Nature of the building stones of the Koh Ker monuments
The sandstone used for the construction of the Koh Ker monuments is a gray to yellowish brown sandstone (feldspathic arenite), which also was used to build the Angkor monuments. It is generally believed that this material is derived from the Jurassic Grés Rouge Formation. However, the use of sandstone as a major building material for the construction of sanctuaries, such as those at Prasat Balang, Prasat Thneng, Prasat G, Prasat Trapean Rosei and Prasat Khna, is rare. In the Koh Ker area, outcrops of gray to yellowish brown sandstone can be found near Prasat Khna, Prasat Neang Khmau, and several other temples. In addition, ancient quarries of gray to yellowish brown sandstone have been found 6 km to the north of Prasat Thom along the Rongea River, as well as 5 km to the northeast of Prasat Thom in the area east of Prasat Trapean Russei [8, 16]. Hence, the sandstone blocks of the Koh Ker monuments likely originate from the surrounding area rather than from Mt. Kulen, which was used to supply sandstone blocks to the Angkor area. The sandstone blocks of the Koh Ker monuments had magnetic susceptibilities in the range 0.7 to 1.3 × 10-3 SI units (Figure 4). In contrast, the magnetic susceptibilities of the sandstone blocks used in the Angkor monuments during the Koh Ker period were much greater, ranging from 2.3 to 3.0 × 10-3 SI units . These results support the idea that the sandstone blocks used in the Koh Ker monuments were locally supplied. However, a distinct difference was observed between the magnetic susceptibilities of the sandstone blocks used in the Koh Ker monuments and those derived from the ancient quarries along the Rongea River and the area surrounding Prasat Trapean Russei (0.3 to 0.7 × 10-3 SI units). A decrease in the magnetic susceptibility could be owed to alteration of the sandstones in the outcrops, else there were other important sandstone quarries in addition to those along the Rongea River and around Prasat Trapean Russei that have not yet been found.
Pisolitic to slightly porous laterite was used in the construction of the Koh Ker monuments. Laterite was used extensively in the construction of the sanctuaries, enclosures, and libraries. It is not possible to visually detect any differences between the types of laterite used in these buildings. X-ray diffraction analysis and observations with a polarizing microscope confirm that the mineral content of the laterite used for the Koh Ker monuments is the same as that found in laterites from the Angkor monuments. The material is predominantly of kaolinite, quartz, goethite and hematite. In addition, a small amount of a strongly magnetic mineral was separated with a magnet, which was later identified as maghemite by X-ray diffraction analysis. The magnetic susceptibility of the laterite could be caused by this small amount of maghemite, and the presence of goethite and hematite, which are weakly magnetic, but abundant in laterite.
Close to the Srayang village, immediately south of the Koh Ker monuments, there are many laterite outcrops. Given that there are the remains of several ancient laterite quarries in this area (Figure 5), it is possible that these quarries provided the laterite material used for the construction of the Koh Ker monuments . Hence, we have investigated both the laterite blocks used in the Koh Ker monuments, as well as the laterite outcrops close to the Srayang village.
The average magnetic susceptibilities of the laterite blocks used in the Koh Ker monuments ranged from 1.0 to 5.2 × 10-3 SI units (Figure 6). Lowest magnetic susceptibility was recorded in the palaces of Prasat Thom, whereas the highest value was recorded in the sanctuary of Prasat Neang Khmau. The values were mostly in the range of 1.3 to 3.5 × 10-3 SI units. At the Angkor monuments, the average magnetic susceptibilities were found to be low for the porous laterite (0.3 to 0.6 × 10-3 SI units), but relatively high for the pisolitic laterite (0.7 to 1.6 × 10-3 SI units) . Average magnetic susceptibilities of the laterites in the Koh Ker monuments tended to be higher than those in the Angkor monuments. This difference is attributed to the richer iron content (Fe as Fe2O3: 42 to 53 wt.%) of the laterites in the Koh Ker monuments compared with those of the Angkor monuments (Fe as Fe2O3: 24 to 48 wt.%) (Table 1).
Magnetic susceptibilities of the laterites distributed around the Srayang village ranged from 1.3 to 3.1 × 10-3 SI units, covering the same range as laterites from the Koh Ker monuments.
Chemical analyses by Activation Laboratories Ltd. of representative laterite samples collected from the Koh Ker monuments are summarized in Table 1. Of all the elements analyzed, Sr showed the most remarkable variation between samples. Hence, we carried out non-destructive analyses using the pXRF on the laterites used in each building of the Koh Ker monuments, focusing on the Sr content (Additional file 1). Using these results, the laterites could be classified into two types based on their average Sr content: one had a low Sr content of less than 300 ppm, and the other had a high Sr content of greater than 400 ppm (Figure 7).
Even in the same temple, laterite blocks from the sanctuaries, libraries and enclosures generally had different average Sr contents. In Prasat Thom, the largest temple of the Koh Ker monuments, the Sr content of the laterite was low in all blocks (Figure 4). In contrast, the Sr contents of the laterite blocks of Prasat Chrap, a middle-scale temple, were high in the sanctuaries and the outer enclosure, but low in the inner enclosure. These results suggest that the laterite quarries used to provide materials in the construction of these temples changed over time. Although it is not possible to distinguish between low Sr and high Sr laterite types visually, the low Sr type is much richer in quartz under the microscope. In the Angkor monuments, we noticed that laterites with high Sr content tended to be rich in As . However, the As content was generally less than 30 ppm (below the detection limit of As by the pXRF) in all of the laterite blocks analyzed from the Koh Ker monuments.
The laterite from outcrops distributed around the Srayang village was found to be rich in Sr (1093 ± 475 ppm). High Sr content laterites from the Koh Ker monuments are relatively rich in Ti, P and light rare earth elements, but low in Pb and V (Table 1). Likewise, the laterites from outcrops around the Srayang village also were rich in Sr, Ti, P, and light rare earth elements. These results suggested that the high Sr content laterite used in the Koh Ker monuments was supplied from an area close to the Srayang village. Quarries that match the composition of the low Sr content laterites are yet to be found.