The Formation of Blue-Colored Sapphire Crystals in Volcanic Melts

Sapphires are known for their beauty and rarity, with their characteristically blue-colored crystals being highly sought after. Researchers at Heidelberg University have been delving into the formation of these precious gems in volcanic melts. While sapphires are composed of chemically “contaminated” aluminum oxide, or corundum, they are found predominantly in association with silicon-poor volcanic rocks.

The connection between sapphires and volcanic deposits has long been established, with the prevailing theory suggesting that sapphires originate in deep crustal rocks and are brought to the Earth’s surface through volcanic activity. However, geoscientists at Heidelberg University have conducted geochemical analyses that challenge this conventional wisdom. They have discovered that millimeter-sized sapphire grains found in the Eifel region in Germany formed in association with volcanism.

According to Prof. Dr. Axel Schmitt, who conducted the research, sapphires in the Earth’s crust may have originated from clayey sediments at high temperatures and pressure. As magma ascends towards the surface, it serves as an elevator for the crystals to reach the top. This hypothesis was put to the test by examining 223 sapphires from the Eifel region, where volcanic activity has been ongoing for nearly 700,000 years.

To determine the age of the sapphires, researchers used the uranium-lead method on mineral inclusions in the crystals. This analytical approach also allowed them to identify the composition of oxygen isotopes, providing valuable insight into the origin of the sapphires. By comparing the relative abundances of O-16 and O-18 isotopes, researchers were able to trace the sapphires’ formation process.

The research findings revealed that sapphires in the Eifel region formed concurrently with the volcanic activity. Some sapphires inherited their isotopic signature from mantle melts contaminated by crustal rock at depths of five to seven kilometers. Other sapphires formed in contact with subterranean melts, where the interaction between melts and adjacent rock led to sapphire crystallization.

The formation of blue-colored sapphire crystals in volcanic melts is a complex process that involves a combination of magmatic and metamorphic activities. The research conducted at Heidelberg University sheds light on the geological processes that give rise to these precious gems, challenging existing theories about sapphire formation. Further studies in this field could provide a deeper understanding of the Earth’s geological history and the formation of valuable minerals.