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Thermogravimetric analysis, PXRD, EDX and XPS study of

(Cu,Al)2H2Si2O5(OH)4·nH2O -structural implications

 Ray L. Frost, Yunfei Xi and Barry J. Wood*

School of Chemistry, Physics and Mechanical Engineering, Science and Engineering Faculty, Queensland University of Technology, GPO Box 2434, Brisbane Queensland 4001, Australia.

*Centre for Microscopy and Microanalysis (CMM),The University of Queensland, St. Lucia, QLD 4072, Australia

Abstract

Selected chrysocolla samples from different origins have been studied by using PXRD, SEM, EDX and XPS. The XRD patterns show that the chrysocolla mineral samples are non-diffracting and no other phases are present in the , thus showing the chrysocolla samples are pure. SEM analyses show the chrysocolla surfaces are featureless. EDX analyses enable the formulae of the chrysocolla samples to be calculated.

The thermal decomposition of the mineral chrysocolla has been studied using a combination of thermogravimetric analysis and derivative thermogravimetric analysis. Five thermal decomposition mass loss steps are observed for the chrysocolla from Arizona (a) at 125°C with the loss of water (b) at 340°C with the loss of hydroxyl units (c) at 468.5°C with a further loss of hydroxyls (d) at 821°C with oxygen loss (e) at 895°C with a further loss of oxygen. The thermal analysis of the chrysocolla from Congo shows mass losses at 125, 275.3, 805.6 and 877.4°C and for the Nevada chrysocolla, mass loss steps at 268, 333, 463, 786.0 and 817.7°C are observed. The thermal analysis of spertinitte is very different from that of chrysocolla and thermally decomposes at around 160°C.

XPS shows that there are two different copper species present, one which is bonded to oxygen and one to a hydroxyl unit. The O 1s is broad and very symmetrical suggesting two O species of equal number. The bond energy of 102.9eV for the Si 2p suggests that it is in the form of a silicate. The bond energy is much higher for silicas around ~103.5eV. The reported

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

value for silica gel has Si 2p at 103.4eV. The combination of TG, PXRD, EDX and XPS adds to our fundamental knowledge of the structure of chrysocolla.

Keywords: chrysocolla, copper silicate, spertiniite, thermal analysis, PXRD, EDX, XPS.

2

Introduction

Chrysocolla is a hydrated hydroxy silicate of copper and aluminium of formula

(Cu, Al)2H2Si2O5(OH)4·nH2O [1]. It is one of several copper silicates [2]. There are a significant number of silicate minerals which have copper as one of the main cations. These include kinoite Ca2Cu2Si3O10(OH)4, chrysocolla (Cu,Al)2H2Si2O5(OH)4·nH2O,

CuSiO3·H2O, planchéite Cu8Si8O22(OH)4·H2O, Cu5(SiO3)4(OH)2,whelanite

Ca5Cu2(OH)2CO3,Si6O17·4H2O, (K,Na)Cu7AlSi9O24(OH)6·3H2O, apachite

Cu9Si10O29·11H2O, CaCuAlSi2O6(OH)3. Apart from chrysocolla which appears as a normally amorphous mineral, all of these copper silicate minerals are crystalline; however the crystallinity may vary between the minerals. All of the minerals contain either hydroxy units or water units or both. These water and OH units are important for the stability of the minerals.

Chrysocolla is of an uncertain structure but is possibly orthorhombic with a non-determined point group [3, 4]. Chrysocolla has a distinctive X-ray diffraction pattern (ICDD Powder Diffraction file No. 27-188) which differs from that of spertiniite (ICDD Powder Diffraction File No. 35-505), and the unit cell dimensions of each are very different. Van Oosterwyck- Gastuche and Grégoire [5] suggested a chain structure for fibrous microcrystals from 0.5 to 3 microns long and 60–70 nm across, with corresponding idealized formula of

Cu2H2(Si2O5)(OH)4·nH2O. Recent studies have brought into question whether chrysocolla is

a mineral at all, but rather is a mesoscopic assemblage of dominantly spertiniite Cu(OH)2, water and amorphous silica [6, 7]. Farges et al. [8] supported this concept showing that chrysocolla is an assemblage using XAFS and μ-XAFS spectroscopy. The results suggest that the local structure around Cu is similar to that in Cu(OH)2 (spertiniite). The structure of

chrysocolla may resemble that of planchéite which consists of brucite-like (CuO2) layers with

pyroxene-type (SiO3)n chains joined to their surfaces.

There have been a few studies on the thermal decomposition of chrysocolla. The results of these analyses are somewhat conflicting and the conclusions vary among these thermal analytical studies. Chukhrov and Anosov [9] reported the thermal curve for chrysocolla 3

which shows characteristic endothermal effects at 120-140, 450-530, and 690-700°, and an exothermic reaction at 1040°, with a typical montmorillonite character. Chukhrov et al. [10] reported the DTA patterns of some seven chrysocolla mineral samples from Russia. Toussaint [11] reported the thermal analysis of chrysocolla and stated that no OH units were in the structure of chrysocolla. Martinez [12] studied the differential thermal analysis of chrysocolla. Bisengalieva et al. [13] determined the heat capacity of chrysocolla but such an assessment may depend upon the chemical composition of the chrysocolla and the actual origin of the mineral. Chrysocolla presents no long range structural order, thus thermal analysis is a suitable tool for the study of chrysocolla. Thermal analysis has proved most useful for the study of the thermal decomposition of minerals and the assessment of their stability.

Experimental

Minerals

The minerals were supplied by the Mineralogical Research Company. Three mineral samples were selected for study: (a) Kaluruluka Mine, Katanga Provence, Democratic Republic of Congo (b) Inspiration Mine, Globe-Miami District, Gila County, Arizona (c) Empire-Nevada Mine, Yerington, Lyon, Nevada, USA. Each of the mineral samples was used for the thermogravimetric analysis.

X-ray diffraction

The chrysocolla mineral samples were prepared as pressed powders and mounted in stainless steel sample holders. The powder X-ray diffraction (XRD) patterns were recorded on a Philips PANalytical X’Pert PRO diffractometer using Cu Kα radiation operating at 40 kV and 40 mA. XRD diffraction patterns were taken in the range of 10-70o at a scan speed of 2◦ min−1 with 0.5o divergence slit size. Phase identification was carried out by comparison with XRD patterns included in the Inorganic Crystal Structure Database (ICSD).

Scanning electron microscopy (SEM) and EDX analysis

The minerals were mounted on SEM mounts with carbon tape and coated with a thin layer of evaporated gold. The secondary electron images were obtained using a scanning electron microscope (FEI Quanta 200 SEM, FEI Company, Hillsboro, Oregon, USA), operating at 30 kV. EDX analysis procedure is as follows: the samples were crushed into tablets and then 4

coated with a thin layer of evaporated carbon to enhance surface conductivity. After this preparation, samples were examined in a JEOL 840A analytical SEM at 20 kV accelerating voltage. The EDX analysis was done using a JEOL 2300 micro-analyser.

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.

X-ray photoelectron spectroscopy (XPS)

Data was acquired using a Kratos Axis ULTRA X-ray Photoelectron Spectrometer incorporating a 165 mm hemispherical electron energy analyser. The incident radiation was Monochromatic Al Kα X-rays (1486.6 eV) at 150 W (15 kV, 10 mA) and at 45 degrees to the sample surface. Photoelectron data was collected at take off angle of theta = 90 o. Narrow high-resolution scans were run with 0.05 eV steps and 250 ms dwell time. Base pressure in the analysis chamber was 1.0×10-9 torr and during sample analysis 1.0×10-8 torr.

A small amount of each finely-powdered sample was carefully applied to double sided adhesive tape on a standard Kratos Axis Ultra sample bar. This was attached to the sample rod of the Load Lock system for initial evacuation to ~ 1 × 10-6 torr. The sample bar was then transferred to the UHV Sample Analysis Chamber (SAC) for collection of X-ray Photoemission spectra. Spectra were subjected to a Shirley baseline. Various data handling procedures were carried out using the CasaXPS version 2.3.14 software.

Results and Discussion

X-ray diffraction

5

The PXRD results are reported in Figure 1. Only two XRD patterns were obtained as there was insufficient sample remaining of the third sample to obtain a XRD pattern. It is clear from the XRD patterns that the chrysocolla mineral samples are non-diffracting and no other phases are present in the minerals, thus showing the chrysocolla samples are pure. An XRD pattern is given in the RRUFF data base (see http://rruff.info/chrysocolla/display=default/R050053). No peaks are observed in this XRD pattern.

EDX results

The results of the EDX analyses of the chrysocolla samples from Congo and Arizona is given in the following Table.

Chrysocolla (Congo) Chrysocolla (Arizona)

Weight% Atom% Weight% Atom%

O 38.53 63.9 20.39 45.57

Al 1.21 1.2 0.46 0.61

Si 18.51 17.48 13.07 16.64

Cu 41.75 17.42 66.08 37.19

If we use the formula of chrysocolla as (Cu2-x,Alx)(HSi2O5(OH)4·nH2O with x<1, then the formula of the two samples is estimated as chrysocolla-Congo: Cu1.87Al0.13(H1.87Si1.88O4.7)

(OH)x·nH2O and chrysocolla-Arizona: Cu1.97Al0.032(H1.97Si0.88O2.2) (OH)x·nH2O.

Thermogravimetry

The thermal decomposition of chrysocolla from three different origins is reported in Figures 2-4. Figure 2 shows the TG and DTG patterns of the thermal analysis of the chrysocolla sample from Arizona, Figure 3 reports the thermal analysis of the chrysocolla from the Congo, Figure 4 illustrates the TG and DTG patterns of the chrysocolla sample from Nevada.

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Five thermal decomposition steps are observed for the thermal decomposition of chrysocolla from Arizona. The first mass loss step occurs over the range ambient to 125 °C. This mass loss is attributed to the loss of water. Such a result is in harmony with the results of Martinez [12] who suggested using TG analysis that water was evolved over the 50 to 150°C

temperature range. If we accept the formula of chrysocolla as (Cu,Al)2H2Si2O5(OH)4·nH2O, then a chemical reaction is envisaged as: Cu2H2Si2O5(OH)4·nH2O → Cu2H2Si2O5(OH)4+ nH2O. The mass loss percentage for this initial step attributed to the evolution of water in the decomposition of chrysocolla is 13.7%. If the initial mass is 23.04 mg, it means that 3.3156 mg are lost in this decomposition step. Many minerals contain water and the first loss step is more often than not due to the loss of water [14-17] although water molecules can be retained in the structure to significantly higher temperatures.

The next two mass loss steps of chrysocolla from Arizona are observed at 340.3 and 468.5°C with mass losses of 2.9 and 2.1%. Mass spectrometry shows that these mass losses are due to the loss of hydroxyl units. The following reaction is proposed: Cu2H2Si2O5(OH)4→ 2CuO +

2SiO2 + 3H2O. The reaction simply serves to show the loss of hydroxyl units and formation of water. A small mass loss is observed at 639.9°C and this may be attributed to the presence of a small amount of carbonate present in the mineral. It is probable that the copper is bonded to the hydroxyl units in an octahedral structure or a distorted octahedral structure [18, 19]. Lakshman et al. [20] used electronic absorption spectra to show that the Cu2+ ion in chrysocolla is tetragonal. Sarma et al. [18, 19] based upon EPR and optical absorption studies found that the Cu2+ ion in copper silicate minerals is an elongated octahedral environment. The mineral chrysocolla is possibly orthorhombic and the two minerals shattuckite and planchéite are also orthorhombic. Thus, some of the conclusions based upon shattuckite and planchéite may be extended to chrysocolla especially in terms of the molecular structure. The next decomposition step of chrysocolla from Arizona is observed at 821.3°C with a mass loss of 1.5%. This decomposition step differs for the other two chrysocolla samples. The DTG curve is quite broad for each of the decompositions. This suggests that oxygen is lost over a wide temperature range. (Mass spectrometry shows that oxygen is lost at this temperature). A further mass loss step is observed at 895.4°C with a percentage mass loss of 1.4%. This mass loss is attributed to oxygen evolution. The reaction proposed is as follows 2CuO → Cu2O

+1/2O2 . Torocheshnikov et al. [21] measured the thermal decomposition of copper oxide 7

and determined the decomposition range to be over 825 to 950°C. Frost et al. [22] showed that the final decomposition steps of azurite and with the loss of oxygen occurred at 840°C. Such a result is not in agreement with work of Kleber and Guertzsch [23] who concluded that the thermal decomposition of copper oxide with loss of oxygen occurred at 660°C. Ding et al. 24 also reported the thermal activation of copper carbonates and showed that oxygen was evolved at 850°C.

A comparison may be made with the thermal decomposition of chrysocolla from the Kaluruluka Mine (Figure 3). The TG analysis is different to that observed for chrysocolla from Arizona. This may be attributed to the different compositions of the two minerals, although the EDX analyses show that the formulae of the two minerals are very similar. A large mass loss of 14.7% is observed over the temperature range ambient to 125°C. This mass loss is attributed to the evolution of water. The DTG curve shows a mass loss peak at 275.3°C. The mass loss over the 125 to 600°C temperature range of 7% is observed. Two DTG peaks are observed at 805.6 and 877.4°C. These maxima are at somewhat lower temperatures than those observed for the decomposition of the Inspiration Mine sample from Arizona.

The thermal analysis of chrysocolla from Nevada is reported in Figure 4. The thermal analysis pattern more closely resembles that shown in Figure 2. The thermogravimetric analysis is very similar to that of chrysocolla from Arizona. A large mass loss of 16.3% occurs over the temperature range ambient to 130°C. Mass spectrometry shows that this mass loss is due to the evolution of water. Three peaks in the DTG curve are observed at 268, 333 and 463 °C with a total mass loss of 7.6%. These mass loss steps are attributed to the loss of the hydroxyl units. Two mass loss steps are observed at 786.0 and 817.7°C. The

attribution of these peaks is as follows Cu2Si2O5(OH)4 → 2CuO + 2SiO2 + 2H2O +1/2O2.

The question arises as to whether chrysocolla is a true mineral or simply a colloidal mixture

of spertiniite (Cu(OH)2) and amorphous silica [8]. The mineral spertiniite decomposes in a single step at 160°C with the loss of water and the formation of (CuO). The 8

following reaction is envisaged: Cu(OH)2 → CuO + H2O. This mineral has a further mass

loss at around 850°C with the loss of oxygen according to the reaction 2CuO → Cu2O +

1/2O2. According to a data base reference the cupric hydroxide decomposes at 185°C [

http://digitalfire.com/4sight/material/copper_hydroxide_2252.html]. Guenter and Oswald

[25] determined the topotactical relationship between Cu(OH)2, CuO and Cu2O. These

researchers found that that a sudden, reversible elongation of the c axis of Cu(OH)2 at 50° has been found, and the topotactic relation is 1-dimensionally controlled, as only 1 crystallographic axis of the initial hydroxide is conserved as a structural element throughout the decomposition reaction. Fricke and Kubach [26] found that thermal decomposition of

pure Cu(OH)2 as followed by mass loss and by the emanation method takes place at 150-

160°. Since all the H2O is not lost, a Cu hydroxide containing less than one H2O per mole is

indicated [26]. The Cu(OH)2 was found to decompose at ~160°C. Based upon the thermogravimetric analysis of chrysocolla and spertiniite it is concluded that the mineral chrysocolla is not based upon spertiniite. The thermal analysis patterns of chrysocolla differ greatly from that of spertiniite.

XPS studies

X-ray photoelectron spectroscopy (XPS) has been undertaken on the finely crushed chrysocolla mineral samples, from (a) Congo and (b) Arizona (c) Nevada. The XPS spectra are shown in Figure 5. Two Cu species are present one with a Cu 2p3/2 binding energy of 933.2eV and the other with a Cu 2p3/2 binding energy of 935.4eV. These are consistent with

Cu(II) species and the latter is consistent with Cu(OH)2. This bond energy could be due to a Cu Si O bond. The O 1s is broad and very symmetrical suggesting two O species of similar intensities. Controlled curve fitting resulted in two O 1s species with binding energies of 531.5 and 532.4 eV. The higher bond energy is due to OH units. The latter is consistent with Cu(OH)2. . The bond energy of 102.9eV for the Si 2p suggests that it is in the form of a silicate. The bond energy is much higher for silicas around ~103.5eV. The reported value for silica gel has Si 2p at 103.4eV. Mosser et al. measured the XPS of natural and synthetic copper silicate minerals [27]. These researchers found a binding energy of 935.4 eV for Cu(II) in copper phyllosilicates.

9

Conclusions

The mineral chrysocolla has been studied by a combination of powder X-ray diffraction, SEM, EDX and XPS. The XRD patterns show that the chrysocolla mineral samples are non- diffracting. No additional XRD peaks were observed thus showing no other phases are present in the minerals. This shows the chrysocolla samples are pure. EDX analyses enable the formulae of the chrysocolla samples to be calculated. The formula is calculated as

chrysocolla-Congo: Cu1.87Al0.13(H1.87Si1.88O4.7) (OH)x·nH2O and chrysocolla-Arizona:

Cu1.97Al0.032(H1.97Si0.88O2.2) (OH)x·nH2O.

The thermal decomposition of the mineral chrysocolla has been studied by thermogravimetric analysis in association with mass spectrometry. Variation in the thermal analysis is observed between samples of different origin. This is attributed to variation in chemical composition. Decomposition of the chrysocolla mineral sample from Arizona occurred in five mass loss steps: (a) at 125°C with the loss of water (b) at 340°C with the loss of hydroxyls (c) at

468.5°C with a further loss of hydroxyls and formation of SiO2 (d) at 821°C and 895°C with

the loss of oxygen and formation of Cu2O. The thermal analysis of spertinitte is very different from that of chrysocolla and thermally decomposes at low temperatures at around 160°C. The thermal analysis of the chrysocolla from Congo shows mass losses at 125, 275.3, 805.6 and 877.4°C and for the Nevada chrysocolla, mass loss steps at 268, 333, 463, 786.0 and 817.7°C are observed. The thermal analysis of spertinitte is very different from that of chrysocolla and thermally decomposes at around 160°C.

XPS shows that there are two different copper species present, one which is bonded to oxygen and one to a hydroxyl unit. The O 1s is broad and very symmetrical suggesting two O species. The bond energy of the Si 2p suggests that it is in the form of a silicate. The bond energy is much higher for silicates than silica above 103.5eV.

2+ These results show that Cu is bonded to both SiO3 units and OH units. These results support the concept that chrysocolla is a defined mineral and not a colloid based upon spertiniite. If the two minerals were related it would be expected that the two minerals would decompose at similar temperatures. This does not occur. Spertiniite decomposes at significantly lower temperatures than chrysocolla. There is a difference of ~180°C in temperature between the

10

decomposition of chrysocolla and spertiniite. It is concluded that chrysocolla is a mineral in its own right.

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.

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List of Figures

Figure 1 XRD of two of the chrysocolla minerals

Figure 2 Thermogravimetric and differential thermogravimetric analysis of chrysocolla from the Inspiration Mine, Gila County, Arizona.

Figure 3 Thermogravimetric and differential thermogravimetric analysis of chrysocolla from the Kaluruluka Mine, DR of Congo.

Figure 4 Thermogravimetric and differential thermogravimetric analysis of chrysocolla from the Empire-Nevada Mine, Yerington, Lyon County, Nevada.

Figure 5 (a) XPS of Cu in chrysocolla (Congo) (b) XPS of oxygen in chrysocolla (Congo) (c) XPS of Cu in chrysocolla (Arizona) (d) XPS of oxygen in chrysocolla (Arizona) (e) XPS of copper in chrysocolla from Nevada (f) XPS of oxygen in chrysocolla from Nevada

15

Figure 1

16

Figure 2

17

Figure 3

18

Figure 4

19

Figure 5a XPS of copper in chrysocolla Figure 5b XPS of oxygen in chrysocolla from Congo from Congo

Figure 5c XPS of copper in chrysocolla Figure 5d XPS of oxygen in chrysocolla from Arizona from Arizona

20

Figure 5e XPS of copper in chrysocolla Figure 5f XPS of oxygen in chrysocolla from Nevada from Nevada

21