166-12 Distribution of Spectroscopic Defects in Rare Natural Diamonds Colored by Isolated Nitrogen

Session: Mineralogical Characterization of Economic Resources: From Critical Minerals to Gemstones

Presenting Author:

Authors:

Abstract:

Nitrogen is a common impurity in the lattice of natural diamond at the parts per million or parts per billion level, introduced as single substitutional atoms (C-centers). Rare natural stones colored by C-centers can exhibit strong yellow-to-orange color, adding economic value. Experimentation has shown that with sufficient temperature C-centers will aggregate to A-centers (pairs) and eventually B-centers (four nitrogen atoms surrounding a vacancy). In nature, this process happens to more than 98% of gem diamonds over geologic time in the Earth’s mantle. Diamonds with dominant C-centers detected by Fourier-Transform Infrared (FTIR) spectroscopy are termed type Ib. Type Ib diamonds can easily be grown in a laboratory but are also occasionally preserved naturally. Some of these diamonds may contain unaggregated nitrogen atoms due to short mantle residence times, and/ or at relatively low temperatures. However, anomalous samples have been documented--including some that have both aggregated and unaggregated nitrogen atoms-- indicating there is still much to learn about these diamonds geologically.

In this study, we cut four diamonds into parallel plates in order to examine the spatial distribution of spectroscopic defects evaluated using photoluminescence (PL) and FTIR spectroscopy. These samples represent a wide variety of natural C-center bearing diamonds: two of these samples are type Ib, one type IaA+Ib and one type IaAB+Ib. The plates were FTIR and PL mapped, revealing variability in nitrogen aggregation states and lattice defects. The IaAB+Ib diamond has a colorless core and a thick yellow rim in which the C-centers are concentrated, revealing episodic growth conditions. One type Ib sample has concentric cloud inclusions in which PL peaks such as the 905 nm defect are highly localized. The other type Ib diamond contains slip planes revealing a history of plastic deformation, and the type IaA+Ib sample displays distinct growth sectors. This study highlights the complexity of C-center-bearing diamonds and reveals that many can have highly spatially localized defects, even in stones without visible color zonation. Correlating PL-active defects to distinct color zones, growth sectors, or plastic deformation- related features provide important geologic context to the nondestructive bulk spectroscopy used more commonly in gemological applications.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-8033

© Copyright 2025 The Geological Society of America (GSA), all rights reserved.