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Variscite (AlPO4 2H2O) from Cioclovina Cave (Åžureneau Mountains, Romania): A tale of a missing phosphate

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Variscite (AlPO4 2H2O) from Cioclovina Cave (Åžureneau Mountains, Romania): A tale of a missing phosphate
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Cinta Panzaru, Simona
Kearns, Joe
Breban, Radu
Onac, Bogdan P.
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Cioclovina Cave (Romania) ( 45.571, 23.231 )
Geology ( local )
Anthropology ( local )
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Romania
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45.571 x 23.231
46 x 25

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Recent investigations on a phosphatized sediment sequence in the Cioclovina Cave led to the identification of a second occurrence in Romania (first time in the cave environment) of variscite, AlPO4 2H2O. The mineral exists as dull-white, tiny crusts and veinlets within the thick argillaceous material accumulated on the cave floor. Under scanning electron microscope (SEM) variscite appears as subhedral to euhedral micron-size crystals. The {111} pseudo-octahedral form is rather common. Variscite was further characterized by means of X-ray diffraction, thermal, vibrational FT-IR and FT-Raman spectroscopy, and by SEM energy-dispersive spectrometry (EDS). The calculated orthorhombic cell parameters are a = 9.823(4), b = 8.562(9), c = 9.620(5) ?, and V = 809.167(6) A3. The ED spectrum of variscite shows well-resolved Al and P lines confirming thus the presence of the major elements in our compound. The formation of variscite is attributed to the reaction between the phosphate-rich leachates. -- Authors
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Description
Recent investigations on a phosphatized sediment
sequence in the Cioclovina Cave led to the identification of
a second occurrence in Romania (first time in the cave
environment) of variscite, AlPO4 2H2O. The mineral exists as
dull-white, tiny crusts and veinlets within the thick
argillaceous material accumulated on the cave floor. Under
scanning electron microscope (SEM) variscite appears as
subhedral to euhedral micron-size crystals. The {111}
pseudo-octahedral form is rather common. Variscite was
further characterized by means of X-ray diffraction, thermal,
vibrational FT-IR and FT-Raman spectroscopy, and by SEM
energy-dispersive spectrometry (EDS). The calculated
orthorhombic cell parameters are a = 9.823(4), b = 8.562(9),
c = 9.620(5) ?, and V = 809.167(6) A3. The ED spectrum of
variscite shows well-resolved Al and P lines confirming thus
the presence of the major elements in our compound. The
formation of variscite is attributed to the reaction between
the phosphate-rich leachates. --
Authors



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STUDIA UNIVERSITATIS BABE -BOLYAI, GEOLOGIA, XLIX, 1, 2004, 3-14 VARISCITE (AlPO 4 H 2 O) FROM CIOCLOVINA CAVE ( UREANU MOUNTAINS, ROMANIA): A TALE OF A MISSING PHOSPHATE BOGDAN P. ONAC 1 JOE KEARNS 2 RADU BREBAN 3 SIMONA CNT PNZARU 4 ABSTRACT. Recent investigations on a phosp hatized sediment sequence in the Cioclovina Cave led to the identification of a second occurrence in Romania (first time in the cave environment) of variscite, AlPO 4 H 2 O. The mineral exists as dullwhite, tiny crusts and veinlets within the thick argillaceous material accumulated on the cave floor. Under scanning electron microscope (SEM) variscite appears as subhedral to euhedral micron-size crystals. The {111} pseudo-octahedral form is rather common. Variscite was further char acterized by means of X-ray diffraction, thermal, vibrational FT-IR and FT-R aman spectroscopy, and by SEM energydispersive spectrometry (EDS). The ca lculated orthorhombic cell parameters are a = 9.823(4), b = 8.562(9), c = 9.620(5) and V = 809.167(6) 3 The ED spectrum of variscite shows well-resolved Al and P lines confirming thus the presence of the major elements in our compound. The formation of variscite is attributed to the reaction between the phosphate-rich leachates derived from guano and the underlying clay sediments. Keywords: phosphate minerals, variscite, Cioclovina Cave, Romania. GENERAL DATA The Cioclovina Cave is located 40 km southeast of the city of Hunedoara, in the west-southwest side of ureanu Mountains, Romania (Fig. 1, inset), at an altitude of 770 m asl. The investigated par t of the cave develops in reef limestone of Lower Cretaceous age (Stilla, 1981; P op et al., 1985). The total length of the cave is 1400 m (Breban et al., 2003), but only the first 450 m (Fig. 1) are of great interest with respect to the occurrence of authigenic phosphate minerals. Mining operations undertaken in this cave during the first half of the 20 th century have exposed impressive outcrops of interstratified alluvial sediments (clay, sand, and gravels) that were subsequently phosphatized through the action of P-rich solutions provided by the br eakdown of guano and bones. Depending on whether the percolating water passing thr ough guano reacts with carbonate rocks or clay minerals, Ca-, Mg-, and Al-rich, or K-Fe phosphates have been deposited. Over the last five years, several descriptive and/or detailed mineralogical investigations were undertaken, each of them pointing out a remarkable rich and divers assemblage of authigenic phosphate minerals. So far, 15 phosphate minerals were described by Schadler (1929, 1932), Halla (1931), Constantinescu et al. (1999), Dumitra & Marincea (2000), Marincea et al. (2002, 2003), Breban (2002), Onac et al. (2002), Onac & White (2003), and Dumitra et al. (2004a, b). 1 Department of Mineralogy, Babe -Bolyai University & Emil Racovi Institute of Speleology, Clinicilor 5, 400006, Cluj, Romania (bonac@bioge.ubbcluj.ro). 2 Materials Research Institute, The Pennsylvania State University, University Park, PA 16801, U.S.A. 3 S.C. Ro ia Montan Gold Corporation S. A., Ro ia Montan Romania. 4 Department of Molecular Spectroscopy, Babe -Bolyai University, Kog lniceanu 1, 400084, Cluj, Romania.

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BOGDAN P. ONAC, JOE KEARNS, RADU BREBAN, SIMONA CNT PNZARU 4 Variscite, ideally (AlPO 4 H 2 O), is a relatively common cave mineral (Nriagu & Moore, 1984; Hill & Forti, 1997), but its presence in the Cioclovina Cave was not reported so far in any of the prev ious studies, although, five other Al-rich phosphates have been documented from this cave. When berlinite (AlPO 4 ) was identified, the authors hypothesized the possibility that variscite acted as precursor and its dehydration/calcination (due to guano combustion) ultimately led to berlinite (Onac & White, 2003). However, the identification of variscite in the phosphate association close to the berlinite occurrence, failed, and therefore its presence was simply a tale. The focus of the present paper is to characterize the variscite sample we recently identified in Cioclovina Cave The only other identified occurrence of variscite in Romania is from the Iacobeni manganese deposit (B lan, 1976) hence, this study is the first to report vari scite in a Romanian cave environment. OCCURRENCE AND PHYSICAL PROPERTIES In the room located just across the plac e where the artificial gallery penetrates the cave (Fig. 1), a huge deposit of phosphatizated argillaceous material (clays and silty-clays) was exposed during the mining activities. On the upper part of this outcrop, within the clay-rich sequence, crus ts and veinlets of dull white clay-like aggregates, intimately associated with detrital quartz were sampled. The mineral is brittle, shows waxy luster, possesses a splintery fracture, and white streak. The thickness of these occurren ces never exceeds 3-4 mm. Fig. 1. Location of the Cioclovina Cave (inset ) and the map of the near-entrance cave passage showing the location of the sampling points.

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VARISCITE (AlPO4H2O) FROM CIOCLOVINA CAVE ( UREANU MOUNTAINS, ROMANIA) 5 Carbon-coated fragments of variscite samples examined by a JEOL JSM 5510LV scanning electron microscope (SEM ) showed the aggregates are generally composed of tiny euhedral and subhedral crystals that never exceed 2 m across (measured along the c axis) (Fig. 2, left). Nicely developed individual crystals having pseudo-octahedral habit {111} can also be observed (Fig. 2, right). Fig. 2. General SEM image of variscite masses (left) and details of well developed pseudo-octahedral {111} crystals (right). Several qualitative chemical analyses were performed using the energydispersive X-ray spectrometer facility of the SEM. Invariably, the obtained spectra showed well-resolved Al and P lines (Fig. 3). Because the only possible ambiguity between K lines of these elements is that with L lines of Br and Zr, respectively, we are confident our compound is an aluminum phosphate. The Si peak it is not surprising considering the material with which the investigated mineral is intermixed. Fig. 3. ED spectrum of variscite.

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BOGDAN P. ONAC, JOE KEARNS, RADU BREBAN, SIMONA CNT PNZARU 6 Quantitative analyses were conducted with an inductively coupled plasmaatomic emission spectrometer (ICP-AES, Jobin-Yvon ULTIMA) for K, Mn, Mg, Na, Ti, and Fe and with an Jeol JSM-6310 (equipped with EDand WD-spectrometer) electron microprobe (EMP-Oxford) for Al, P, Ca, Si, and Fe, operated at an acceleration voltage of 15 kV and a beam current of 10 nA with corundum (Al), YPO 4 (P), adular (K), apatite (Ca), and garnet (Fe) as standards. The crystal was analyzed at 5 different points. The results of the chemical analyses are listed in Table 1. The empirical formula for variscite is: Al 0.695 P 0.683 O 4 H 2 O. Table 1 The composition of variscite as obtained from combined ICP-AES and EMP analyses (in brackets the standard deviations; n = 5). 1 2 3 Al 2 O 3 P 2 O 5 CaO SiO 2 FeO MnO MgO K 2 O Na 2 O TiO 2 H 2 O 35.47 48.47 1.54 0.57 0.69 0.04 0.27 0.36 0.09 0.06 10.61 98.18 33.87-36.91 (0.1) 44.25-49.57 (0.11) 0.67-2.21 (0.03) 0.43-0.69 (0.3) 0.42-0.84 (0.06) 0.695 0.683 0.055 0.019 0.019 0.001 0.013 0.015 0.006 0.002 2.219 1. mean analytical results for variscite wt% (ICP-AES) 2. electron microprobe ranges 3. cations based on O = 4 X-RAY CRYSTALLOGRAPHY X-ray data on several samples were obtained by means of Scintag Pad V and Philips X-pert diffractometers using Cu K radiation ( = 1.5406 ). NBS 640b Si was used as internal standard. Peak positions were determined by fitting the numerical profiles with a Pearson VII func tion. Operating conditions were 40 kV and a beam current of 40 mA, and 40 kV at 30 mA, respectively. All samples were continuously scanned from 3 to 85 and 5 to 70 2 with a step scan of 0.01 2 and 2s per step. The diffraction spectra obt ained with both instruments are almost identical to the one illustrated in Fig. 4. Cell dimensions were calculated as average of two least-square refinements of 45 corrected d values (Tab. 2) using the UnitCell Program of Holland & Redfern (1997). The orthorhombic unit cell (space group Pbca ) was found to be: a = 9.823(4), b = 8.562(9), c = 9.620(5) and V = 809.167(6) 3

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VARISCITE (AlPO4H2O) FROM CIOCLOVINA CAVE ( UREANU MOUNTAINS, ROMANIA) 7 Table 2 Comparison between the powder diffraction pattern of Cioclovina variscite and the standard reference pattern from ICDD file 33-33. Variscite (this study) Variscite (ICDD card 33-33) Nr. crt. d meas. () d calc. () I/I 0 d () I/I 0 1 5.3604 5.3599 72 5.36 65 2 4.812 4.8102 43 4.815 25 3 4.269 4.2604 100 4.26 70 4 3.908 3.911 32 3.903 25 5 3.634 3.633 15 3.632 20 6 3.195 3.197 11 3.190 7 7 3.043 3.0408 55 3.041 100 8 2.914 2.914 22 2.914 45 9 2.877 2.8719 20 2.871 35 10 2.633 2.6358 24 2.633 25 11 2.566 2.566 9 2.564 9 12 2.475 2.483 15 2.483 20 13 2.449 2.455 8 2.457 12 14 2.4029 2.405 5 2.406 2 15 2.397 2.3903 5 2.390 8 16 2.337 2.336 6 2.337 12 17 2.2907 2.292 3 2.293 5 18 2.211 2.213 3 2.213 1 19 2.141 2.1405 8 2.1401 12 20 2.100 2.0996 4 2.097 4 21 2.085 2.0834 5 2.0839 13 22 2.054 2.0506 3 2.054 6 23 2.026 2.020 4 2.0202 6 24 1.962 1.9622 9 1.9608 14 25 1.945 1.947 7 1.947 10 26 1.917 1.918 5 1.918 10 27 1.845 1.8439 4 1.8447 6 28 1.784 1.786 3 1.786 3 29 1.753 1.755 6 1.755 13 30 1.727 1.7223 4 1.7225 4 31 1.651 1.644 1 1.644 2 32 1.6002 1.608 6 1.608 6 33 1.596 1.5919 6 1.591 18 34 1.578 1.5782 4 1.578 8 35 1.563 1.564 6 1.563 4 36 1.525 1.5251 7 1.5247 3 37 1.503 1.4989 3 1.497 4 38 1.4716 1.484 1 1.483 1 39 1.4447 1.447 7 1.446 13 40 1.432 1.431 4 1.4309 6 41 1.411 1.411 2 1.4109 4 42 1.4035 1.404 2 1.404 6 43 1.395 1.397 2 1.396 3 44 1.354 1.355 2 1.353 1 45 1.3434 1.342 3 1.3434 4

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BOGDAN P. ONAC, JOE KEARNS, RADU BREBAN, SIMONA CNT PNZARU 8 0 100 200 300 400 500 600 700 800 900 1000 1100 0 10 20 30 40 50 60 70VarisciteCioclovina Cave counts /a.u.2 theta /deg.raw data Fig. 4. X-ray diffraction pattern of vari scite from the Cioclovina Cave. THERMAL ANALYSIS Simultaneous thermogravimetric analys is (TGA) and differential thermal analysis (DTA) were performed on about 21 mg of the sample in a Mettler Toledo instrument. The temperature ranged from 30 to 1000C, using a constant flow of nitrogen (100 mL/min) and a heating rate of 5C/min. TGA/DTA trace obtained for variscite is reproduced in Fig. 5. The TG curve shows one single dehydration step. The weig ht loss of 10.0 % that occurs between 90 and 145C was due to the expulsion of the two molecules of water. The corresponding endothermic effect appears at 120C on the DTA curve (Fig. 5). Our TG analysis indicated a total amount of 2.1 H 2 O groups per AlPO 4 unit, i.e., slightly more than the expected stoichiometric amount. This extra water (0.1) was attributed to water absorption on the parti cles surface. A similar situation was observed by Reale & Scrosati (2003) when investigating strengite, FePO 4 H 2 O. INFRARED ABSORPTION SPECTRUM Fourier-transform infrared spectra were obtained with a Perkin-Elmer 1760X FT-IR spectrometer middle (NIR spectral range, 400 4000 cm -1 ) using the KBr pellet technique. The spectral resolution was 2 cm -1 FT-IR spectrum of the dull white phosphate material (Fig. 6) presents the characteristic bands assigned to variscite (Lehr et al., 1967).

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VARISCITE (AlPO4H2O) FROM CIOCLOVINA CAVE ( UREANU MOUNTAINS, ROMANIA) 9 Fig. 5. TGA and DTA curves of variscite (sample #1535a) from Cioclovina Cave. The bands observed in the spectral range between 2500 and 3500 cm -1 (3300, 3195, 3108, and 2945) are due to symmetric or asymmetric O-H stretching of water (Ross, 1974; Socrates, 2001; Fros t & Weier, 2004). The two very sharp peaks at 3583 cm -1 and 1384 cm -1 respectively, can tentatively be ascribed to Ncontaining organic compounds (aromatic or aliphatic) that could be present as contaminants of the title compound. This la st band is a stretching mode specific for the -C=Nbonds or C-N-C from aromatic rings. Amine/amide-containing organic compounds exhibit also strong bands in IR spectra in the 3100-3500 cm -1 spectral range, assigned as N-H stretching modes, whereas the bending modes are located around 1600 cm -1 They are weak to medium in the Raman spectra. Variscite absorption bands at ~1600 and 1574 cm -1 are located in the water deformation region, and their presence was interpreted by Salvador & Fayos (1972) as indicating the existence of two different types of water molecules. The nature of the two water molecules in variscite has always been of special interest to scientists (Kniep et al., 1977; Falk, 1984). However, in our variscite sample the DTA and TG curves do not show a two-step wa ter loss so that this assumption is not substantiated. It seems more likely to accept Pques-Ledent & Tarte (1969) opinion that suggests the presence of OH and H 3 O + in the variscite structure. The accuracy in precisely locating the hydrogen atoms is very poor and usually the O water -H distances are shorter than the theoretical one (~0.96 ). It is believed that three of the H atoms fo rm single hydrogen bonds to phosphate O atoms, whereas the fourth does not partic ipate in a hydrogen bond (Kniep et al., 1977). Considering our case if we assume that the 1575 cm -1 bending mode and

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BOGDAN P. ONAC, JOE KEARNS, RADU BREBAN, SIMONA CNT PNZARU 10 one of the 2500 to 3500 cm -1 stretching mode bands are associated to the same water molecules, at least one of its tw o hydrogen atoms should not be bonded to the structure. Such an interpretation is in good agreement with the studies undertaken for the isotypic indium and galliu m analogs of variscite (Mooney-Slater, 1961, 1966; Loiseau et al. 1998), and with the more recent investigations on hydrogen bond lengths (Libowitzky, 1999) and valence bond calculations (Brese & OKeeffee, 1991; Huminiki & Hawthorne, 2002). Fig. 6. FT-IR spectrum of variscite (sample #1535a). Variscite absorption bands at 1160, 1063, and 934 cm -1 are comparable in position and intensity to other Al-phosphates compiled by Lehr et al. (1967) and Ross (1974). According to Adler (1964, 1968 ) the number of infrared-active modes in this part of the spectrum depends on the PO 4 3group symmetry. The two bands at 1160 and 1063 cm -1 assignable to the PO 4 3: 3 asymmetric mode, and a unique band at 934 cm -1 (PO 4 3: 1 symmetric stretching) may indicate a C 3v symmetry for at least one of the two independent PO 4 3anions. The following four significant absorption bands occur at 866, 799, 646, and 574 cm -1 These wave numbers correspond to vibrational modes involving M-OH 2 but PO 4 3: 4 can also contribute to bands between 500 and 600 cm -1 (Tarte, 1967; Salvador & Fayos, 1972; Ross, 1974). The IR absorption spectrum between 550 and 400 cm -1 is dominated by bands at 514, 449, 422, and cm -1 which were interpreted (based on comparison with other published data) as being due to O-P-O ( 4 in-plane; 2 out-of plane) bending modes.

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VARISCITE (AlPO4H2O) FROM CIOCLOVINA CAVE ( UREANU MOUNTAINS, ROMANIA) 11 RAMAN SPECTROSCOPY OF VARISCITE The FT-Raman spectrum of variscite was recorded using an Equinox 55 FT-IR Bruker spectrometer with an integrated FRA 106 S Raman module. For the excitation of the spectra the 1064 nm line from a Nd:YAG laser was used with an output power of 350 mW. The InGaAs detector was used. For each spectrum 200 scans were accumulated. The spectral resolution was 4 cm -1 Table 3 lists the wavenumbers, intensity, and the assignment s of the identified bands. The positions of the bands observed on the variscite spectrum are in agreement with the characteristic Raman data of the phosphates (Socrates, 2001). Table 3 Vibrational Raman data of varisicte /cm -1 and their assignment (s-strong, m-medium, w-weak, sh-shoulder). Wavenumber (cm -1 ) & Intensity Assignments 3400-3100 broad band OH asym, sym, stretch, Water stretching modes 1634 wm OH bend, water bending mode 1079 sh PO 4 3, often complex, broad P=O stretch 1055 m P-O stretch, depending on the inductive effect of the substituent 1026 m P-O stretch 605 w 562 w 434 m-s 225 m Phosphate bending 168 sh Lattice vibrations 144 m Lattice vibrations The most intense bands in the stre tching vibrations region of the PO 4 3units (all assigned to the 3 ) are at 1023 cm -1 and 1055 cm -1 with less defined other component bands. According to Frost et al. (2004), the number of bands observed in the stretching (symmetric and antisymmetric vibrations) regions of the PO 4 3units indicates a combination of two effects: symmetry reduction and multiple PO 4 species. Based on the later one, individual crystals of the variscite mineral group (but not only) can readily be determined by Raman spectroscopy. CONCLUSIONS The formation of variscite is attributed to the reaction between the phosphate-rich leachates derived from thick guano deposits and the underlying Alrich residual or water-laid sediments under acid conditions. With continued leaching and in the presence of excess alkali, crandallite may be the dominant phase. In fact, there are evidences from different locations worldwide that variscite is replaced by crandallite (Alt schuler, 1973). Therefore, a similar scenario could be applicable to the geochemical situation within the Cioclovina Cave, where crandallite has already been identified by Constantinescu et al. (1999).

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BOGDAN P. ONAC, JOE KEARNS, RADU BREBAN, SIMONA CNT PNZARU 12 Powder X-ray diffraction of the dull white crusts and ve inlets recovered from a phosphatized sequence indicate t he presence of variscite, of which orthorhombic cell was found to be in good agreement with other published results on this mineral. The presence of this Al -phosphate was also c onfirmed by a set of qualitative (EDS) and quantitative (ICP-AES and EMP) analyses. Additionally, the SEM investigations on crystal morphology, along with thermal and vibrational IR and Raman analyses confirm the existence of variscite in this unique Romanian cave. Acknowledgements. Permission for entering and sampling the Cioclovina Cave phosphate deposit was granted by the Romanian Academy through the Romanian Commission of Natural Protection. Part of this research was supported by MEC-CNCSIS grants 101/1696 to B. P. Onac. L. Zaharia first tested the FT-IR spectrum of variscite at Institute of Mi neralogy and Crystallography, University of Wien. D. Keravis and D. Vere are thanked for the ICP-AES analyses at Institute des Sciences de la Terre, Universit d'Orlans (CNRS). The EMP analyses were conducted at the Institute of Mineralogy and Petrology in Graz by the senior author. The help of K. Ettinger with EMP analyses is acknowledged. Useful suggestions were provided by two anonymous referees during the review process. Variscite specimen is preserved in the Cave Minerals collection of the Mineralogical Museum, Babe -Bolyai University in Cluj R E F E R E N C E S Adler, H. H. 1964, Infrared spectra of phosphate minerals: symmetry and substitutional effects in the pyromorphite series. The American Mineralogist, 49: 1002-1015. Adler, H. H. 1968, Infrared spectra of phosphate minerals: splitting and frequency shifts associated with substitution of PO 4 for AsO 4 3in mimetite. The American Mineralogist 53: 1740-1744. Altschuler, Z. S. 1973, The weathering of phosphate deposits-geochemical and environment aspects. In Environmental phosphorus handbook (Griffith, E. J., Beeton, A., Spencer, J. M., Mitchell, D. T ., Eds.), New York: John Wiley & Sons, 33-96. B lan, M. 1976, Mineralogia z c mintelor manganifere de la Iacobeni. Ed. Academiei R. S. Romnia, Bucure ti, 124 pp. Breban, R. C. 2002. Studiul fosfa ilor din Pe tera Ciclovina Uscat (Mun ii ureanu) B.Sc. Thesis. University of Cluj, 92 pp. Breban, R., erban, M., Viehmann, I., B icoan M. (Eds.) 2003, Istoria exploat rii de guanofosfat i a descoperirii omului fosil din pe terile de la Ciclovina (Hunedoara). Clubul de Speologie Proteus, Hunedoara, 138 pp. Brese, N. E., O'Keeffe, M. 1991, B ond-valence parameters for solids. Acta Cryst. B47: 192-197. Constantinescu, E., Marincea, ., Cr ciun, C. 1999, Crandallite in the phosphate association from Cioclovina cave ( ureanu Mts., Romania). In Mineralogy in the system of the earth sciences (Anastasiu, N., Ilinca, G., Eds.), London: Imperial College Press, 1-5.

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VARISCITE (AlPO4H2O) FROM CIOCLOVINA CAVE ( UREANU MOUNTAINS, ROMANIA) 13 Dumitra D., Marincea, 2000, Phosphates in the bat guano deposit from the "Dry" Cioclovina Cave, ureanu Mountains, Romania. Rom. J. Mineral. Dep. 79 (1): 43-45. Dumitra D. G., Marincea, ., Fransolet, A. M. 2004 a, Brushite in the bat guano deposit from the "dry" Cioclovina Cave ( ureanu Mountains, Romania). N. Jb. Miner. Abh. 180 (1): 4564. Dumitra D.-G., Hatert, F., Bilal, E., Marincea, 2004 b, Gypsum and bassanite in the bat guano deposit from the "Dry" Cioclovina Cave ( ureanu Mountains, Romania). Rom. J. Mineral. Dep. 81: 84-87. Falk, M. 1984, The frequency of the H---O---H bending fundamental in solids and liquids. Spectrochimica Acta A40: 43-48. Frost, R. L., Weier, M. L. 2004, Vibrational spectroscopy of natural augelite. Journal of Molecular Structure 697: 207-211. Frost, R. L., Weier, M. L., Erik son, K. L., Carmody, O., Mills, S. J. 2004, Raman spectroscopy of phosphates of the variscite mineral group. J. Raman Spectrosc. 35 (12): 1047-1055. Halla, F. 1931, Isomorphe Beziehungen und Doppelsalzbildung zwischen Gips und Brushit. Z. Krist., 80: 349-352. Hill, C. A., Fo rti, P. 1997. Cave minerals of the world (2 nd ed.) National Speleological Society, Huntsville, Alabama, pp. 163-176. Holland, T. J. B., Redfern, S. A. T. 1997, Unit cell refinement from powder diffraction data: the use of regression diagnostics. Mineral. Mag. 61: 65-77. Huminicki, D. M. C., Hawthorne, F. C. 2002, The crystal structure of the phosphate minerals. In Phosphates geochemical, geobiological, and materials importance (Kohn, M. J., Rakovan, J., Hughes, J. M., Eds.), In Reviews in Mineralogy & Geochemistry 48, Washington: Mineralogical Society of America / Geochemical Society, 123-253. Johan, Z., Slansky, E., Povondra, P. 1983, Vashegyite, a sheet aluminum phosphate: new data. The Canadian Mineralogist 21: 489-498. Kniep, R., Mootz, D., Vega s, A. 1977, Variscite. Acta Cryst. B33: 263-265. Lehr, J. R., Brown, E. H., Frazier, A. W., Smith, J. P., Thrasher, R. D. 1967. Crystallographic properties of fertilizer compounds National Fertilizer Development Center, Muscle Shoals, AB., 166 pp. Libowitzky, E. 1999, Correlation of O-H st retching frequencies and O-HO hydrogen bond length in mineral. Monatshefte fr Chemie 130: 1047-1059. Loiseau, T., Paulet, C., Frey, G. 1998, Crystal structure determination of the hydrated gallium phosphate GaPO 4 H 2 O, analog of variscite. C. R. Acad. Sc. Paris, II(1): 667-674. Marincea, ., Dumitra D. G. 2003, The occurrence of tarana kite in the "dry" Cioclovina Cave ( ureanu Mountains, Romania). N. Jb. Miner. Mh. 2003 (3): 127-144. Marincea, ., Dumitra D., Gibert, R. 2002, Tinsleyite in the "dry" Cioclovina Cave ( ureanu Mountains, Romania): the second occurrence. Eur. J. Mineral. 14: 157-164. Mooney-Slater, R. C. L. 1961, X-ray diffrac tion study of indium phosphate dihydrate and isostructural thallic compounds. Acta Cryst., 14: 1140-1146. Mooney-Slater, R. C. L. 1966, The crystal structure of hydrated gallium phosphate of composition GaPO 4 H 2 O. Acta Cryst., 20: 526-534. Nriagu, J. O., Moore, P. B. (Eds.) 1984, Phosphate minerals. Springer-Verlag, Berlin, 442 pp.

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