XRPD patterns of opals: A brief review and new results from recent studies

Ghisoli, Christian; Caucia, Franca; Marinoni, Luigi

doi:10.1154/1.3478554

Cited by 25 publications

(17 citation statements)

References 40 publications

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“…All other samples can be classified as opals‐CT, because their X‐ray diffraction patterns (Fig. b) show five well‐resolved peaks at about 20.5° ( d = 4.30 Å), 21.6° ( d = 4.11 Å), 35.8° ( d = 2.50 Å), 44.0° ( d = 2.03 Å) and 56.5° ( d = 1.62 Å) . Moreover, we notice that going from Ethiopia‐FO to Honduras (that is from bottom to top of Fig.…”

Section: Resultsmentioning

confidence: 87%

“…3b) show five well-resolved peaks at about 20.5°(d = 4.30 Å), 21.6°( d = 4.11 Å), 35.8°(d = 2.50 Å), 44.0°(d = 2.03 Å) and 56.5°( d = 1.62 Å). [40] Moreover, we notice that going from Ethiopia-FO to Honduras (that is from bottom to top of Fig. 4b) the main peaks sharpen indicating an increasing degree of order.…”

Section: Xrd Measurementsmentioning

confidence: 83%

“…The X-ray diffraction patterns of the samples from Australia, Madagascar and Slovakia (Fig. 4a) show only a very broad peak at about 21.7°(d = 4.1 Å), that according to, [40] identifies these samples as opals-A. All other samples can be classified as opals-CT, because their X-ray diffraction patterns (Fig.…”

Section: Xrd Measurementsmentioning

confidence: 99%

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Raman, FT‐IR and XRD investigation of natural opals

Sodo

Municchia

Barucca

et al. 2016

J Raman Spectroscopy

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Opals are naturally occurring hydrous silica materials (SiO2*nH2O), characterized by different degrees of crystallinity and crystal structure. Because of their optical properties, opals have been largely used in jewelry and as decorative elements in artworks. For this reason, a complete characterization and a provenance study of this kind of materials is mandatory in order both to avoid frauds and to reconstruct ancient and modern trade routes of gems. In this work, we present a combined spectroscopic (Raman, FTIR) and X-ray powder diffraction (XRD) investigation of nine opals from the main deposits around the world (Australia, Madagascar, Slovakia, Mexico, Honduras and Ethiopia). Four of these samples are the rare and precious fire opals, characterized by an intense red-orange color. Ethiopia, Honduras and Mexico opals showed spectra and diffraction patterns typical of Opal-CT, generally associated to volcanic genesis, while Australia, Madagascar and Slovakia opals are Opal-A type, associated to sedimentary origin. Unexpectedly the fire opal from Brazil behaves as a CT one. The presence of CO2 was detected only in the latter group, and exceptionally in the Honduras sample; FTIR-FPA imaging showed carbon dioxide to be homogeneously distributed inside the gems. Opals-CT are CO2-free and give much more complex FT-IR spectra in the NIR region where H2O combination modes occur. The obtained results are discussed in terms of relevance of the above experimental techniques for geosourcing opals, and contribute to increase the database of the chemical-physical properties of opals

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Section: Resultsmentioning

confidence: 87%

Section: Xrd Measurementsmentioning

confidence: 83%

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Raman, FT‐IR and XRD investigation of natural opals

Sodo

Municchia

Barucca

et al. 2016

J Raman Spectroscopy

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“…Conventionally, opals are classified into three types [10]: opal-A (further divided into opal-AG (opal) and opal-AN (hyalite)), opal-CT and opal-C [11,12]. For nearly 50 years, the classification of opals proposed by Jones and Segnit [1], based on X-ray powder diffraction (XRD), has been widely adopted, e.g., [13][14][15]. The pertinent features of the XRD classification are:…”

Section: Introductionmentioning

confidence: 99%

A Review of the Classification of Opal with Reference to Recent New Localities

et al. 2019

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Our examination of over 230 worldwide opal samples shows that X-ray diffraction (XRD) remains the best primary method for delineation and classification of opal-A, opal-CT and opal-C, though we found that mid-range infra-red spectroscopy provides an acceptable alternative. Raman, infra-red and nuclear magnetic resonance spectroscopy may also provide additional information to assist in classification and provenance. The corpus of results indicated that the opal-CT group covers a range of structural states and will benefit from further multi-technique analysis. At the one end are the opal-CTs that provide a simple XRD pattern (“simple” opal-CT) that includes Ethiopian play-of-colour samples, which are not opal-A. At the other end of the range are those opal-CTs that give a complex XRD pattern (“complex” opal-CT). The majority of opal-CT samples fall at this end of the range, though some show play-of-colour. Raman spectra provide some correlation. Specimens from new opal finds were examined. Those from Ethiopia, Kazakhstan, Madagascar, Peru, Tanzania and Turkey all proved to be opal-CT. Of the three specimens examined from Indonesian localities, one proved to be opal-A, while a second sample and the play-of-colour opal from West Java was a “simple” Opal-CT. Evidence for two transitional types having characteristics of opal-A and opal-CT, and “simple” opal-CT and opal-C are presented.

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“…According to this model, a larger peak suggests the presence of more tridymite, 65-70%. Conversely, Ghisoli and his colleagues offered an alternative method for the estimate (GHISOLI et al 2010). The ratio of tridymite and cristobalite in opaline polymorphs defined as "C/T= d(4.11)-d (measured) / d(4.11)-d(4.04)".…”

Section: Proportions Of Cristobalite and Tridymite Stacking Using Thementioning

confidence: 99%

Nano-structure of the cristobalite and tridymite stacking sequences in the common purple opal from the Gevrekseydi deposit, Seyitömer-Kütahya, Turkey

Hatipoğlu

Kibici²,

Yanık³

et al. 2015

Orient. J. Chem

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The strata that include the fine purple opal formations examined in this paper from the magmagenetic hydrothermal dissolutions. The opals are locally known as Gevrekseydi purple opals. These opal-bearing strata are deposited in volcanic lavas and tuffs in the Seyitömer-Kütahya region of western Turkey. The purple opals are common-type and attractive gem-quality, and they are sold on the worldwide gem markets. We conducted mineralogical investigations to clarify their silica building components, measure their nano-size, and determine their origins. The opals are an opal-CT (opal-cristobalite/tridymite)-type silica polymorph with the variable number of cristobalite and tridymite layers. The cristobalite and tridymite stacking sequences were evaluated in terms of maturation, crystallite size, and genesis of the opaline silica material. These sequences are modelled using X-ray diffraction patterns. The relationship between the crystallite size and full width at half maximum values of the Gevrekseydi purple opals were also examined using X-ray diffraction patterns. The crystallite sizes were found to be L=17 nm for the main opal-CT peak (4.09 Å), L=23 nm for the shoulder opal-CT peak (4.29 Å), and L=27 nm for the opal-CT/C peak (2.51 Å).Given the relationship between the crystallite size and maturation of opal-CT, the Gevrekseydi volcanic common purple opals are most likely in a stage of early to mid maturation

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XRPD patterns of opals: A brief review and new results from recent studies

Cited by 25 publications

References 40 publications

Raman, FT‐IR and XRD investigation of natural opals

Raman, FT‐IR and XRD investigation of natural opals

A Review of the Classification of Opal with Reference to Recent New Localities

Nano-structure of the cristobalite and tridymite stacking sequences in the common purple opal from the Gevrekseydi deposit, Seyitömer-Kütahya, Turkey

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