MICROSCOPICAL AND XRD STUDY ON THE GEMS FROM THE AREA WITHIN GURASADA LOCALITY (HUNEDOARA COUNTY)

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1 MICROSCOPICAL AND XRD STUDY ON THE GEMS FROM THE AREA WITHIN GURASADA LOCALITY (HUNEDOARA COUNTY) Ciprian CONSTANTINA 1, Terry MOXON 2 1 Romaltyn Exploration Baia Mare, cconstantina@yahoo.com; 2 Department of Earth Sciences, University of Cambridge, UK Geological background Gurasada locality is situated north from Mureş culoir, in South Apuseni Mountains, and from an administrative point of view belongs to Hunedoara County. The rocks making up this area are represented by laramian volcanic rocks (lava flows, pyroclastic breccia and rare levels of tuffs) and were radiometric-dated as Senonian age (Constantina, 2008). There is a bentonite deposit, near Gurasada, which resulted from the argillization of a volcanic tuff having an age older than that of the previously mentioned rocks. The area is crossed by Gurasada valley, which together with its tributaries, concentrates miscellaneous gems from the silica group (agates, chalcedonies, jasper, silicified wood and opal). Microscopical study Usually, agates and chalcedonies are lightly coloured, with the central part most frequently represented by white-milky crystalline quartz that grades into microcrystalline and fibrous silica of grey and red colours. Microscopically, the quartz microcrystals have been classified according to their structure; the granular one prevails, and in addition, fibrous and lamellar varieties have been identified. The microscopic structures evidenced in our samples were represented by: granular structures, structures based on long fibres, fan-like structures, rosette-like structures, hematite interlayers (fig 1). As regards the jasper, the colour is mainly given by oxides and iron hydroxides (hematite). The microscopic study of silicified wood highlights the vegetal (cellular) structure of the wood fully replaced by silica (fig. 2). 49

2 Fig. 1. Chalcedony consisting of fibrous silica (rosettes) and microgranular and fibrous (rosette-fan type) silica (Microphotograph, N, 35 X); Tisei V. Fig. 2. licified wood with visible vegetal structure cross section (Microphotograph, 1N, 35 x);câmpuri Brook. X-ray diffraction study of the gems Besides the identification of the mineral species, we have also tried a novel interpretation for Romania, which connects the age of the agates (with possible implications on the age of their host rocks) to the quartz crystallinity degree. Thus, we have applied the calibration method proposed by Moxon (2002), tested and completed in further studies (Moxon and Rios, 2004; Moxon and Reed, 2006; Moxon et al., 2006). The mineralogenetic mechanism that explains the basis of this interpretation is related to the transformation, at geologic scale, of moganite into. The correlation of these evolutionary trends, evidenced in a large number of representative samples, carefully selected from world wide occurrences, allowed the elaboration of a quantitative work hypothesis concerning the agate formation and evolution, schematically presented in fig. 3. The first set of measurements was taken for moganite identification in the θ interval, at 0,01 0 for 20 sec/step. In the investigated samples we have noticed a moganite doublet around 20 o 2θ. It is worth noticing that all the diffractograms on our samples display an additional phase of approximately θ, which does not allow a semi-quantitative routine assessment for the moganite content. Nevertheless, by comparing them with other diffractograms of the same type, we estimate a moganite concentration of around 5%. 50

3 Early stage Intermediary stage Final stage Age of the host rock: ~ 275 My > 400 My The amount of internal water decreases due to the internal water free water conversion Free water (constant) Free water (constant) Free water (constant) Internal water (~ 1 %) moganite ( ~ 13 %) (~ 86 %) Fig. 3. Scheme for the quantitative evaluation of the agates age based on the content of internal water, and moganite, and crystal. internal standard and measurements in the θ interval, at 0,02 0 for 10 sec/step were used at the the second set for the identification of the mineral phases. In samples Ro 156wh and Ro 158wh representing the white areas of the corresponding chalcedonies, the peaks of an additional phase have been noticed, around the values of 22 and 36 o 2θ. We assume that this phase is represented by cristobalite, which has the main peak at o 2θ, followed by three less intense peaks at 28,32; and respectively 36,08 o 2θ (according to the ICDDD - International Centre for Diffraction Data on synthetic cristobalite). The values for the natural opal-c are close to these reference ones (fig. 4). ~ 320 Ǻ Internal water (~ 0.7 %) moganite (~ 6 %) ( ~ 93 %) ~ 470 Ǻ Internal water (0.4 %) moganite 0 % (~ 99 %) ~ 550Å Counts 600 Cr Cr Ro 156 wh theta (deg) Fig. 4. Diffractogram of the white sample Ro 156 with addition. 51

4 Ro Counts / 10 secs Mog Q Cris Q Calc theta (deg) Fig. 5. Diffractogram of the chalcedony sample Ro 155 consisting of - quartz (Q), moganite (Mog), calcite (Calc) and cristobalite (Cris). The brownish, chocolate-like chalcedony Ro 155 (N.B. the vein chalcedony described in situ from the intensely altered pyroclastic breccia from Şcolii Valley, Gurasada) has indicated a minimum amount of -quartz, a large concentration of moganite besides which calcite and cristobalite have been identified. It is worthy to mention that the addition represents only 13 % of the sample mass, but in the diffractogram it appears as dominant phase (fig. 5). We state that these mineral phases of gems from Gurasada were also noticed by RAMAN spectroscopy analyses (Pop et al, 2004). The third set of measurements, using scans in the θ interval, at 0,010 for 10 sec/step, with a view to calculate the quartz crystallites sizes based on the 26,640 2θ peak, was performed for the calculation of the quartz crystallites sizes based on the peak width at half-heights of the main quartz peak at 26,640 2θ that was subsequently used for evaluating the age of the agates and chalcedonies under study. The dimension of quartz crystallites Cs(101) from our samples are tabulated further on. Table 1. ze of quartz crystallites in the agates and chalcedonies from Gurasada Sample Crystallite size C s (101) / Å Ro Ro Ro Ro Ro Ro Average 567 (72) 52

5 As compared to the values obtained by Moxon and Rios, (2004), the average value of 567 Å obtained for our studied samples would suggest a geological age in the interval 30 and 40 My. The difference of about de million years between the formation of the silica gem varieties (30-40 My) and the radiometric ages obtained for the host rocks by using the K-Ar method (76-80 My) suggests, as in the case of the agates from Brazil, that the agates and chalcedonies from the area under study have formed in a subsequent stage of the volcanic activity, in relationship with the bentonitization processes that have affected the pyroclastic rocks. Conclusions The theory connecting the transformation of moganite into quartz in chalcedonies and agates at geological scale, corroborated with the increase of the quartz crystallites size was applied in a study of this type for the first time in Romania; based on this method we could estimate ages of million years (Lower Eocene) for the agates. These values, to which the nature of mineral phases is added, may suggest that, in the area, the agates and chalcedonies were the outcome of exogenous processes (bentonitization of the pyroclastic rocks). References Constantina, C. (2008) Studiul complexului vulcanoclastic paleocen din regiunea Sârbi-Gurasada- Burjuc (Valea Mureşului), cu privire specială asupra mineralelor cu valoare gemologică. PhD thesis 135 p. Univ. Babeş-Bolyai, Cluj-Napoca. Moxon, T. (2002) Agate: A study of ageing. European Journal of Mineralogy 14, p , Stuttgart, Germany. Moxon, T., Rios, S. (2004) - Moganite and water content as a function of age in agate: an XRD and thermogravimetric study. European Journal of Mineralogy 16, p , Stuttgart, Germany. Moxon, T., Nelson, D. R., Zang, M. (2006) Agate recrystallisation: evidence from samples found in Archaean and Proterozoic host rocks, Western Australia. Australian Journal of Earth Sciences (2006) 53, p Moxon, T., Reed, S.J.B. (2006) Agate and chalcedony from igneous and sedimentary hosts aged from 13 to 3480 Ma: a cathodoluminescence study. Mineralogical Magazine, 70(5), pp Pop Dana, Constantina, C., Tătar, D., Kiefer, W. (2004) RAMAN Spectroscopy on gem-quality microcristaline and amorphous silica varieties from Romania. Studia Univ. UBB, Geologia, XLIX, 1, 41-52, Cluj-Napoca. 53