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Frost, Raymond, Weier, Matthew, Erickson, Kristy, Carmody, Onuma,& Mills, Stuart (2004) Raman Spectroscopy of Phosphates of the Variscite Mineral Group. Journal of Raman Spectroscopy, 35, pp. 1047-1055.
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Ray L. Frost a•••, Matt L. Weier a, Kristy L. Erickson a, Onuma Carmody a and Stuart J. Mills b, c a Inorganic Materials Research Program, School of Physical and Chemical Sciences, Queensland University of Technology, GPO Box 2434, Brisbane Queensland 4001, Australia. b Geosciences, Museum Victoria, PO Box 666E, Melbourne, Victoria 3001, Australia. c CSIRO Minerals, Box 312, Clayton South, Victoria 3169, Australia.
Abstract
Raman spectra of phosphates of the variscite mineral group namely strengite, variscite, metavariscite and phosphosiderite have been obtained at 298 and 77 K using a thermal stage and complemented with selected infrared data using a diamond ATR cell. Multiple phosphate stretching bands are observed. Bands for strengite at 1006, 1012, and 1026 cm 1 at 298 K and at 997, 1006, 1014 and 1027 cm 1 at 77 K are observed. Variscite is characterised by PO stretching vibrations at 1023, 1005 and 938 cm 1 and phosphosiderite by bands at 1009 and 993 cm 1. These minerals are readily identified by their Raman spectrum both in the PO stretching region and in the low wavenumber region. Multiple bands attributed to the bending modes of PO 4 are identified for each of the minerals. A model is proposed involving non hydrogen bonded PO 4 and strongly hydrogen bonded PO 4 units together with (Al or + Fe(OH) 2) .(H 2PO 4) type species. Raman spectroscopy shows that at the molecular level multiple species of phosphate anions exist for the variscite phosphate minerals.
Keywords: strengite, variscite, metavariscite, phosphosiderite, Raman spectroscopy, infrared spectroscopy
Introduction
• Author to whom correspondence should be addressed ([email protected])
1 The variscite mineral group are orthorhombic arsenates and phosphate with 3+ 3+ 3+ 3+ the general formula AXO 4.2H 2O, where A = Al , Fe , Cr or In and X = As or P, with space group Pcab. The most common of the phosphate minerals are strengite 3+ 1 (Fe PO 4.2H 2O) and variscite (AlPO 4.2H 2O) , with metavariscite, metastrengite and phosphosiderite (also known as clinostrengite) less common. Strengite forms a series with variscite and is dimorphous with metavariscite. Variscite is dimorphous with 2,3 metavariscite . Metavariscite (AlPO 4.2H 2O) is monoclinic with the space group P21/n .
Strengite is formed as secondary minerals in complex granite pegmatites and in limonite iron ores and gossans, whilst variscites are formed from phosphate bearing waters and also from bat guano, in contact with aluminous rocks. Variscite is commonly found as a cave mineral whereas strengite does not occur in this environment. Metavariscite is a common product of the weathering of phosphate rocks and may form during the addition of phosphate fertilisers to acid soils. Near Iron Knob, South Australia, rare phosphate minerals occur sporadically in hematitic Fe formations of the Eyre Peninsula as a number of different, often monomineralic, minor parageneses: montgomeryite variscite apatite, montgomeryite millisite, cyrilovite strengite, wavellite wardite, turquoise, mitridatite, kidwellite, kleemanite, dufrenite, gorceixite, crandallite, and variscite. These parageneses are interpreted as the precipitates from ground waters in a range of aqueous microenvironments beneath an ancient water table 4. The Moculta phosphate deposit in South Australia contains several of the variscite phosphate group minerals, which have formed during water −rock interaction of Al and Fe rich sediments 5. The formation of variscite group minerals in soils is of importance 6 8. It is probable that colloidal variscite and strengite are important in the uptake of phosphorus by plants 9,10 . Such minerals can be formed through the weathering of phosphate rocks but a more likely to be formed through the addition of rock phosphates to soils 11 . In Australia, the soils are very deficient in phosphates. The most likely factor in the formation of the variscite minerals is that of pH 12 14 .
In aqueous systems, Raman spectra of phosphate oxyanions including phosphate, hydrogen phosphate and dihydrogen phosphate species have been
2 3 published. The Raman spectrum of the oxyanion (PO 4) shows a symmetric 1 1 stretching mode (ν 1) at 938 cm , the antisymmetric stretching mode (ν 3) at 1017 cm , 1 1 15 17 the symmetric bending mode (ν 2) at 420 cm and the ν 4 mode at 567 cm .
Farmer reported the infrared spectra of berlinite (AlPO 4) with PO 4 stretching modes at 1263, 1171, 1130 and 1114 cm 1; bending modes at 511, 480, 451, 379 and 605 cm 1. Al O modes were found at 750, 705, 698 and 648 cm 1 18 . On hydration of the mineral as with variscite (AlPO 4.2H 2O), PO 4 stretching modes were found at 1160, 1075, 1 1 1050 and 938 cm ; bending modes at 515, 450 and 420 cm ; in addition H 2O stretching bands were found at 3588, 3110, 2945 cm 1. For the mineral augelite
(Al 2PO 4(OH) 3), infrared bands were observed at 930 (ν1), 438 (ν2), 1205, 1155, 1079, 1 1015 (ν3) and 615, 556 cm (ν4). For augelite, OH stretching modes were not observed. Other infrared studies have been published 19,20 . However, studies of the vibrational spectrum of these minerals remain incomplete. Few Raman studies of any of these phosphate minerals have been published. In this work we report the Raman at 298 and 77 K of the variscite phosphate minerals and relate the spectra to the mineral structure. The application of the spectroscopy is in the possible determination of colloidal variscite minerals in soils 21 23 .
Experimental
Minerals
The minerals used in this study were obtained from the collection of Museum Victoria. The minerals were analysed, wherever possible, for phase purity by X ray diffraction techniques and for composition by electron probe analyses.
Mineral Formula Museum Number Origin
Variscite (AlPO 4.2H 2O) M45772 Lavers Hill, Fish Creek, Victoria
Variscite (AlPO 4.2H 2O) M37991 Iron Monarch, Middleback Ranges,
3 South Australia 3+ Strengite (Fe PO 4.2H 2O) M44244 Lavers Hill, Fish Creek, Victoria 3+ Strengite (Fe PO 4.2H 2O) M34613 Iron Monarch, Middleback Ranges, South Australia
Metavariscite (AlPO 4.2H 2O) M31413 Mt Lucia, Utah, USA
Phosphosiderite (FePO 4.2H 2O) M38005 Hacks Quarry, Gladstone, South Australia
Phosphosiderite (FePO 4.2H 2O) M33897 McMahon’s Quarry, Mingary Station, South Australia
Raman microprobe spectroscopy
The crystals of the mineral were placed and orientated on the stage of an Olympus BHSM microscope, equipped with 10x and 50x objectives and part of a Renishaw 1000 Raman microscope system, which also includes a monochromator, a filter system and a Charge Coupled Device (CCD). Raman spectra were excited by a HeNe laser (633 nm) at a resolution of 2 cm 1 in the range 100 −4000 cm 1. Repeated acquisition using the highest magnification was accumulated to improve the signal to noise ratio. Spectra were calibrated using the 520.5 cm 1 line of a silicon wafer. In order to ensure that the correct spectra are obtained, the incident excitation radiation was scrambled. Previous studies by the authors provide more details of the experimental technique. Spectra at controlled temperatures were obtained using a Linkam thermal stage (Scientific Instruments Ltd, Waterfield, Surrey, England). Details of the technique have been published by the authors 15,24 28 .
Infrared spectroscopy
4 Infrared spectra were obtained using a Nicolet Nexus 870 FTIR spectrometer with a Smart Endurance single bounce diamond ATR cell. Spectra in the region 4000 −525 cm 1 were obtained by the co addition of 64 scans with a resolution of 4 cm 1 and a mirror velocity of 0.63 cm/s.
Spectra were analysed using the Galactic software package GRAMS. Band component analysis was undertaken using the Jandel ‘Peakfit’ software package, which 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 Gauss Lorentz cross product function with the minimum number of component bands used for the fitting process. The Gauss Lorentz ratio was maintained at values greater than 0.7 and fitting was undertaken until reproducible results were obtained with squared regression coefficient of R 2 greater than 0.995.
Results and discussion
The Raman spectra of the region 900 −1200 cm 1 of strengite, variscite, metavariscite and phosphosiderite at 298 K and 77 K are shown in Figures 1 and 2 respectively, with the band component analyses reported in Table 1. Bands in this spectral region are assignable to the stretching vibrations of the PO 4 units. The Raman 1 spectrum of strengite shows a strong band at 985 cm assigned to the ν1 symmetric 1 stretching vibration of the PO 4 units. Farmer (1974) reported a band at 990 cm in the infrared spectrum of strengite 18 . This band shifts to 990 cm 1 at 77 K and is indicative of a strengthening of the hydrogen bonding between the water and the PO 4 units. A number of bands are observed for strengite at 1006, 1012, and 1026 cm 1 at 298 K and at 997, 1006, 1014 and 1027 cm 1 at 77 K. It is proposed that these bands may be due to symmetric stretching vibrations of (H 2PO 4) units. In the infrared spectrum of strengite as reported by Farmer (1974), a band was observed at 870 cm 1 and was identified with the symmetrical P(OH) 2 stretching of the (H 2PO 4) units. Figure 3 shows a comparison of the infrared and Raman spectra of Variscite. A comparatively broad band is observed in the Raman spectra of strengite at 870 cm 1. The band was observed at 879 cm 1 at 77 K. A similar result was obtained for strengite sample
5 although the band was observed somewhat higher at 878 cm 1. An alternative assignation is that the band is a water librational mode. Such bands would be expected in the region 700 −900 cm 1. A strong infrared band is observed around this position. Low intensity bands are observed for strengite at 1357, 1250, 1158 and 1137 cm 1 and are ascribed to the ν3 antisymmetric stretching vibrations of the PO 4 units. These bands are observed at 1337, 1249, 1236 and 1152 cm 1 at 77 K.
The most intense band in the region 900 −1200 cm 1 of variscite is at 1023 cm 1 with component bands observed at 1005 and 938 cm 1. These bands are all assigned to the ν1 symmetric stretching vibrations of the PO 4 units. The bands are well separated at 77 K, with the most intense band observed at 1030 cm 1. In the infrared spectrum of variscite a low intensity band is observed at 930 cm 1 and is assigned to this vibrational mode. Such a mode should be Raman active infrared inactive. However the reduction of symmetry induced through the hydrogen bonding of the water to the
PO 4 units allows the observation of the mode in the infrared spectrum. In the Raman spectrum of variscite at 298 K, a number of low intensity bands are observed at 1250, 1 1157, 1133, 1077 and 1029 cm . These bands are assigned to the ν3 antisymmetric stretching vibrations of the PO 4 units. The multiplicity of bands is even more apparent in the Raman spectrum at 77 K with bands observed at 1361, 1248, 1157, 1131, 1080, 1059 and 1046 cm 1. In the infrared spectrum of variscite bands are observed at 1155, 1117, 1083, 1067, 1031 and 992 cm 1 however Farmer (1974) reported bands at 1160, 1075 and 1050 cm 1 18 . The number of bands observed in the both the symmetric and antisymmetric stretching region of the PO 4 units is an indication of multiple PO 4 species. A model is proposed involving non hydrogen bonded PO 4 and strongly + hydrogen bonded PO 4 units together with (Al(OH) 2) •(H 2PO 4) type species. The model includes the existence of phosphate, dihydrogen phosphate and monohydrogen phosphate species 12 . Some research based upon synthetic variscite has suggested that such species do not exist 1. However, if this is the case, there needs to be a logical explanation of the existence of multiple bands in the phosphate stretching region 1.
Some novel phosphate minerals such as vantasselite (Al 4(PO 4)3(OH) 3.9H 2O) are often formed in slag heaps 29 . In soils gelatinous strengite and variscite may be formed in solution and precipitate from this solution 30 . Such gels may contain mixed variable
6 phosphate anions 30 which will depend upon the temperature and pH of the crystallisation 13,31 . Such phenomena effect the chemistry of soils 14 .
The Raman spectrum of phosphosiderite is characteristically different from either strengite or variscite. This is attributed to the different crystal structure of phosphosiderite. An intense band is observed at 1009 with a second band at 993 1 cm . The bands are attributed to the ν1 symmetric stretching vibrations of the PO 4 1 units. Two other bands are observed at 1250 and 1033 cm and are assigned to the ν3 antisymmetric stretching vibrations of the PO 4 units of phosphosiderite. At 77 K the symmetric stretching modes are observed at 1008, 992 and 983 cm 1 and antisymmetric modes at 1247, 1154, 1043 and 1032 cm 1. The infrared spectrum of phosphosiderite showed bands at 1118, 1049, 1020 and 991 cm 1. Farmer (1974) published the data for the infrared spectrum of phosphosiderite and gave bands at 1160, 1100 and 1020 cm 1.
The Raman spectra of the low wavenumber region of strengite, variscite, metavariscite and phosphosiderite at 298 and 77 K are shown in Figures 4 and 5. The Raman spectrum of strengite displays bands at 590, 568, 491 and 477 cm 1. In the infrared spectrum, bands are observed at 586, 545, 520, 485 and 475 cm 1. These bands may be assigned to the ν4 bending modes of the PO 4 units. Three bands are 1 observed at 446, 424 and 399 cm and are assigned to the ν2 bending modes of the
PO 4 units. The multiplicity of bands in the bending region of PO 4 suggests a combination of two effects: (a) symmetry reduction and (b) multiple phosphate species. The Raman spectrum of variscite shows bands at 563, 534, 483 and 464 1 cm attributed to the ν4 bending modes. These bands are observed at 568, 562, 536, 488 and 467 cm 1 at 77 K. The Raman spectrum of variscite also shows bands at 437, 1 419 and 389 cm which are assigned to the ν2 bending modes. The bands are observed at 444, 424 and 418 cm 1 at 77 K. Farmer (1974) reported bending modes for variscite at 515, 450 and 420 cm 1. The positions of these bands are in reasonable agreement with the Raman data. The Raman spectrum of phosphosiderite shows three 1 bands at 548, 388 and 452 cm assigned to the ν4 bending modes of the PO 4 units. At liquid nitrogen temperature, these bands are observed at 544, 497, 479 and 452 cm 1. Two bands are observed at 435 and 400 cm 1 at 298 K and at 448 and 405 cm 1 at 77
K and are attributed to the ν2 bending modes of the PO 4 units.
7
For the infrared technique used in these experiments the wavenumber limiting value is 550 cm 1. The reason for using the single bounce diamond ATR technique is that the method is not destructive which is most important for museum samples. Importantly a number of bands are observed in the infrared spectrum of variscite at 870, 799, 779, 698, 651, 646 and 637 cm 1. Farmer (1974) also reported bands at 870, 800, 705, 646, 612 and 577 cm 1 and he assigned these bands to vibrational modes involving Al OH’s. Whilst these bands are observed in the infrared spectra, they are not observed in the Raman spectra. Two low intensity bands are observed for variscite at 790 and 613 cm 1 in the 298 K spectrum and are found at 803 and 615 cm 1 in the 77 K spectrum. It is apparent that these Al OH modes are either infrared or Raman active.
The spectra of the low wavenumber region of the variscite phosphate minerals become extremely complex at liquid nitrogen temperature. This is especially pronounced for both strengite and variscite. The number of bands in this spectral region may indicate multiple anionic species involving phosphate as detailed above. An intense sharp band is observed in the spectra at around 326 cm 1. This band is assigned to Fe O or Al O stretching band. This band shows a slight shift (329 cm 1) at 77 K. The Raman spectrum of strengite displays an intense band at 193 cm 1. This band may be assigned to an OFeO symmetric bending mode. An equivalent band is observed for variscite at 230 cm 1and at 229 cm 1 at 77 K. This same band is observed at 203 cm 1 for phosphosiderite. One of the strong features of Raman spectroscopy is that bands below 400 cm 1 are readily determined. This means that intense bands attributable to M O vibrations can be ascertained.
Conclusions
This work has shown that at the molecular level multiple species of phosphate anions exist for the variscite phosphate minerals. This work shows that minerals of the variscite group are readily determined by Raman spectroscopy. The minerals are pargenically related and often found in close proximity. Individual crystals of the variscite mineral group may be identified by Raman spectroscopy.
8
Acknowledgements
The financial and infrastructure support of the Queensland University of Technology Inorganic Materials Research Program of the School of Physical and Chemical Sciences is gratefully acknowledged. The Australian Research Council (ARC) is thanked for funding. Museum Victoria is thanked for the loan of the mineral. SJM wishes to thank the support of CSIRO Minerals.
References
1. Arlidge, EZ, Farmer, VC, Mitchell, BD, Mitchell, WA. Journal of Applied Chemistry 1963 ; 13 : 17. 2. Kniep, R, Mootz, D. Acta Crystallographica, Section B: Structural Crystallography and Crystal Chemistry 1973 ; 29 : 2292. 3. Kniep, R, Mootz, D, Vegas, A. Acta Crystallographica, Section B: Structural Crystallography and Crystal Chemistry 1977 ; B33 : 263. 4. Segnit, ER, Watts, J. Porodoobrazuyushchie Miner., Mater. S'ezda MMA, 11th 1981: 273. 5. Henderson, WA, Jr., Peisley, V. Mineralogical Record 1985 ; 16 : 477. 6. Ashley, PM, Lottermoser, BG, Scott, KM. Neues Jahrbuch fuer Mineralogie, Monatshefte 1997: 309. 7. Beauchemin, S, Hesterberg, D, Chou, J, Beauchemin, M, Simard, RR, Sayers, DE. Journal of Environmental Quality 2003 ; 32 : 1809. 8. Gonez Morales, J, Rodriguez Clemente, R, Matijevic, E. Journal of Colloid and Interface Science 1992 ; 151 : 555. 9. McDowell, RW, Sharpley, AN. Geoderma 2003 ; 112 : 143. 10. McDowell, RW, Mahieu, N, Brookes, PC, Poulton, PR. Chemosphere 2003 ; 51 : 685. 11. Kudeyarova, AY, Trubin, AI. Proc. Int. Semin. Soil Environ. Fertil. Manage. Intensive Agric. 1977: 816.
9 12. Malunda, JJ. (2000) PhD Univ. of California,Davis,CA,USA. Dissolution of synthetic strengite (FePO4.2H2O) and synthetic variscite (AlPO4.2H2O) as functions of pH and citrate level 2000. 13. Griffioen, J. Environmental Science and Technology 1994 ; 28 : 675. 14. Hsu, PH, Jackson, ML. Soil Science 1960 ; 90 : 16. 15. Frost, RL, Martens, W, Williams, PA, Kloprogge, JT. Mineralogical Magazine 2002 ; 66 : 1063. 16. Frost, RL, Martens, WN, Kloprogge, T, Williams, PA. Neues Jahrbuch fuer Mineralogie, Monatshefte 2002: 481. 17. Frost, RL, Williams, PA, Martens, W, Kloprogge, JT, Leverett, P. Journal of Raman Spectroscopy 2002 ; 33 : 260. 18. Farmer, VC Mineralogical Society Monograph 4: The Infrared Spectra of Minerals , 1974. 19. Povarennykh, AS, Gevork'yan, SV. Dopovidi Akademii Nauk Ukrains'koi RSR, Seriya B: Geologiya, Geofizika, Khimiya ta Biologiya 1973 ; 35 : 597. 20. Tarte, P, Paques Ledent, MT. Bulletin de la Societe Chimique de France 1968: 1750. 21. Taylor, AW, Gurney, EL. Soil Science 1964 ; 98 : 9. 22. Taylor, AW, Lindsay, WL, Huffman, EO, Gurney, EL. Soil Science Society of America Proceedings 1963 ; 27 : 148. 23. Zhang, M, Alva, AK, Li, YC, Calvert, DV. Soil Science 2001 ; 166 : 940. 24. Frost, RL, Crane, M, Williams, PA, Kloprogge, JT. Journal of Raman Spectroscopy 2003 ; 34 : 214. 25. Frost, RL, Williams, PA, Martens, W. Mineralogical Magazine 2003 ; 67 : 103. 26. Martens, W, Frost, RL, Kloprogge, JT. Journal of Raman Spectroscopy 2003 ; 34 : 90. 27. Martens, W, Frost, RL, Kloprogge, JT, Williams, PA. Journal of Raman Spectroscopy 2003 ; 34 : 145. 28. Frost, RL, Martens, W, Kloprogge, JT, Williams, PA. Journal of Raman Spectroscopy 2002 ; 33 : 801. 29. Fransolet, AM. Bulletin de Mineralogie 1987 ; 110 : 647. 30. Greben'ko, NV, Eshchenko, LS, Pechkovskii, VV. Izvestiya Akademii Nauk SSSR, Neorganicheskie Materialy 1978 ; 14 : 136. 31. Kudeyarova, AY, Trubin, AI. Agrokhimiya 1978: 18.
10
11 variscite strengite phosphoside phosphos metavaris Suggested rite ideite cite assignmen ts M37991 M44244 M38005 M33897 M31413 Raman Raman IR IR Raman IR Raman Raman IR IR Raman IR published publish published publish 18 ed 18 ed 18 18 298 77 K 298 K 77 K 298 K 77 K 298 K 77 K 298 K 77 K 3580 3588 3565 3377 ~3500 ~3300 OH 3564 3110 3130 ~3300 ~3000 stretching 3282 2945 ~3100 vibrations 3094 3059 2945 2910 1886 1672 1585 1886 ~1600 1617 1887 1594 1887 1622 ~1660 1887 ~1670 HOH 1573 1577 1620 1603 1613 1583 1628 bending 1359 1528 1453
1250 1361 1155 1160 1357 1337 1125 1250 1247 1253 1372 1118 1160 1362 1170 PO 4 1157 1248 1117 1075 1250 1249 1024 1033 1154 1204 1353 1049 1100 1249 1120 antisymm 1133 1157 1083 1050 1158 1236 1009 1043 1200 1271 1020 1020 1150 1100 etric 1077 1131 1067 1137 1152 993 1032 1156 1215 991 1081 1029 1080 1031 1008 1140 1195 1063 1023 1059 992 1005 992 1034 1144 1033 1005 1046 1010 1126 1018 1030 1024 790 803 870 870 744 870 790 775 850 840 Water 613 615 799 800 694 760 756 790 740 librational 604 779 705 650 700 698 646 660 651 612 646 577 637
563 568 568 515 560 586 616 643 PO 4 534 562 450 487 545 548 544 489 579 585 Out of 483 536 447 520 488 497 539 574 plane 464 488 485 452 479 553 bends 437 467 475 435 460 499 444 400 448 460
12 424 405 446 427 400 387 419 418 420 434 430 456 453 PO 4 389 400 398 400 In plane 356 394 bends 340 387 374 367 360 353 347 340 317 389 383 386 327 380 326 329 303 349 365 322 374 299 302 249 333 352 367 288 297 311 339 360 258 290 315 353 237 273 347 258 340 253 329 244 239 230 227 211 204 290 291 292 302 177 201 172 277 286 260 297 143 187 153 259 276 243 290 122 180 135 225 263 223 280 171 229 273 152 258 147 253 133 244 124 239 230
203 204 205 153 211 185 180 187 201 156 160 172 187 129 153 153 171 134 135 152 129 147 117 133 124
13 Table 1 Raman and infrared spectroscopic analysis of strengite, variscite, metavariscite and phosphosiderite
14 List of Figures
Figure 1 Raman spectra of the 900-1200 cm -1 region of strengite, variscite and phosphosiderite at 298 K.
Figure 2 Raman spectra of the 900-1200 cm -1 region of strengite, variscite, metavariscite and phosphosiderite at 77 K.
Figure 3 Raman and infrared spectra of Variscite and strengite.
Figure 4 Raman spectra of the low wavenumber region of strengite, variscite, metavariscite and phosphosiderite at 298 K.
Figure 5 Raman spectra of the low wavenumber region of strengite, variscite, metavariscite and phosphosiderite at 77 K.
List of Tables
Table 1 Raman and infrared spectroscopic analysis of strengite, variscite, metavariscite and phosphosiderite
15
Figure 1
298 K
Strengite Raman Intensity Variscite
Phosphosiderite
1200 1150 1100 1050 1000 950 900 Wavenumber /cm -1
16 77 K
Strengite
Variscite RamanIntensity
Metavariscite
Phosphosiderite
1200 1150 1100 1050 1000 950 900 Wavenumber /cm -1
Figure 2
17 IR
Intensity 298K Raman
77K Raman
1300 1200 1100 1000 900 800 700 Wavenumber /cm -1
Figure 3a
18
ATR-IR
298K Raman Relative Intensity Relative
77K Raman
1200 1100 1000 900 800 700 Wavenumber /cm -1
Figure 3b
19
Figure 4
298 K
Strengite
Variscite Raman Raman Intensity
Metavariscite
Phosphosiderite
700 600 500 400 300 200 100 Wavenumber /cm -1
20 77 K
Strengite
Variscite
Metavariscite Raman Intensity
Phosphosiderite
700 600 500 400 300 200 100 Wavenumber /cm -1
Figure 5
21
22