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On 16 July 1945, the first atomic bomb was detonated at the Alamogordo Bombing range in New Mexico, USA. Swept up into the nuclear cloud was the surrounding desert sand, which melted to form a green glassy material called 'trinitite'. Contained within the glass are melted bits of the first atomic bomb and the support structures and various radionuclides formed during the detonation. The glass itself is marvelously complex at the tens to hundreds of micrometre scale, and besides glasses of varying composition also contains unmelted quartz grains. Air transport of the melted material led to the formation of spheres and dumbbell shaped glass particles. Similar glasses are formed during all ground level nuclear detonations and contain forensic information that can be used to identify the atomic device.At 05:29:45 am local time on Monday, 16 July 1945, the nuclear age began. In a most graphic example of Einstein's famous equation (E = mc 2 or the original form m = L/V 2 where L = mass and V = the speed of light) a plutonium bomb, referred to as the 'Gadget', with a yield of 21 kilotons (equivalent to the explosive power of 21 000 tons of TNT) was detonated ( Fig. 1) at the Alamogordo Bombing range, 210 miles south of Los Alamos, New Mexico. The genie was out of the bottle and has been our uneasy companion ever since. One of the products of this nuclear explosion was a green glassy material formed by melting of the surrounding desert sand. Robert Oppenheimer had chosen the name 'Trinity' for the nuclear test and the name 'trinitite' was adopted for this green glassy material. The nuclear explosionThere are two fissionable isotopes (U-235 and Pu-239) that can be used to make atomic (fission) bombs. In both cases the basic principle is the same. A neutron interacts with a nucleus which leads to an increase in atomic mass and an unstable nucleus that splits into several pieces (fission fragments) + neutrons with a concomitant release in energy due to a loss in mass. The 235 U fission reaction is: U-235 + neutron → U-236 → fission fragments + neutrons + energyThe ejected neutrons cause further fission events leading to a chain reaction. For an explosive nuclear reaction billions of fission events need to occur in microseconds. The key is to have a sufficient mass of U-235 or Pu-239 in close proximity for this explosive reaction to occur.Two types of nuclear weapons were developed near the end of World War II. One, the 'gun type' consisted of an enriched uranium (U-235) bullet that was fired at an enriched uranium spike. The impact of these two pieces of enriched uranium produced the necessary mass density for a thermal nuclear explosion. Physicists were convinced that this type of bomb would work and only one was built and subsequently dropped on Hiroshima, on 6 August 1945. Fig. 1. The only known colour photograph of the Trinity Test.

Editor: P. DeMenocalKeywords: mollusc seasonality Gibraltar palaeoclimate oxygen isotopesThe seasonal cycle is a fundamental aspect of climate, with a significant influence on mean climate and on human societies. Assessing seasonality in different climate states is therefore important but, outside the tropics, very few palaeoclimate records with seasonal resolution exist and there are currently no glacial-age seasonal-resolution sea-surface-temperature (SST) records at mid to high latitudes. Here we show that both Mg/Ca and oxygen isotope (δ 18 O) ratios in modern limpet (Patella) shells record the seasonal range of SST in the western Mediterranean -a region particularly susceptible to seasonal change. Analysis of a suite of fossil limpet shells from Gibraltar shows that SST seasonality was greater during the last glacial by~2°C as a result of greater winter cooling. These extra-tropical seasonal-resolution SST records for the last glacial suggest that the presence of large ice-sheets in the northern hemisphere enhances winter cooling. This result also indicates that seasonality in the Mediterranean is not well-represented in most palaeoclimate models, which typically show little change in seasonal amplitude, and provides a new test for the accuracy of climate models.

Trinitite is the glass formed during the first atomic bomb test near Socorro, New Mexico, on July 16, 1945. The protolith for the glass is arkosic sand. The majority of the glass is bottle green in color, but a red variety is found in the northern quadrant of the test site. Glass beads and dumbbells, similar in morphology to micro-tektites, are also found at the Trinity site. The original description of this material, which appeared in American Mineralogist in 1948, noted the presence of two glasses with distinctly different indices of refraction (n = 1.46 and 1.51-1.54). Scanning electron microscopy (SEM) and Quantitative Evaluation of Minerals by SCANning electron microscopy (QEMSCAN) analysis is used to investigate the chemical composition and fine-scale structure of the glass. The glass is heterogeneous at the tens of micrometer scale with discrete layers of glass showing flow-like structures. The low index of refraction glass is essentially SiO 2 (high-Si glass), but the higher index of refraction glass (low-Si glass) shows a range of chemical compositions. Embedded in the glass are partially melted quartz (α-quartz as determined by X-ray diffraction) and feldspar grains. The red trinitite consists of the same two glass components along with additional Cu-rich, Fe-rich, and Pb-rich silicate glasses. Metallic globules are common in the red trinititeIn terms of viscosity, the high-Si and low-Si glasses differ by several orders of magnitude, and there is minimal mixing between the two glasses. QEMSCAN analysis reveals that there are several chemical subgroups (that can be characterized as simple mixtures of melted mineral components) within the low-Si glasses, and there is limited mixing between these glass subgroups. The red trinitite contains regions of Fe-rich glass, which show sharp contact with surrounding Fe-poor glass. Both the textural and chemical data suggest that these two glasses existed as immiscible liquids. The metallic droplets in the red trinitite, which consist of variable amounts of Cu, Pb, and Fe, show textural evidence of unmixing. These metals are largely derived from anthropogenic sources-Cu wire, Pb bricks, and the steel tower and bomb casing. The combination of mineralogical and chemical data indicate that temperatures on the order of 1600 °C and pressures of at least 8 GPa were reached during the atomic detonation and that there was a reducing environment during cooling, as evidenced by the presence of native metals, metal sulfides, and a low-Fe 3+ /Fe 2+ ratio. Independent estimates of maximum temperature during the detonation are on the order of 8000 K, far higher than suggested by the mineral data. This discrepancy is probably due to the very short duration of the event. In all respects, the trinitite glasses are similar to tektites and fulgurites, and by analogy one conclusion is that temperature estimates based on mineralogical observations for these materials also underestimate the maximum temperatures.

Biotite is a mineral of widespread occurrence and considerable petrological importance in metamorphic rocks. Numerous substitutions occur in biotite, its composition reflecting both rock bulk composition and metamorphic grade. Equilibria involving these substitutions can be used to monitor metamorphic conditions, provided that reliable thermodynamic data exist for biotite end-members. However, successful thermodynamic modeling of biotite solid solutions must be based on a sound knowledge of the cation and anion substitutions in order to define an appropriate set of end-members.The nature of the titanium substitution in biotite, however, is still an open question despite a long history of analytical and experimental investigation (e.g., Forbes and Flower 1974;Dymek 1983;Patino Douce 1993). Ti 4+ substitutes for cations of lower charge in the octahedral layer of the mica structure, but the manner in which this charge is compensated is not clear (Dymek 1983;Guidotti 1984). Suggested coupled substitutions are of various types: (1) within octahedral sites, by creation of vacancies, e.g., Ti 4+ + ■ ■ AE 2Mg 2+ ; (2) between octahedral and tetrahedral sites, as in variants of the Tschermak substitution such as Ti 4+ + 2 IV Al 3+ AE Mg 2+ + 2Si 4+ ; and (3) by loss of hydrogen from the hydroxyl site: Ti 4+ + 2O 2-+ H 2 AE M 2+ + 2OH -. This third type is sometimes called an "oxy-substitution" because it can also be written Ti 4+ + 2O 2-+ H 2 O AE M 2+ + 2OH -+ 1/2O 2 . However, according to Dyar et al. (1993), it is more appropriately termed a "deprotonation", a description made more obvious by writing Ti 4+ + H 2 AE M 2+ + 2H + .Similar considerations apply to other high-charge cations such as Al 3+ and Fe 3+ in the octahedral sites of dioctahedral mica. The nature of the substitutions cannot easily be deduced from routine analytical studies by electron microprobe, because the oxidation state of Fe is unknown, and hydrogen cannot be determined directly. The normal practice of assuming full occupancy of the hydroxyl site by monovalent anions (OH, F, Cl) means that substitutions of type 3 cannot be assessed, and the importance of octahedral vacancies may have been overstated. Recent studies in which both Fe 3+ and H + content have been independently determined (e.g., Dyar et al. 1993;Virgo and Popp 2000) suggest the probable importance of deprotonation substitutions.This study describes a natural occurrence where biotite grew and equilibrated in local domains of greatly varying Ti content as the result of the polymetamorphic transformation of granulite-facies garnet-cordierite gneiss into amphibolite-facies biotite-garnet-sillimanite rock. First, we describe the petrography and petrology of the occurrence, to establish the conditions * ABSTRACT Secondary metamorphic biotite grew with sillimanite and garnet at the expense of cordierite during a Pan-African metamorphic event overprinting Grenville age granulite-facies metapelites at the western margin of the Namaqualand Metamorphic Complex, South Africa. Fe-Mg exchange ther...