The cause of color on blue sapphire from different gem deposits before and after heat treatment

Name: Wanvisa Thengthong.

LCSH: Kasetsart University -- Theses. M.S. (Earth Science and Technology) 2023.

Classification :.LCCS: TN997.S24

LCSH: Kasetsart University. -- Department of Earth Sciences -- Theses.

LCSH: Sapphires -- Coloring.

LCSH: Sapphires -- Heat treatment.

Abstract: The blue color of sapphire is commonly related to the amount of Fe and Ti impurities replacing Al3+ in Al2O3 structure. Generally, the blue color intensity on sapphires is related to the gem deposits including the basaltic-related and metamorphic-related ones. The blue color of sapphires has been turned after heating under oxidizing atmosphere. However, the explanation about the color mechanism from some previous research contradicted each other and it was still wondered. For example, the faded blue intensity on sapphires after heating was reported that caused by the transition between Fe2+ and Fe3+ along with the weakened intensity of the Fe2+ -Ti4+ intervalence charge transfer band. Meanwhile, an evidence about Fe3+ and Ti4+ on blue sapphire whether or not undergone heating was reported. For this reason, this research is focused on the role of Fe and Ti oxidation states as well as the blue color mechanism on sapphires before and after heating under oxidizing atmosphere. In this study, the sapphires were collected from different gem deposits including basaltic-related sapphires from Kanchanaburi province, Thailand and metamorphic-related ones from Sri Lanka before and after heating at 1100 oC under oxidizing atmosphere. Thus, the research experiments were conducted to verify the blue color mechanism by using basic and advanced instruments. As a result, the basaltic-related sapphires show darker blue than metamorphic-related ones due to a higher Fe content relating the higher amount of Fe-Ti pairs assigned to Fe3+ -Ti4+ mixed acceptor states inside an energy band gap and generating a hole color center. The blue color on sapphires before heating can be described that a hole color center that could be received an electron from the valence band for charge-balancing after excitation. After heating at 1100 oC under oxidizing atmosphere, the basaltic-related sapphires turned from dark blue to light blue and the metamorphic-related ones turned from light blue to colorless because the Fe3+ -Ti4+ mixed acceptor states were decreased because a hole color center was filled by an electron from oxygen during the heating process instead of an electron from the valence band. Therefore, it can be concluded that the blue color mechanism on sapphires before and after heating under oxidizing atmosphere can be explained by an energy band model involving the presence or absence of Fe3+ -Ti4+ mixed acceptor states as well as a hole color center inside an energy band gap.

Kasetsart University. Office of the University Library

Address: Bangkok

Email: tdckulib@ku.ac.th

Name: Natthapong Monarumit.

Role: Thesis Advisor

Name: Somruedee Sakkaravej.

Role: Thesis Co-Advisor

Name: Wiwat Wongkokua.

Role: Thesis Co-Advisor

Created: 2023

Modified: 2025-06-25

Issued: 2025-06-25

วิทยานิพนธ์/Thesis

วิทยานิพนธ์/Thesis

application/pdf

URL: http://www.lib.ku.ac.th/KUthesis/2566/wanvisa-the-all.pdf

CallNumber: TN997.S24.W36

eng

DegreeName: Master of Science

Level: Master's Degree

Descipline: Earth Science and Technology

Grantor: Kasetsart University

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