactions occur.

Over the 400 to 730°C temperature range, a mass loss of 11.3% is measured. This value may

be compared with the theoretical mass loss based upon the formula Cu6[Si6O18]·6H2O of 11.428%.

Cu6[Si6O18]·6H2O → Cu6[Si6O18] + 6H2O

At the higher temperatures, the two decomposition steps are associated with the loss of oxygen as is confirmed by evolved gas mass spectrometry. The first mass loss step is at 793.7°C and the second is at 835.7°C.

The following reaction is proposed: Cu6[Si6O18] → 6SiO2 + 6CuO + O2. This step is

attributed to the decomposition of the copper silicate CuSiO3. This result is in agreement with the work of Rozinova and Muratov [6] who showed the product of the reaction was tenorite (CuO).

The second mass loss occurs at 835.7°C and is attributed to the decomposition of CuO. This mass loss is attributed to oxygen evolution. The reaction proposed is as follows 2CuO →

Cu2O +1/2O2 . Torocheshnikov et al. [10] measured the thermal decomposition of copper oxide and determined the decomposition range to be over 825 to 950°C. Frost et al. [11] showed that the final decomposition steps of azurite and malachite with the loss of oxygen occurred at 840°C. Such a result is not in agreement with work of Kleber and Guertzsch [12] who concluded that the thermal decomposition of copper oxide with loss of oxygen

occurred at 660°C. Ding et al. 13 also reported the thermal activation of copper carbonates and showed that oxygen was evolved at 850°C.

Mass loss Mass loss Mass loss Mass loss

Step 1 Step 2 Step 3 Step 4

Namibia, 472.2°C 626.1°C 793.7°C 835.7°C Tsumeb 11.3% 1.2% 3.5%

Arizona 485.4 612.0 771.8 815.6

10.8% 0.8% 4.1%

Namibia, 489.9 614.5 779.2 812.0 Okatumba 11.1% 0.7% 4.8%

Table 1 Summary of the results of the three dioptase minerals

A summary of the thermal analysis of the three dioptase samples is given in Table 1. The thermal analysis of the dioptase mineral from Arizona shows a broad decomposition step with peaks at 485.4 and 612.0°C. The mass loss observed over the 400 to 730°C temperature range is 10.8% which may be compared with the calculated mass loss of 11.4%. The two higher temperature mass losses occur at 771.8 and 815.6°C. A broad mass loss for the dioptase from Okatumba occurs over the 400 to 730°C temperature range with a distinct sharp peak at 489.9°C and a broad almost indistinguishable peak at 614.5°C. The measured mass loss is 11.1% which compares favourably with the theoretical mass loss of 11.4%. Evolved gas mass spectrometry shows that water vapour is evolved over this temperature range. Two further mass loss steps are observed at 779.2 and 812.0°C with total mass loss of 4.8%, due to evolved oxygen.

The shape of the broad DTG peak 472 - 626 °C suggests that in fact it is an effect of the overlapping of two the peaks (about 470 and 612 °C). This means that dioptase dehydration has the two stage mechanism. The left side of the DTG peak has a specific steep shape

(Figs.1 and 2) or is narrow and high (Fig. 3). This indicates the very rapid expelling of water from the mineral structure. This suggests that the first step of dioptase dehydration is belongs to the rare but interesting so called “explosive dehydration processes “[14, 15]. Dehydroxylation and dehydration of some layer silicates and borates belongs to explosive dehydration processes. It takes place, when molecules of water, existing in a mineral and trapped in the structure or formed during the heating of OH groups, under the influence of the increasing temperature, generate water vapour pressure, able to destroy the mineral structure to go out. It can be accompanied by a complete disintegration of mineral grains and is used among others in borate minerals processing [14, 15].

The kinetics of explosive dehydration depends on the crystal structure perfection or crystallinity of the mineral. This may be observed in our DTG curves. The sample from Okatumba has sharpest shape peak and its temperature is the highest. This means that the mineral has the highest strength in terms of the structure. Any defects or imperfections weaken the structure of minerals such as dioptase. Moreover these imperfections are the mechanism of escape for water molecules. It indicates that crystals of Okatumba dioptase sample are the most perfect as compared with the other two. The sandwich-like building of the dioptase structure can favour this " popcorn -like " dehydration effect. Usually explosive dehydration is accompanied by the reconstruction of the parent structure [14, 15], leaving the possibility to next part of water loss, which may be observed as a second DTG peak.

The infrared spectra of the water stretching region of the three dioptase minerals are shown in Figures 4, 5 and 6. In each case the spectrum is broad with a harp peak at around 3363 cm-1. Infrared peaks for the Tsumeb sample are observed at 3035, 3214, 3290, 3363 and 3489 cm-1. These peaks are assigned to water stretching vibrations. The infrared spectrum of the dioptase from Arizona displays peaks at 2921, 3140, 3227, 3363 and 3486 cm-1. The infrared spectrum of the Okatumba dioptase sample shows bands at 2880, 3168, 3363, 3498 cm-1. Again these infrared peaks are attributed to water stretching vibrations.

Tsumeb dioptase Arizona dioptase Okatumba dioptase

Infrared band Calculated Infrared band Calculated Infrared band Calculated position Hydrogen position Hydrogen position Hydrogen

-1 bond -1 bond -1 bond /cm distance /Å /cm distance/Å /cm distance/Å

3489 2.8805 3486 2.8805 3498 2.8925

3363 2.7499 3363 2.7499 3363 2.7499

3290 2.7384 3291 2.7384 3283 2.7353

3214 2.7807 3227 2.7133 3209 2.7402

3108 2.6761 3140 2.6851 3168 2.6935

3035 2.6575 2921 2.6329 2880 2.6251

Table 2 Table of infrared band positions and the corresponding hydrogen bond distances

The infrared spectra of the three dioptase samples show a number of overlapping bands in the hydroxyl stretching region. Table 2 reports the band positions in the infrared spectra and the corresponding calculated hydrogen bond distance. Studies have shown a strong correlation between OH stretching frequencies and both O…O bond distances and H…O hydrogen bond distances [16-19]. Libowitzky (1999) showed that a regression function can be employed relating the hydroxyl stretching frequencies with regression coefficients better than 0.96 using infrared spectroscopy [20]. The function is described as: ν1 =

d (OO) (3592  304) 109 0.1321 cm-1. Thus OH---O hydrogen bond distances may be calculated using the Libowitzky empirical function. These hydrogen bond distances are reported in Table 2.

A number of conclusions may be made. Firstly there is a wide range of hydrogen bond distances. These hydrogen bond distances vary from 2.625 to 2.892Å. There is a certain consistency of the band positions of the three dioptase samples. This variation in hydrogen bond strength of the water in the dioptase structure accounts for the broad mass loss over the temperature range 400 to 730°C. Because there are water molecules in different molecular environments, the evolution of water occurs over such a wide temperature range. 7

Conclusions

Dioptase Cu6[Si6O18]·6H2O [1, 2] is a cyclosilicate sometimes written with the formula

CuSiO2(OH)2. According to Ribbe et al. [2] who state that the structure consists of puckered trigonal rings of 6 water molecules with an ice-like configuration sandwiched between similarly puckered trigonal rings of 6 silicate tetrahedra bonded laterally and vertically by Cu atoms. In this work we have measured and analysed the thermogravimetric patterns of three dioptase mineral samples. Three mass loss steps are found, the first with a loss of water and the latter two with loss of oxygen. The mass loss of water occurred over a very wide temperature range.

We propose that dioptase decomposes through a ‘explosive dehydration processes.’ The shape of the DTG curve suggests that we have two overlapping DTG peaks. The firs peak is the result of water exploding out of the crystal. Dehydroxylation and dehydration of some layer silicates and borates belongs to explosive dehydration processes. It takes place, when molecules of water, existing in a mineral and trapped in the structure or formed during the heating of OH groups, under the influence of the increasing temperature, generate water vapour pressure, able to destroy the mineral structure to go out. It can be accompanied by a complete disintegration of mineral grains and is used among others in borate minerals processing [14, 15].

Infrared spectra of dioptase were collected for the three minerals. The position of the OH stretching bands enabled the hydrogen bond distances of water in the dioptase structure to be calculated. A wide range of hydrogen bond distances were found. This range accounts for the broad thermal decomposition step with water loss.

Acknowledgments

The financial and infra-structure support of the Queensland University of Technology, Chemistry discipline is gratefully acknowledged. The Australian Research Council (ARC) is thanked for funding the instrumentation.

References

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(Cu6[Si6O18]·6H2O), American Mineralogist 1977; 62: 807-811. [3] Reddy, KM, Jacob, AS, Reddy, BJ, EPR and optical spectra of copper(2+) in dioptase, Ferroelectrics, Letters Section 1986; 6: 103-112. [4] Kiseleva, IA, Ogorodova, LP, Melchakova, LV, Bisengalieva, MR, Thermodynamic

properties of copper silicate: dioptase: Cu6Si6O18·6H2O, Journal of Chemical Thermodynamics 1993; 25: 621-630. [5] Toussaint, J, Thermal (differential-thermal analysis) study of the natural hydrated copper silicates, Annales de la Societe Geologique de Belgique 1957; 80: 287-295. [6] Rozinova, EL, Muratov, IG, DTA and differential thermogravimetric analysis diffraction patterns of some copper minerals, Zapiski Vsesoyuznogo Mineralogicheskogo Obshchestva 1976; 105: 232-236. [7] Breuer, KH, Eysel, W, Structural and chemical varieties of dioptase,

Cu6[Si6O18]·6H2O. Thermal properties, Zeitschrift fuer Kristallographie 1988; 184: 1-11. [8] Breuer, KH, Eysel, W, Mueller, R, Structural and chemical varieties of dioptase,

Cu6[Si6O18]·6H2O. II. Structural properties, Zeitschrift fuer Kristallographie 1989; 187: 15- 23.

[9] Goryainov, SV, Change of vibrational states of dioptase Cu6[Si6O18]·6H2O under dehydration, Zhurnal Strukturnoi Khimii 1996; 37: 68-74. [10] Torocheshnikov, NS, Ketov, AN, Mikulina, OG, Thermal decomposition of copper oxide in a nitrogen stream, Zhurnal Vsesoyuznogo Khimicheskogo Obshchestva im. D. I. Mendeleeva 1966; 11: 117-118. [11] Frost, RL, Ding, Z, Kloprogge, JT, Martens, WN, Thermal stability of azurite and malachite in relation to the formation of mediaeval glass and glazes, Thermochimica Acta 2002; 390: 133-144. [12] Kleber, W, Guertzsch, W, Thermal decomposition and topotaxy of azurite and malachite, Berichte der Deutschen Gesellschaft fuer Geologische Wissenschaften, Reihe B: Mineralogie und Lagerstaettenforschung 1966; 11: 305-315.

[13] Ding, Z, Frost, RL, Kloprogge, JT, Thermal activation of copper carbonate, Journal of Materials Science Letters 2002; 21:981-983. [14] Stoch, L, On a model of thermal internal decomposition of solids, Thermochimica Acta 1992; 203: 259-267. [15] Stoch, L, Internal structure rebuilding reactions of crystalline and amorphous solids, Journal of Thermal Analysis 1992; 38: 131-139. [16] Emsley, J, Very strong hydrogen bonding, Chemical Society Reviews 1980; 9: 91- 124. [17] Lutz, H, Hydroxide ions in condensed materials - correlation of spectroscopic and structural data., Structure and Bonding (Berlin, Germany) 1995; 82:85-103. [18] Mikenda, W, Stretching frequency versus bond distance correlation of O-D(H)...Y (Y = N, O, S, Se, Cl, Br, I) hydrogen bonds in solid hydrates, Journal of Molecular Structure 147 1986; 147: 1-15. [19] Novak, A, Hydrogen bonding in solids. Correlation of spectroscopic and crystallographic data, Structure and Bonding (Berlin) 1974; 18: 177-216. [20] Libowitsky, E, Correlation of the O-H stretching ferequencies and the O-H...H hydrogen bond lengths in minerals,Monatschefte fÜr chemie 1999; 130: 1047-1049.

List of Figures

Figure 1 Thermogravimetric and derivative thermogravimetry of dioptase from the Tsumeb Mine.

Figure 2 Thermogravimetric and derivative thermogravimetry of dioptase from the NW extension Arizona.

Figure 3 Thermogravimetric and derivative thermogravimetry of dioptase from the Okatumba, Namibia.

Figure 4 Infrared spectrum of the water stretching region of dioptase from the Tsumeb Mine.

Figure 5 Infrared spectrum of the water stretching region of dioptase from the NW extension Arizona.

Figure 6 Infrared spectrum of the water stretching region of dioptase from the Okatumba, Namibia

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6