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1333691 B631-Klar---Kopia.Pdf UNIVERSITY OF GOTHENBURG Department of Earth Sciences Geovetarcentrum/Earth Science Centre Chemical characteristics of granites with a discolouration potential Joachim Andersson ISSN 1400-3821 B631 Bachelor of Science thesis Göteborg 2011 Mailing address Address Telephone Telefax Geovetarcentrum Geovetarcentrum Geovetarcentrum 031-786 19 56 031-786 19 86 Göteborg University S 405 30 Göteborg Guldhedsgatan 5A S-405 30 Göteborg SWEDEN Abstract The chemistry of six leucogranites with a clear discolouration was studied using SEM-EDS and the polarizing microscope. The Bohus granite was used as a reference, because it is considered as a durable natural stone. All samples are currently sold as natural stones. Three of them are slightly weathered before use. The granites are fractionated I-type granites, which mean that they have a magmatic source and evolved through magmatic processes. They have low contents of Ti, Mn, Mg, Ca, high field strength elements and rare earth elements compared to most granites. It is uncertain how or if the fractionated nature of the granites affects their overall resistance to weathering. Iron leakage from biotite is most likely the cause of discolouration in all samples, although the cause of discolouration in Granite 2 remains more uncertain because it is the only fresh sample. Weathering products high in Mn were found in Granite 3 and are probably caused by biotite which has a relatively high Mn content in relation to the bulk concentration of Mn. The Mn content in biotite is 60% higher (1.67 wt.% MnO) compared to the other samples. Mn is generally concentrated in a few minerals and could explain a part of the discolouration. High density of large cracks in large quartz grains is probably the major cause of high weathering rates and rapid propagation of the discolouration. Sammanfattning Kemin hos sex leucograniter med tydlig missfärgning har analyserats med SEM-EDS och polarisationsmikroskopering. Bohusgraniten har används som referens, eftersom den anses vara av god kvalitet. Alla prover säljs för tillfället som natursten. Tre av dem är något vittrade innan användning. Graniterna är fraktionerade I-graniter, vilket innebär att de har en magmatisk källa och utvecklats genom magmatiska processer. De har låga halter av Ti, Mn, Mg, Ca, high field strength elements och sällsynta jordartsmetaller jämfört med de flesta graniter. Det är oklart hur eller om detta påverkar provernas generella vittringsbenägenhet. Järn läckage från biotit är den mest troliga orsaken till missfärgningarna i samtliga prover, men orsaken till missfärgning i Granite 2 är mer osäker eftersom provet är det enda ovittrade som analyserats. Vittringsprodukter med hög Mn-halt hittades i Granite 3 och beror troligtvis på biotit, vilken är Mn-rik i förhållande till bulkkoncentrationen av Mn. Mn-koncentrationen i biotit är 60% högre (1.67 viktprocent MnO) än i övriga prover. Mn är koncentrerat i få mineral och skulle kunna förklara en del av missfärgningen. Hög koncentration av stora sprickor i stora kvartskorn är troligen den dominerande orsaken till de höga vittringshastigheterna och missfärgningen. Keywords: granite; natural stone; weathering; discolouration; biotite 1 Contents Abstract ..................................................................................................................................... 1 Sammanfattning ....................................................................................................................... 1 Contents ..................................................................................................................................... 2 Introduction .............................................................................................................................. 3 Commercial granites ............................................................................................................................ 3 Granite weathering .............................................................................................................................. 4 Methods ..................................................................................................................................... 6 Results ....................................................................................................................................... 7 Whole-rock chemistry ......................................................................................................................... 7 Mineral compositions ........................................................................................................................ 13 Petrography ....................................................................................................................................... 13 Discussion ................................................................................................................................ 19 Conclusions ............................................................................................................................. 20 Acknowledgements ................................................................................................................. 21 References ............................................................................................................................... 21 Appendix ................................................................................................................................. 23 Contents 2 Introduction The aim of this study is to investigate six leucogranites (light-colored granites), that have become discoloured when used as natural stone, and try to find the causes for their high weathering and discolouration rate. If common characteristics that could cause the rapid discolouration are found, a test procedure or recommendation can be worked out in order to prevent such granites from entering the Swedish market or being used in applications that can trigger discolouration. The granites will be classified using the S-I-A-M classification for granitoid rocks (Winter 2001) in order to find similarities or differences in their paragenisis. The physical and micro-structural characteristics of these rocks such as cracks and grain boundaries are not primarily a part of this work, but will be paid some attention. This study is a joint project in the collaboration between Earth Sciences Centre, Göteborg University (GVC) and the Swedish Cement and Concrete Research Institute (CBI), Borås. Samples were chosen by CBI and the additional Bohus granite was donated by Fredrik Henriksson, Henrikssons stenhuggeri. All six granites are currently used as natural stones, and all have become discoloured after only few months in their application/environments respectively (Fig. 1.). Three of the granites were yellow even before use (Granite 1, 4 and 5). Low-cost, yellow granites from China have become fairly common in Sweden recently, obviously due to the price but also due to their unusual color which appeals to architects among others. As geologists know, yellow rocks are rarely of good technical quality for to natural stone usage. The origin of the studied rocks is, in cases, poorly known. Sample Granite 4 is from Leizhou, southeastern China and is the only located rock. The other five samples are either from China, Italy or Portugal. Unfortunately there are not both before and after weathering samples from the same rock. All samples are from different rocks and can be grouped into three categories; one fresh unused rock, two weathered used rocks and three unused yellow rocks. Because of this inconvenience the samples will not be considered as six separate problems, instead there will be more of a “put the puzzle together” approach to this study. The Bohus granite is considered a high quality natural stone and will therefore be used for comparisons. The samples and the Bohus granite are shown in (Fig. 2.) Commercial granites In the natural stone industry the term granite has a much wider meaning and can roughly be defined as a hard polishable natural stone significantly harder than marble. 64% of the world’s commercial granites are extracted in three countries: India (32%), Brazil (22%) and USA (10%). Precambrian rocks are overrepresented. Scandinavia supplies 10.5% of the world’s commercial granites (Sweden supplies 1%) and most of them are Precambrian. Italy, China and Portugal provide 4%, 3.5% and less than1% respectively of the world’s commercial granites. For those countries phanerozoic rocks dominate. Commercial granites from Italy and Portugal are primarily Carboniferous in age and are associated with the Hercynian orogeny (~300 Ma). The geological positions of Chinese granites are not as well known to European buyers as European granites. There are several Cretaceous granites from the Fujian province in southeastern China (Pivko 2004). Fujian is a neighbor province to Guangdong, where Leizhou is situated. Two of the non-yellow granites included in this study are very similar to granites from this region, but most pictures are of low quality and rocks can have up to 50 synonym names. Given the recent problems regarding yellow granites from China, the possibility exists that all samples are of Chinese origin, but there is a great 3 uncertainty. The problems with discoloured granites from China and southern Europe are discussed by Schouenborg et al. (2010). Granite weathering The weathering rate of granites depends on many variables. Oxidation is often the first form of weathering to affect
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