Gemmological Characteristic and Heat Treatment of Trapiche-Like Sapphire from Shandong Province

The sapphire from Changle, Shandong Province have received widespread attention for the high yield, large crystalline grains, complete crystal shapes, and diverse varieties^[1-2]. However, there are fewer studies on the trapiche-like sapphire, a special variety, and the cause of its phenomenon is still inconclusive^[3-4]. In addition, The value of the sapphire from Shandong Province is inexpensive because of the dark colour and poor transparency. The research hotspot on improving the colour and market value of the sapphire from Shandong Province has been discussed. However, the features of inclusions and spectral characteristics of sapphire from Shandong Province during the heating process were not thoroughly documented or summarized in the earlier studies^[5-6]. Thus, in this study, the gemmological characteristics and heat treatment of trapiche-like sapphires from Changle, Shandong Province were studied using standard gemmological instruments, Raman spectroscopy, infrared spectroscopy, laser ablation-inductively coupled plasma-mass spectrometry, scanning electron microscopy-energy dispersive spectrometer, ultraviolet visible near-infrared spectroscopy and high-temperature tube furnaces.

The surface of trapiche-like sapphire samples from Shandong Province typically have visible erosion pits and a black film^[1]. There is a hexagonal colour zoning and trapiche phenomenon on the cross section perpendicular to the c-axis, that is a combination of the blue-black hexagonal "core" and six silvery fibrous "arms"(Fig. 1). The mineral componets of the "arm" and fan-shaped areas is corundum, indicating that it should be trapiche-like sapphire with many kinds of different inclusions. The dark "core" is mainly caused by a large number of brown, black irregular flake-shape, needle-shape or point-shape inclusions that are densely dispersed. The "arms" is composed of a significant amount of thin-film fluid inclusions, fine fissures, and healing fissures that are parallel to the direction of the extension^[7].

The results of chemical composition show that the sapphire samples from Shandong Province are typical basalt-type genesis^[8-11]. The contents of Fe and Ti elements is the highest in the "core" area. Colour bands are related to the cyclic fluctuations in the elemental composition of Fe and Ti in the primary magma. It is easier to distinguish the trace element characteristics between sapphire samples from Shandong Province and the sapphires with mostly various basalt origins in the world. The IR spectra of the trapiche-like sapphire samples from Shandong Province all have OH-related infrared absorption peaks at 3 696, 3 624, 3 309 cm^-1, indicating that they had not undergone heat treatment. The micro-UV-Vis-NIR absorption spectra show that Fe²⁺-Ti⁴⁺ pair is closely linked to the shift in blue hue, and Fe³⁺ is a weak chromophore. In short, sapphire samples from Shandong Province have deeper colour because of the selective absorption of these chromogenic ions in practically all visible light ranges, as well as their high total absorption.

The temperature range of the heat treatment in oxidation circumstances is 1 000-1 600℃, with the temperature step of 100℃. That experiment can make the colour of sapphire samples lighter with producing gray tones. Additionally, as the temperature increased, the trapiche-like phenomenon and colour band progressively faded. The colour and types of inclusions, as well as variations in infrared transmission and reflection spectra, have changed at different temperatures. Therefore, the identification of heated sapphire (heating to 1 000 ℃ or above) can be identified by internal and external characteristics as well as spectroscopic features.