Explanation on Yellow, Green and Brown Coloration Series of Basalt Related Bangkaja Sapphires

The results confirm the pure yellow component is caused by Fe3+ with increasing intensity to the bright brown coloration so called the Mekong whisky colour, the most preferable yellow sapphires from Bangkaja, Chanthaburi, Thailand, which also commands high pricing in the market among Thai people. This colour series shows UV-Vis absorptions at 377 nm, 388 nm and 450 nm similar to their polychromatic UV-Vis excitation spectra confirming that the samples are natural without beryllium heat treatment (Monarumit et al., 2020 and 2023). The more intense brown stones also show pronounced absorption at 450 nm which is also designated for the Fe3+/Fe3+ forbidden state (Häger, 2001). The tinted green samples, i.e., greenish yellow gradually ranges to green stones tend to have an increasing of Ti content. The three series have proven that the green is caused by gradually adding the blue component to the yellow which finally turns to blue with elevated Ti together with Fe content. Though this is rather commonly understandable, however, when the blue component has been added to the yellow, and the green (also blue-green till blue) varieties visually appear, the 580 nm and 710 nm absorptions present with a remarkably additional broad absorption at around 890 nm. Besides, their FTIR absorptions display the structural Ti-OH peak at 3309 cm-1 (Phlayrahan et al., 2018a, 2018b and 2019) while the greenish tint increases, finally the whole 3309 cm-1 series (3309 cm-1, 3232 cm-1 and 3184 cm-1) appears in the blue Bangkaja stones. It is thus revealed that the 580 nm, 710 nm and 890 nm absorptions are related to Ti. The increasing 890 nm absorption also normally indicates green/blue sapphires of basalt hosted known by most gem scientists (e.g., Palke et al., 2019). Hence, it is time to explain and understand, why there is no 3309 cm-1 absorption and its series in yellow/brown sapphires whereas those with blue tint do; this is because the Fe3+ and Fe3+/Fe3+ pairs provide balance substitution of Al3+ and 2Al3+ in the adjacent octahedral sites. Indeed, the Fe3+/ Ti4+ pairs proposed by our previous work using the XANES technique (e.g., Wongrawang et al., 2016; Mogmued, et al., 2017), are responsible for the blue component which is increasingly added to Bangkaja yellow sapphires resulting in greenish yellow and green stones. Therefore, an OH- is required to balance the one-more excess positive charge of octahedral structural units which contain the Fe3+/Ti4+ pair (7+) instead of Fe2+/Ti4+ (6+) as the IVCT concept previously claimed and widely followed (e.g., Townsend, 1968; Nassau, 1978; Burns, 1981; Fritsch and Rossman, 1988).

The band models of yellow, green, and brown colour series of Bangkaja sapphires have been proposed; the yellow component gradually ranges to the intense-brownish yellow to brown colorations due to Fe3+ and Fe3+/Fe3+ states in the yellow sapphire band gap (Figure 1a and c). Besides, the Fe3+/Ti4+ mixed acceptor states are additionally present in the green colour series (Figure 1b).