This may be the author’s version of a work that was submitted/accepted for publication in the following source: Frost, Ray& Xi, Yunfei (2013) A vibrational spectroscopic study of the so-called ‘healing’ mineral pa- pagoite CaCuAlSi2O6(OH)3. Spectroscopy Letters, 46(5), pp. 344-349. This file was downloaded from: https://eprints.qut.edu.au/58784/ c Consult author(s) regarding copyright matters This work is covered by copyright. Unless the document is being made available under a Creative Commons Licence, you must assume that re-use is limited to personal use and that permission from the copyright owner must be obtained for all other uses. If the docu- ment is available under a Creative Commons License (or other specified license) then refer to the Licence for details of permitted re-use. It is a condition of access that users recog- nise and abide by the legal requirements associated with these rights. If you believe that this work infringes copyright please provide details by email to [email protected] Notice: Please note that this document may not be the Version of Record (i.e. published version) of the work. Author manuscript versions (as Sub- mitted for peer review or as Accepted for publication after peer review) can be identified by an absence of publisher branding and/or typeset appear- ance. If there is any doubt, please refer to the published source. https://doi.org/10.1080/00387010.2012.733477 1 A vibrational spectroscopic study of the so-called ‘healing’ mineral papagoite 2 CaCuAlSi2O6(OH)3 3 4 Ray L. Frost, Yunfei Xi 5 6 School of Chemistry, Physics and Mechanical Engineering, Science and Engineering 7 Faculty, Queensland University of Technology, GPO Box 2434, Brisbane Queensland 8 4001, Australia. 9 10 11 Abstract 12 Papagoite is a silicate mineral named after an American Indian tribe and was used as a 13 healing mineral. Papagoite CaCuAlSi2O6(OH)3 is a hydroxy mixed anion compound with 14 both silicate and hydroxyl anions in the formula. The structural characterisation of the 15 mineral papagoite remains incomplete. Papagoite is a four membered ring silicate with Cu2+ 16 in square planar coordination. -1 17 The intense sharp Raman band at 1053 cm is assigned to the ν1 (A1g) symmetric stretching 18 vibration of the SiO4 units. The splitting of the ν3 vibrational mode offers support to the 19 concept that the SiO4 tetrahedron in papagoite is strongly distorted. A very intense Raman -1 -1 20 band observed at 630 cm with a shoulder at 644 cm is assigned to the ν4 vibrational modes. -1 21 Intense Raman bands at 419 and 460 cm are attributed to the ν2 bending modes. 22 Intense Raman bands at 3545 and 3573 cm-1 are assigned to the stretching vibrations of the 23 OH units. Low intensity Raman bands at 3368 and 3453 cm-1 are assigned to water 24 stretching modes. It is suggested that the formula of papagoite is more likely to be 25 CaCuAlSi2O6(OH)3·xH2O. Hence, vibrational spectroscopy has been used to characterise the 26 molecular structure of papagoite. 27 28 Key words: papagoite, stringhamite, cupric ions, healing mineral, vibrational spectroscopy 29 30 31 32 Author to whom correspondence should be addressed ([email protected]) P +61 7 3138 2407 F: +61 7 3138 1804 1 33 Introduction 34 The mineral papagoite CaCuAlSi2O6(OH)3 is known from two locations in narrow 35 veinlets in altered granodiorite porphyry from Ajo, Arizona and in quartz crystals, 36 Messina, South Africa [1, 2]. The mineral is a hydroxy silicate of calcium, copper and 37 aluminium. It is one of a number copper silicates [3]. 38 39 There are a significant number of silicate minerals which have copper as one of the main 40 cations. These include chrysocolla (Cu, Al)2H2Si2O5(OH)4·nH2O, dioptase CuSiO3·H2O, 41 planchéite Cu8Si8O22(OH)4·H2O, shattuckite Cu5(SiO3)4(OH)2,whelanite 42 Ca5Cu2(OH)2CO3,Si6O17·4H2O, ajoite (K,Na)Cu7AlSi9O24(OH)6·3H2O, apachite 43 Cu9Si10O29·11H2O, papagoite CaCuAlSi2O6(OH)3. Apart from chrysocolla which 44 appears as an amorphous non-diffracting mineral, all of these copper silicate minerals are 45 highly crystalline. All of the minerals contain either hydroxy units or water units or both. 46 These water and OH units are important for the stability of the minerals. All these 47 minerals are of various shades of blue. 48 49 The mineral papagoite is named after the Papago tribe of Arizona. The native American 50 tribes have been known to use copper silicate minerals as healing antiseptic medicines. A 51 common feature of these silicate minerals is that these copper containing silicate 52 minerals have been traditionally used as ‘healing’ minerals [4]. Copper is very important 53 in human health but is toxic at higher concentrations [5]. Copper ions are of course a 54 very powerful antibacterial agent [4, 6, 7]. Copper compounds are used as antibacterial 55 agents. Estimation of the amount of copper in water and foods can be measured [8, 9]. 56 57 The mineral papagoite is monoclinic with point group 2/m [3]. The crytal structure was 58 refined by Groat and Hawthorne [10].The cell data is: Space Group: C2/m: a = 12.926(3), b 59 = 11.496(3), c = 4.696(1), β = 100:81(2)± Z = 4 [11]. Papagoite is one of only a few four 60 membered ring silicates [11]. These silicates have four silicate tetrahedrons linked into a ring 61 forming a distorted square-like structural element [3]. 62 Raman spectroscopy has proven very useful for the study of minerals [12-19]. Indeed Raman 63 spectroscopy has proven most useful for the study of diagenetically related minerals as often 64 occurs with minerals containing copper and silicate groups. This paper is a part of systematic 65 study of vibrational spectra of minerals of secondary origin in the oxide supergene zone. The 2 66 objective of this research is to report the Raman spectra of papagoite and to relate the spectra 67 to the molecular structure of the mineral. Papagoite is known as a healing mineral. The 68 question arises as to whether there is any evidence that papagoite is actually a healing 69 mineral. 70 71 Experimental 72 Mineral 73 The mineral papagoite CaCuAlSi2O6(OH)3 was supplied by the Mineralogical Research 74 Company. The mineral originated from the new Cornelia Mine, Ajo, Pima County, 75 Arizona, USA. The mineral sample is defined as a ‘type’ mineral and is used as a 76 reference for this type of mineral and its structure. Details of the mineral have been 77 published (page 617) [20]. 78 79 Raman spectroscopy 80 81 Crystals of papagoite were placed on a polished metal surface on the stage of an Olympus 82 BHSM microscope, which is equipped with 10x, 20x, and 50x objectives. The spectra are 83 collected from a mixture of non-oriented crystals. The microscope is part of a Renishaw 1000 84 Raman microscope system, which also includes a monochromator, a filter system and a CCD 85 detector (1024 pixels). The Raman spectra were excited by a Spectra-Physics model 127 He- 86 Ne laser producing highly polarised light at 633 nm and collected at a nominal resolution of 2 87 cm-1 and a precision of ± 1 cm-1 in the range between 200 and 4000 cm-1. Repeated 88 acquisitions on the crystals using the highest magnification (50x) were accumulated to 89 improve the signal to noise ratio of the spectra. Spectra were calibrated using the 520.5 cm-1 90 line of a silicon wafer. A spectrum of papagoite is provided on the RRUFF data base. The 91 spectra have been downloaded and are shown in the supplementary information. 92 93 Infrared spectroscopy 94 95 Infrared spectra were obtained using a Nicolet Nexus 870 FTIR spectrometer with a smart 96 endurance single bounce diamond ATR cell. Spectra over the 4000525 cm-1 range were 97 obtained by the co-addition of 128 scans with a resolution of 4 cm-1 and a mirror velocity of 98 0.6329 cm/s. Spectra were co-added to improve the signal to noise ratio. The infrared spectra 99 are given in the supplementary information. 3 100 101 Spectral manipulation such as baseline correction/adjustment and smoothing were performed 102 using the Spectracalc software package GRAMS (Galactic Industries Corporation, NH, 103 USA). Band component analysis was undertaken using the Jandel ‘Peakfit’ software package 104 that enabled the type of fitting function to be selected and allows specific parameters to be 105 fixed or varied accordingly. Band fitting was done using a Lorentzian-Gaussian cross-product 106 function with the minimum number of component bands used for the fitting process. The 107 Gaussian-Lorentzian ratio was maintained at values greater than 0.7 and fitting was 108 undertaken until reproducible results were obtained with squared correlations of r2 greater 109 than 0.995. 110 111 Results and Discussion 112 113 The Raman spectrum of papagoite in the 100 to 4000 cm-1 range is displayed in Figure 1a. 114 This spectrum shows the relative intensities of the different bands in the Raman spectrum of 115 papagoite. It is obvious that there are large parts of the spectrum where no peaks are 116 observed. The spectrum is then subdivided into sections depending upon the type of 117 vibrations being recorded. The infrared spectrum of papagoite is reported in Figure 1b. 118 Again the relative intensities of the infrared bands may be observed and a comparison of 119 intensities with the Raman spectrum observed.