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Colours in Minerals and Rocks
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Colours in Minerals and Rocks

A special issue of Minerals (ISSN 2075-163X). This special issue belongs to the section "Crystallography and Physical Chemistry of Minerals & Nanominerals".

Deadline for manuscript submissions: closed (19 November 2021) | Viewed by 57058

Special Issue Editors

Dr. Lluís Casas

E-Mail Website
Guest Editor
Department de Geologia, Universitat Autònoma de Barcelona (UAB), Edifici C, 08193 Cerdanyola del Vallès, Spain
Interests: archaeometry; crystallography; magnetism; petrography; physical–chemical characterization; X-ray microdiffraction; Raman microscopy (RM); magnetic nanoparticles; 3D-printing
Special Issues, Collections and Topics in MDPI journals
Dr. Roberta Di Febo

E-Mail Website
Guest Editor
Department de Geologia, Universitat Autònoma de Barcelona (UAB), Edifici C, 08193 Cerdanyola del Vallès, Catalonia, Spain
Interests: archaeometry, petrography of archaeological materials, pottery, glazes, marbles; X-ray microdiffraction; Raman microscopy (RM)
Special Issues, Collections and Topics in MDPI journals

Special Issue Information

Dear Colleagues,

Colour appears as a result of the interaction between light and matter and is one of the most eye-catching properties of some minerals and rocks. In some occasions this property is the essential feature that has historically determined the application or the uses of certain minerals and rocks. Besides, new modern uses sometimes rely on particular colours or chromatic-related effects. Colours of rocks are basically associated to concentration of colour-bearing minerals. A wavelength-dependable absorption coefficient is responsible for colours in minerals and this depends both on their chemical composition and crystallographic structure (and their corresponding chemical bonds). For allochromatic minerals, trace impurities or structural defects can determine the colour and sometimes this can be altered by exposing a mineral to light, heat or radiation.

Colour, pleochroism and interference colour are fundamental properties observable using a petrographic microscope. There are also a number of techniques specifically aimed at colour characterisation and related properties, among others, colorimetry, UV-VIS spectroscopy or multispectral photogrammetry; sometimes the characterised colour appears as a response to an excitation source, as in cathodoluminescence, thermoluminiscence, fluorescence, etc.

In this special issue we propose to group papers that focus, on the whole, on colour in minerals and rocks in a wide variety of contexts, e.g. gems, pigments for paints, pigments in ceramics, decorated glazes, stained glasses, decorative building stones, mosaics, colourful rock formations, as well as fundamental studies on rare colour varieties of minerals and on chromatic effects.

Dr. Lluís Casas
Dr. Roberta Di Febo
Guest Editors

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All submissions that pass pre-check are peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Minerals is an international peer-reviewed open access monthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 2400 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

  • Colour
  • Mineral Pigments
  • Heritage Science
  • Gems
  • Characterization
  • Construction Materials
  • Landscape

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Published Papers (16 papers)

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Editorial

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4 pages, 203 KiB  
Editorial
Editorial for Special Issue “Colours in Minerals and Rocks”
by Lluís Casas and Roberta Di Febo
Minerals 2022, 12(4), 473; https://0-doi-org.brum.beds.ac.uk/10.3390/min12040473 - 13 Apr 2022
Viewed by 1945
Abstract
Colour is one of the most eye-catching properties of some minerals and rocks [...] Full article
(This article belongs to the Special Issue Colours in Minerals and Rocks)

Research

Jump to: Editorial

27 pages, 6154 KiB  
Article
Colours of Gemmy Phosphates from the Gavà Neolithic Mines (Catalonia, Spain): Origin and Archaeological Significance
by Yael Díaz-Acha, Marc Campeny, Lluís Casas, Roberta Di Febo, Jordi Ibañez-Insa, Tariq Jawhari, Josep Bosch, Ferran Borrell, Susana Esther Jorge-Villar, Jean-Marc Greneche, Esperança Tauler and Joan Carles Melgarejo
Minerals 2022, 12(3), 368; https://0-doi-org.brum.beds.ac.uk/10.3390/min12030368 - 17 Mar 2022
Cited by 2 | Viewed by 2596
Abstract
In the Neolithic Gavà mines, variscite and turquoise were exploited for ornaments manufacturing, although some prospective pits and tunnels were dug on other similar greenish minerals such as smectite or kandite. A 3D study of the distribution of mineral phases allows us to [...] Read more.
In the Neolithic Gavà mines, variscite and turquoise were exploited for ornaments manufacturing, although some prospective pits and tunnels were dug on other similar greenish minerals such as smectite or kandite. A 3D study of the distribution of mineral phases allows us to determine the parameters involved in variscite colours. Methods are comprised of quantitative colourimetry, thin section petrography, SEM-BSE-EDS, EMPA, XRD, Raman spectroscopy, and 57Fe Mössbauer spectrometry. Mapping of the mines indicates that colour is not directly dependent on depth. Although variscite from Gavà is poor in Cr3+ and V+3 compared with gemmy variscite from other localities, the deep green samples content has the highest values of Cr3+. In the case of cryptocrystalline mixtures with jarosite, phosphosiderite, or goethite, variscite tends to acquire a greenish brown to olivaceous hue. If white minerals such as quartz, kandite, crandallite, or alunite are involved in the mixtures, variscite and turquoise colours become paler. Full article
(This article belongs to the Special Issue Colours in Minerals and Rocks)
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19 pages, 6649 KiB  
Article
Influence of Chemical Composition and Microvesiculation on the Chromatic Features of the Obsidian of Sierra de las Navajas (Hidalgo, Mexico)
by Paola Donato, Sandro Donato, Luis Barba, Gino Mirocle Crisci, Maria Caterina Crocco, Mariano Davoli, Raffaele Filosa, Vincenzo Formoso, Giancarlo Niceforo, Alejandro Pastrana, Andrea Solano and Rosanna De Rosa
Minerals 2022, 12(2), 177; https://0-doi-org.brum.beds.ac.uk/10.3390/min12020177 - 29 Jan 2022
Cited by 6 | Viewed by 2452
Abstract
The obsidian of Sierra de las Navajas is well known for its green color and gold hue. In order to relate these features with compositional and microtextural characteristics, we have carried out a microanalytical study by Wave Dispersion System associated to Electron Probe [...] Read more.
The obsidian of Sierra de las Navajas is well known for its green color and gold hue. In order to relate these features with compositional and microtextural characteristics, we have carried out a microanalytical study by Wave Dispersion System associated to Electron Probe Micro-Analyzer, Scanning Electron Microscope observation, and X-rays micro-tomographic analyses of samples showing different colors (dark to light green, sometimes with bands of different color intensity) and hues (changing, uniform, no hue). In accordance with previous studies, the green color of the obsidian seems to be related to a high iron content, probably in its reduced state. However, no significant difference in composition occurs between dark and light green samples. The SEM observation and microtomographic study revealed the absence of microcrystals and the occurrence of vesicles of different size, shape, and orientation. Lighter green colors are shown by highly vesiculated surfaces, whereas non-vesiculated samples are darker. On the surfaces with a high concentration of coarse vesicles, a uniform golden hue is observed. Decreasing vesiculation gives a hue changing with the angle of incident light. However, when the vesicularity approaches zero, no hue is visible. The iso-orientation of vesicles along preferential directions and their distribution in bands determine the variation in color intensity and hue on differently oriented surfaces. Microvesiculation also influences other characteristics that were important features for the use of obsidian in the past, such as fracture, transparency, and roughness. Full article
(This article belongs to the Special Issue Colours in Minerals and Rocks)
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21 pages, 10251 KiB  
Article
Automatic Gemstone Classification Using Computer Vision
by Bona Hiu Yan Chow and Constantino Carlos Reyes-Aldasoro
Minerals 2022, 12(1), 60; https://0-doi-org.brum.beds.ac.uk/10.3390/min12010060 - 31 Dec 2021
Cited by 21 | Viewed by 7067
Abstract
This paper presents a computer-vision-based methodology for automatic image-based classification of 2042 training images and 284 unseen (test) images divided into 68 categories of gemstones. A series of feature extraction techniques (33 including colour histograms in the RGB, HSV and CIELAB space, local [...] Read more.
This paper presents a computer-vision-based methodology for automatic image-based classification of 2042 training images and 284 unseen (test) images divided into 68 categories of gemstones. A series of feature extraction techniques (33 including colour histograms in the RGB, HSV and CIELAB space, local binary pattern, Haralick texture and grey-level co-occurrence matrix properties) were used in combination with different machine-learning algorithms (Logistic Regression, Linear Discriminant Analysis, K-Nearest Neighbour, Decision Tree, Random Forest, Naive Bayes and Support Vector Machine). Deep-learning classification with ResNet-18 and ResNet-50 was also investigated. The optimal combination was provided by a Random Forest algorithm with the RGB eight-bin colour histogram and local binary pattern features, with an accuracy of 69.4% on unseen images; the algorithms required 0.0165 s to process the 284 test images. These results were compared against three expert gemmologists with at least 5 years of experience in gemstone identification, who obtained accuracies between 42.6% and 66.9% and took 42–175 min to classify the test images. As expected, the human experts took much longer than the computer vision algorithms, which in addition provided, albeit marginal, higher accuracy. Although these experiments included a relatively low number of images, the superiority of computer vision over humans is in line with what has been reported in other areas of study, and it is encouraging to further explore the application in gemmology and related areas. Full article
(This article belongs to the Special Issue Colours in Minerals and Rocks)
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15 pages, 15961 KiB  
Article
Colour Quality Evaluation of Bluish-Green Serpentinite Based on the CIECAM16 Model
by Zitong Zhao and Ying Guo
Minerals 2022, 12(1), 38; https://0-doi-org.brum.beds.ac.uk/10.3390/min12010038 - 28 Dec 2021
Cited by 6 | Viewed by 2576
Abstract
The CIECAM16 colour appearance model is currently a model with high prediction accuracy. It can solve the problem of predicting the influence of different observation conditions on the colour of gemstones. In this study, a computer vision system (CVS) was used to measure [...] Read more.
The CIECAM16 colour appearance model is currently a model with high prediction accuracy. It can solve the problem of predicting the influence of different observation conditions on the colour of gemstones. In this study, a computer vision system (CVS) was used to measure the colour of 59 bluish-green serpentinite samples, and the tristimulus values were input into the CIECAM16 forward model to calculate the colour appearance parameters of serpentinite under different surrounds, illuminances, and light sources. It was found that the darkening of the surround causes the lightness and brightness to increase. Pearson’s r of brightness and colourfulness with illuminance is 0.885 and 0.332, respectively, which predicts the Stevens and Hunt effects. When the light source changes from D65 to A, the calculated hue angle shifts to the complementary area of the A light source, which is contrary to the CVS measurement result. The D65 light source is more suitable for the colour presentation and classification of bluish-green serpentinite. Full article
(This article belongs to the Special Issue Colours in Minerals and Rocks)
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18 pages, 4016 KiB  
Article
Comparative Study on the Origin and Characteristics of Chinese (Manas) and Russian (East Sayan) Green Nephrites
by Jiaxin Wang and Guanghai Shi
Minerals 2021, 11(12), 1434; https://0-doi-org.brum.beds.ac.uk/10.3390/min11121434 - 18 Dec 2021
Cited by 9 | Viewed by 2634
Abstract
Green nephrites are widely pursued for their mild texture and vivid color. In recent years, many Russian green nephrites appeared in China (the world’s largest nephrite market) and competed with the Chinese Manas green nephrites, which are traditionally highly valued. In this study, [...] Read more.
Green nephrites are widely pursued for their mild texture and vivid color. In recent years, many Russian green nephrites appeared in China (the world’s largest nephrite market) and competed with the Chinese Manas green nephrites, which are traditionally highly valued. In this study, we compared the appearance, mineralogy and geochemical features (with EPMA and LA-ICP-MS) of Chinese (Manas) and Russian (East Sayan) green nephrites to objectively characterize and distinguish between these two nephrites. Chinese (Manas) and Russian (East Sayan) green nephrites are mined from serpentinized ultramafic units in the northern Tian Shan and East Sayan orogen, respectively. In terms of appearance, the Manas green nephrites are slightly bluish or grayish, whilst their East Sayan counterparts are brighter (duck-egg cyan). The Manas nephrites commonly have a caramel color, crumple structure, characteristic white globules and sinuous veins, green stains and yellow–green veins, together with a local fibrous structure. The East Sayan green nephrites are more transparent, with a gentler fine texture, uniform color, many black spots and a few green spots. Some green nephrites from the Arahushun mine of East Sayan have an ice-like appearance. Microscopic petrography and EPMA analysis indicate that both the Manas and East Sayan green nephrites comprise mainly tremolite with minor actinolite. Minor minerals in the Manas samples include chromite, chlorite-group minerals, and uvarovite; whilst those in the East Sayan samples include actinolite, chromite, chlorite-group minerals, and bornite. Bornite is not found in any other sources of green nephrite, and thus is characteristic of Russian (East Sayan) green nephrites. LA-ICP-MS trace element data in their amphiboles and Single-Factor Analysis of Variance (ANOVA) results suggest that the differences in Cr, Zn, Y, Ba, and Sr contents and values of δEu, Eu/Sm, (La/Yb)N, (La/Sm)N, (Gd/Yb)N, ∑HREE, ∑LREE/∑HREE between the two nephrites are present, and can be used as their origin trace. Full article
(This article belongs to the Special Issue Colours in Minerals and Rocks)
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27 pages, 5434 KiB  
Article
Exploring New Ways to Reconstruct the Forma Urbis Romae: An Archaeometric Approach (CL Color and Stable Isotope Analyses)
by Lluís Casas, Roberta Di Febo, Mauro Brilli, Francesca Giustini, Marco Gozzi, Francesca De Caprariis, Juan Diego Martín-Martín and Claudio Parisi Presicce
Minerals 2021, 11(12), 1400; https://0-doi-org.brum.beds.ac.uk/10.3390/min11121400 - 11 Dec 2021
Cited by 3 | Viewed by 5020
Abstract
The Forma Urbis Romae (F.U.) was a 3rd-century-AD monumental map of ancient Rome consisting of 151 rectangular marble slabs. Several efforts have been made to reconstruct it from its current incomplete and fragmentary condition. In this paper, we explore the potential of an [...] Read more.
The Forma Urbis Romae (F.U.) was a 3rd-century-AD monumental map of ancient Rome consisting of 151 rectangular marble slabs. Several efforts have been made to reconstruct it from its current incomplete and fragmentary condition. In this paper, we explore the potential of an archaeometric approach to serve this purpose. Almost a hundred F.U. fragments have been characterized, particularly focusing on cathodoluminescence (CL) microscopy and stable isotopes (δ18O and δ13C). Different statistical methods have been used to quantify the similarity between samples. The central assumption is that samples from a given slab share similar CL colors and isotopic ratios. The assumption has been verified for samples from single fragments and then it has been used to check ten debated reconstruction hypotheses. The measured isotopic ratios confirm the Proconnesian nature of the F.U. marble, except for a fragment. Beyond provenance, the results cast doubts on four out of the ten checked reconstruction hypotheses and support the other six. The reconstruction of the F.U. remains a fascinating challenge, and both isotopic and CL analyses have demonstrated their potential to tackle it. Further research could extend the presented methodology to a higher number of samples. The innovative use of CL to reconstruct a fragmented artwork could be applied to other projects. Full article
(This article belongs to the Special Issue Colours in Minerals and Rocks)
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28 pages, 11062 KiB  
Article
Origin of the Coloured Karst Fills in the Neogene Extensional System of NE Iberia (Spain)
by Anna Travé, Noelia Rodríguez-Morillas, Vinyet Baqués, Elisabet Playà, Lluís Casas, Irene Cantarero, Juan Diego Martín-Martín, Enrique Gómez-Rivas, Mar Moragas and David Cruset
Minerals 2021, 11(12), 1382; https://0-doi-org.brum.beds.ac.uk/10.3390/min11121382 - 08 Dec 2021
Cited by 8 | Viewed by 3191
Abstract
Karst fills from the onshore Penedès Basin and offshore València Trough display red, pink, orange and ochre colours. Their Mössbauer spectra indicate that Fe3+ contained in goethite is the dominant species in reddish-pink fills, whereas Fe2+ contained in dolomite and clays [...] Read more.
Karst fills from the onshore Penedès Basin and offshore València Trough display red, pink, orange and ochre colours. Their Mössbauer spectra indicate that Fe3+ contained in goethite is the dominant species in reddish-pink fills, whereas Fe2+ contained in dolomite and clays is more dominant in the orange and ochre ones. The lower δ13C values and higher 87Sr/86Sr ratios of the karst fills with respect to their host carbonates can reflect the input of soil-derived CO2 and an external radiogenic source into the karst system. This geochemical composition, together with the non-carbonate fraction of the fills, consists of authigenic and transported illite, illite-smectite interlayers, as well as kaolinite, chlorite, pyrite, quartz, ilmenite, magnetite, apatite and feldspar, account for a mixed residual-detrital origin of fills. This polygenic origin agrees with that of the terra rossa sediments described worldwide. The different colours of karst fills are attributed to fluctuations in the water table, which control the Eh/pH conditions in the karst system. Thus, reddish colours reflect low water table levels and oxidising episodes, and orange and ochre ones reflect high water table levels and more reducing episodes. The greenish colours of fills could be related to fluctuations in the Fe3+/Fe2+ ratio. Full article
(This article belongs to the Special Issue Colours in Minerals and Rocks)
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22 pages, 5253 KiB  
Article
Antimony and Nickel Impurities in Blue and Green Copper Pigments
by Sylwia Svorová Pawełkowicz, Barbara Wagner, Jakub Kotowski, Grażyna Zofia Żukowska, Bożena Gołębiowska, Rafał Siuda and Petras Jokubauskas
Minerals 2021, 11(11), 1236; https://0-doi-org.brum.beds.ac.uk/10.3390/min11111236 - 07 Nov 2021
Cited by 1 | Viewed by 3208
Abstract
Impurities in paint layers executed with green and blue copper pigments, although relatively common, have been studied only little to date. Yet, their proper identification is a powerful tool for classification of paintings, and, potentially, for future provenance studies. In this paper, we [...] Read more.
Impurities in paint layers executed with green and blue copper pigments, although relatively common, have been studied only little to date. Yet, their proper identification is a powerful tool for classification of paintings, and, potentially, for future provenance studies. In this paper, we present analyses of copper pigments layers from wall paintings situated in the vicinity of copper ore deposits (the palace in Kielce, the palace in Ciechanowice, and the parish church in Chotków) located within the contemporary borders of Poland. We compare the results with the analyses of copper minerals from three deposits, two local, and one historically important for the supply of copper in Europe, i.e., Miedzianka in the Holy Cross Mountains, Miedzianka in the Sudetes, and, as a reference, Špania Dolina in the Slovakian Low Tatra. Optical (OM) and electron microscopy (SEM-EDS), Raman spectroscopy, and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) have been used for a detailed investigation of the minute grains. Special attention has been devoted to antimony and nickel phases, as more unusual than the commonly described iron oxides. Analyses of minerals from the deposits helped to interpret the results obtained from the paint samples. For the first time, quantitative analyses of copper pigments’ impurities have been described. Full article
(This article belongs to the Special Issue Colours in Minerals and Rocks)
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13 pages, 8492 KiB  
Article
The Effects of Munsell Neutral Grey Backgrounds on the Colour of Chrysoprase and the Application of AP Clustering to Chrysoprase Colour Grading
by Yuansheng Jiang, Ying Guo, Yufei Zhou, Xiang Li and Simin Liu
Minerals 2021, 11(10), 1092; https://0-doi-org.brum.beds.ac.uk/10.3390/min11101092 - 04 Oct 2021
Cited by 15 | Viewed by 1870
Abstract
Chrysoprase is a popular gemstone with consumers because of its charming apple green colour but a scientific classification of its colour has not yet been achieved. In this research, we determined the most effective background of the Munsell Chart for chrysoprase colour grading [...] Read more.
Chrysoprase is a popular gemstone with consumers because of its charming apple green colour but a scientific classification of its colour has not yet been achieved. In this research, we determined the most effective background of the Munsell Chart for chrysoprase colour grading under a 6504K fluorescent lamp and applied an affinity propagation (AP) clustering algorithm to the colour grading of coloured gems for the first time. Forty gem-quality chrysoprase samples from Australia were studied using a UV-VIS spectrophotometer and Munsell neutral grey backgrounds. The results determined the effects of a Munsell neutral grey background on the observed colour. It was found that the Munsell N9.5 background was the most effective for colour grading in this case. The observed chrysoprase colours were classified into five groups: Fancy Light, Fancy, Fancy Intense, Fancy Deep and Fancy Dark. The feasibility of the colour grading scheme was verified using the colour difference formula DE2000. Full article
(This article belongs to the Special Issue Colours in Minerals and Rocks)
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11 pages, 6869 KiB  
Article
Ancient Pigments in Afrasiab Murals: Characterization by XRD, SEM, and Raman Spectroscopy
by Dong-Hyeok Moon, Na-Ra Lee and Eun-Woo Lee
Minerals 2021, 11(9), 939; https://0-doi-org.brum.beds.ac.uk/10.3390/min11090939 - 29 Aug 2021
Cited by 8 | Viewed by 2401
Abstract
The Afrasiab murals discovered in the northeast of Samarkand, Uzbekistan—the center of the ancient Silk Road—are presumed to date to the mid-seventh century during the Sogdian era. Although previous studies have examined the primary materials of the pigments used in these murals using [...] Read more.
The Afrasiab murals discovered in the northeast of Samarkand, Uzbekistan—the center of the ancient Silk Road—are presumed to date to the mid-seventh century during the Sogdian era. Although previous studies have examined the primary materials of the pigments used in these murals using chemical and microscopic analyses, in-depth investigations of the pigment raw material composition have not been conducted to verify the results of these studies. We applied X-ray diffractometry, Raman spectroscopy, and scanning electron microscopy in conjunction with energy-dispersive X-ray spectroscopy for the first time to identify the raw materials of ancient pigments in fragments obtained from the Afrasiab murals. The results show that lazurite, cinnabar, and amorphous carbon were used as blue, red, and black pigments, respectively. Moreover, we identified that pigments were not directly painted on the wall surface; instead, they were painted on a white undercoat of gypsum plaster, similar to other ancient Silk Road wall paintings. The results of this study can benefit the provision of more accurate information with regard to the composition of raw materials and further support the selection of appropriate substances for the purposes of conservation and restoration of Afrasiab murals. Full article
(This article belongs to the Special Issue Colours in Minerals and Rocks)
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15 pages, 4909 KiB  
Article
Explaining Colour Change in Pyrope-Spessartine Garnets
by Yan Qiu and Ying Guo
Minerals 2021, 11(8), 865; https://0-doi-org.brum.beds.ac.uk/10.3390/min11080865 - 11 Aug 2021
Cited by 15 | Viewed by 4150
Abstract
A colour-changing garnet exhibits the “alexandrite effect”, whereby its colour changes from green in the presence of daylight to purplish red under incandescent light. This study examines this species of garnets as well as the causes of the colour change by using infrared [...] Read more.
A colour-changing garnet exhibits the “alexandrite effect”, whereby its colour changes from green in the presence of daylight to purplish red under incandescent light. This study examines this species of garnets as well as the causes of the colour change by using infrared and ultraviolet visible (UV-Vis) spectroscopy. The infrared spectra show that the colour-changing garnets in this paper belong to the solid solution of pyrope-spessartine type. CIE1931 XYZ colour matching functions are used to calculate the colour parameters influencing garnet colour-changing under different light sources. The UV-Vis spectra show two zones of transmittance, in the red region at 650–700 nm and the blue-green region at 460–510 nm. As they exhibit the same capacity to transmit light, the colour of the gem is determined by the external light source. The absorption bands of Cr3+ and V3+ at 574 nm in the UV-Vis spectra are the main cause of the change in colour. With the increase in the area of peak absorption, the differences in the chroma and colour of the garnet gradually increase in daylight and incandescent light, and it exhibits a more prominent colour-changing effect. Full article
(This article belongs to the Special Issue Colours in Minerals and Rocks)
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11 pages, 1154 KiB  
Article
Measurement of Gem Colour Using a Computer Vision System: A Case Study with Jadeite-Jade
by Sufei Zhang and Ying Guo
Minerals 2021, 11(8), 791; https://0-doi-org.brum.beds.ac.uk/10.3390/min11080791 - 22 Jul 2021
Cited by 10 | Viewed by 3143
Abstract
This paper introduces computer vision systems (CVSs), which provides a new method to measure gem colour, and compares CVS and colourimeter (CM) measurements of jadeite-jade colour in the CIELAB space. The feasibility of using CVS for jadeite-jade colour measurement was verified by an [...] Read more.
This paper introduces computer vision systems (CVSs), which provides a new method to measure gem colour, and compares CVS and colourimeter (CM) measurements of jadeite-jade colour in the CIELAB space. The feasibility of using CVS for jadeite-jade colour measurement was verified by an expert group test and a reasonable regression model in an experiment involving 111 samples covering almost all jadeite-jade colours. In the expert group test, more than 93.33% of CVS images are considered to have high similarities with real objects. Comparing L*, a*, b*, C*, h, and ∆E* (greater than 10) from CVS and CM tests indicate that significant visual differences exist between the measured colours. For a*, b*, and h, the R2 of the regression model for CVS and CM was 90.2% or more. CVS readings can be used to predict the colour value measured by CM, which means that CVS technology can become a practical tool to detect the colour of jadeite-jade. Full article
(This article belongs to the Special Issue Colours in Minerals and Rocks)
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25 pages, 17514 KiB  
Article
The Colors of the Circus Mosaic from Barcino (Roman Barcelona): Characterization, Provenance, and Technology Issues
by Lluís Casas, Roberta Di Febo, Carme Boix, Albert Egea, Oriol Vallcorba, Ignasi Queralt, Anna Anglisano, Isabel Moreno and Lorena Andino
Minerals 2021, 11(7), 746; https://0-doi-org.brum.beds.ac.uk/10.3390/min11070746 - 09 Jul 2021
Cited by 3 | Viewed by 2746
Abstract
Archaeometric studies on mosaics often concentrate only on glass tesserae, while comprehensive studies including both stone and glass tesserae are scarce; however, both types of tesserae can sometimes bring relevant data to elaborate archaeological knowledge on a studied mosaic. In this paper, a [...] Read more.
Archaeometric studies on mosaics often concentrate only on glass tesserae, while comprehensive studies including both stone and glass tesserae are scarce; however, both types of tesserae can sometimes bring relevant data to elaborate archaeological knowledge on a studied mosaic. In this paper, a representative set of tesserae from a large polychrome Roman mosaic retrieved in Barcelona (NE Spain) is investigated using various methods. Most of the techniques were directly applied on samples prepared as petrographic thin sections (including polarized-light, cathodoluminescence and electron microscopies, and synchrotron through-the-substrate μX-ray diffraction). The results indicate that, from the ten sampled stone tesserae, there are (i) seven limestones, one of them identified as Alveolina limestone (early Eocene) from the southern Pyrenees (ii) two sandstones from Barcelona’s Montjuïc hill (Miocene) and, (iii) a Carrara white marble from the Apuan Alps (Italy). The profuse presence of tesserae of both local and imported materials with well-known uses in architecture, epigraphy, and sculpture could imply that tesserae were a by-product of their main use. Two different production technologies were identified for the three sampled glass tesserae. The concurrent use of antimony- and tin-based opacifiers is in agreement with the accepted archaeological chronology of the mosaic (4th century AD). Full article
(This article belongs to the Special Issue Colours in Minerals and Rocks)
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29 pages, 8838 KiB  
Article
Production Technologies of Ancient Bricks from Padua, Italy: Changing Colors and Resistance over Time
by Elena Mercedes Pérez-Monserrat, Lara Maritan, Enrico Garbin and Giuseppe Cultrone
Minerals 2021, 11(7), 744; https://0-doi-org.brum.beds.ac.uk/10.3390/min11070744 - 09 Jul 2021
Cited by 14 | Viewed by 3297
Abstract
Representative and very uneven texturally bricks having yellow/beige or pale or dark red colors from the Renaissance walls (16th century) of Padua, Northeast Italy, were studied by means of colorimetric, petrographic (MOP), chemical (XRF), mineralogical (PXRD) and microstructural analysis (FESEM-EDS). Starting from the [...] Read more.
Representative and very uneven texturally bricks having yellow/beige or pale or dark red colors from the Renaissance walls (16th century) of Padua, Northeast Italy, were studied by means of colorimetric, petrographic (MOP), chemical (XRF), mineralogical (PXRD) and microstructural analysis (FESEM-EDS). Starting from the color measurements of the ceramic bodies, the manufacturing technologies and their influence on the physical behavior and durability of the bricks were established. The porous system was characterized by means of hygric tests and mercury intrusion porosimetry; the compactness and structural anisotropy were defined through ultrasound velocity; the uniaxial compressive strength was determined; and durability to salt crystallization and frost action of the bricks was assessed. Mg- and Ca-rich illitic clays fired at temperatures ≥900 °C were used to manufacture the beige hue bodies, while the pale red bricks were made out with Ca- and Fe-rich illitic clays fired at 850–900 °C. A lower carbonate content on the base clays and a lower firing temperature were the main causes responsible for the changing colors from beige to red hue. The increase of the red color was associated to higher silicate inclusions content and lower development of reaction rims around grains. The low sintering degree achieved yielded highly porous bodies with diverse porous systems, leading to differential physical performance and durability of the bricks that may turn out beneficial for the conservation of the historic walls. Full article
(This article belongs to the Special Issue Colours in Minerals and Rocks)
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Article
Colour Transformation and Textural Change in Biotite: Some Remarks for the Interpretation of Firing Technology in Greyware Pottery Thin-Sections
by Esther Travé Allepuz
Minerals 2021, 11(4), 428; https://0-doi-org.brum.beds.ac.uk/10.3390/min11040428 - 18 Apr 2021
Cited by 7 | Viewed by 4161
Abstract
Firing is a crucial step in the production of pottery, as it irreversibly transforms the clay into ceramic. Clay sintering and subsequent vitrification occur during firing, together with other transformations undergone by specific minerals and rock inclusions according to their optical and physical [...] Read more.
Firing is a crucial step in the production of pottery, as it irreversibly transforms the clay into ceramic. Clay sintering and subsequent vitrification occur during firing, together with other transformations undergone by specific minerals and rock inclusions according to their optical and physical properties, including their colour. Some of these are visible in thin-sections and might be interpreted as technological markers or contribute to the estimation of firing temperatures, although most of them are poorly documented. In this paper, we approach the transformations in colour, texture and optical properties that occurred in biotite inclusions from medieval greyware pottery. Our study considers a batch of 40 pottery samples from medieval Catalonia analysed by XRD. According to the estimated firing temperature ranges and atmospheres, we examined the behaviour of biotite at different temperature ranges from 700 °C to 1000 °C by means of optical microscopy, considering its size, shape and abundance, and compared these features to a wider assemblage of thin-sections from medieval earthenware. The results obtained are interesting, as they offer a valuable reference for petrographic studies on pottery. We discuss the potential of ceramic petrography as a way to perform more precise and refined sample selection for further analysis on archaeothermometry. Full article
(This article belongs to the Special Issue Colours in Minerals and Rocks)
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