TRINITITE – THE ATOMIC ROCK

On Monday, 16 July, 1945, the atomic age began with the detonation of a plutonium bomb at the Alamogordo Bombing range. One of the products of this nuclear explosion was a green glassy material that was called trinitite, the first atomic rock. The trinitite layer had a radius of ~300 m and was ~2 cm thick. The top of the layer was glassy green while the underside was light tan with sandy inclusions. The trinitite occurs as fragments, agglomerated droplets, and 2 to 3 mm beads that are spheroidal, teardropped shaped or oblong. The original trinitite layer was bulldozed under shortly after the blast, but trinitite beads are now found at the surface around anthills.

The sand at the site is arkosic and consists of α-quartz, microcline, albite, muscovite, and actinolite. This material was entrained in the blast and melted to form trinitite. It is estimated that approximately 85% of the melted material was deposited near ground zero and formed the trinitite layer while the remainder was carried some distance downwind. Anthill trinitite has been found several 1000 m from ground zero. The only crystalline phase found in trinitite is α-quartz. The quartz occurs as fragmented and partly embayed grains. A number of glasses can be distinguished which show a wide variation in chemistry, e.g. SiO₂ = 54-66, Al₂O₃ = 13-19, CaO = 2-16, Na₂O = 1-4, and K₂O = 2-12, wt. %. None of the glasses plots near the minimum in the Ab-Or-Qtz ternary and the chemistry of individual glasses correspond roughly to that of the different minerals found in the sand thus indicating disequilibrium melting.

The REE pattern for a composite sand sample is relatively flat with a pronounced negative Eu anomaly. Trinitite samples have similar patterns except they are relatively enriched in LREE due to the addition of these elements as fission products. Fe, Sc, Cr, Co, and Ni are enriched by a factor of 2 or greater in the trinitite and these elements are derived from the bomb casing and tower. The neutron dose for each trinitite sample is determined by measuring the current radioactivity of ¹⁵²Eu, decay correcting this activity to the time of detonation, and then irradiating the sample with a known neutron dose and measuring the resulting ¹⁵²Eu activity. The calculated neutron dose ranges between 6.1 and 6.5 x 10¹⁵ n cm^-2, in agreement with doses calculated using other approaches.