Cracking of Gem Opals
The value of gem opals is compromised by their potential susceptibility to “crazing”, a phenomenon observed either in the form of whitening or cracking. To understand the latter, 26 opal samples were investigated and separated into 2 groups based on handling: “water-stored” opal samples, which are s...
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| Format: | Article |
| Language: | English |
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MDPI AG
2023-03-01
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| Series: | Minerals |
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| Online Access: | https://www.mdpi.com/2075-163X/13/3/356 |
| _version_ | 1797610025565290496 |
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| author | Boris Chauviré Valentin Mollé Florine Guichard Benjamin Rondeau Paul Stephen Thomas Emmanuel Fritsch |
| author_facet | Boris Chauviré Valentin Mollé Florine Guichard Benjamin Rondeau Paul Stephen Thomas Emmanuel Fritsch |
| author_sort | Boris Chauviré |
| collection | DOAJ |
| description | The value of gem opals is compromised by their potential susceptibility to “crazing”, a phenomenon observed either in the form of whitening or cracking. To understand the latter, 26 opal samples were investigated and separated into 2 groups based on handling: “water-stored” opal samples, which are stored in water after extraction, and “air-stored” opal samples, which are stored in air for more than a year. To induce cracking, samples were thermally treated by staged heating and characterized using optical microscopy and Raman spectroscopy before and after cracking. For water-stored opals, cracking was initiated with moderate heating up to 150 °C, while for air-stored opals, higher temperatures, circa 300 °C, were required. In water-stored opals that cracked, polarized light microscopy revealed stress fields remaining around the cracks, and a red shift in the Raman bands suggested tensile stresses. These stresses were not observed in air-stored samples that cracked. Based on these observations, for air-stored samples, cracking was ascribed to super-heated water-induced decrepitation. By contrast, for water-stored samples, cracking was linked to drying shrinkage, which correlates with the anecdotal reports from the gem trade. We thus identify the physical origin of cracking, and by comparing it to current knowledge, we determine the factors leading to cracking. |
| first_indexed | 2024-03-11T06:09:40Z |
| format | Article |
| id | doaj.art-c76eaafff085403ea38d9e5b946d0f8a |
| institution | Directory Open Access Journal |
| issn | 2075-163X |
| language | English |
| last_indexed | 2024-03-11T06:09:40Z |
| publishDate | 2023-03-01 |
| publisher | MDPI AG |
| record_format | Article |
| series | Minerals |
| spelling | doaj.art-c76eaafff085403ea38d9e5b946d0f8a2023-11-17T12:47:28ZengMDPI AGMinerals2075-163X2023-03-0113335610.3390/min13030356Cracking of Gem OpalsBoris Chauviré0Valentin Mollé1Florine Guichard2Benjamin Rondeau3Paul Stephen Thomas4Emmanuel Fritsch5GeoGems, 44500 Guérande, FranceUniversité d’Orléans, CNRS, BRGM, ISTO, UMR 7327, 45071 Orléans, FranceFaculté des Sciences et Technologies, Boulevard des Aiguillettes, 54506 Vandœuvre-lès-Nancy, FranceLaboratoire de Planétologie et Géosciences, Nantes Université, Univ Angers, Le Mans Université, CNRS, UMR 6112, 44000 Nantes, FranceSchool of Civil and Environmental Engineering, University of Technology, Sydney, NSW 2007, AustraliaInstitut des Matériaux de Nantes Jean Rouxel, Nantes Université, CNRS, IMN, 44000 Nantes, FranceThe value of gem opals is compromised by their potential susceptibility to “crazing”, a phenomenon observed either in the form of whitening or cracking. To understand the latter, 26 opal samples were investigated and separated into 2 groups based on handling: “water-stored” opal samples, which are stored in water after extraction, and “air-stored” opal samples, which are stored in air for more than a year. To induce cracking, samples were thermally treated by staged heating and characterized using optical microscopy and Raman spectroscopy before and after cracking. For water-stored opals, cracking was initiated with moderate heating up to 150 °C, while for air-stored opals, higher temperatures, circa 300 °C, were required. In water-stored opals that cracked, polarized light microscopy revealed stress fields remaining around the cracks, and a red shift in the Raman bands suggested tensile stresses. These stresses were not observed in air-stored samples that cracked. Based on these observations, for air-stored samples, cracking was ascribed to super-heated water-induced decrepitation. By contrast, for water-stored samples, cracking was linked to drying shrinkage, which correlates with the anecdotal reports from the gem trade. We thus identify the physical origin of cracking, and by comparing it to current knowledge, we determine the factors leading to cracking.https://www.mdpi.com/2075-163X/13/3/356opalcrackingwaterTGAdryingshrinkage |
| spellingShingle | Boris Chauviré Valentin Mollé Florine Guichard Benjamin Rondeau Paul Stephen Thomas Emmanuel Fritsch Cracking of Gem Opals Minerals opal cracking water TGA drying shrinkage |
| title | Cracking of Gem Opals |
| title_full | Cracking of Gem Opals |
| title_fullStr | Cracking of Gem Opals |
| title_full_unstemmed | Cracking of Gem Opals |
| title_short | Cracking of Gem Opals |
| title_sort | cracking of gem opals |
| topic | opal cracking water TGA drying shrinkage |
| url | https://www.mdpi.com/2075-163X/13/3/356 |
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