Elements Covers

August 2017 - Volume 13, Number 4

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August 2017

Boron: Light and Lively

Edward S. Grew Guest Editor

Table of Contents

Overview

Fifth in the periodic table, boron is a “light” element whose origin has puzzled astronomers because it is not created in stars. It is “lively”, being an essential element for plants, and having medicinal properties, which has stimulated synthesis of organic compounds containing boron. Borates such as colemanite are thought by some to have stabilized ribose, an essential component of ribonucleic acid and critical for the self-assembly of prebiotic organic compounds to constitute life; others have proposed that ribose was stabilized by borate in solution. Boron isotopes provide insight on the processes responsible for the creation of continental crust, and act as a proxy for paleoclimate. Extreme concentrations of boron result in economic evaporitic deposits, and, thus, water-soluble boron minerals, notably borax, have been among the most accessible of useful compounds to humankind, even in antiquity.

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2017 Topics

Thematic Articles

Boron: From Cosmic Scarcity to 300 Minerals

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Boron is rare in the cosmos because its nucleus is “fragile.” So, how does one get from the interstellar medium, where boron was first produced, to Earth’s upper continental crust where boron is concentrated in deposits containing remarkably diverse suites of boron minerals? Processes that led to the formation of continental crust also concentrated boron, which is preferentially incorporated into melts and aqueous fluids. Deposits with high boron-mineral diversity include granitic pegmatites, peralkaline intrusions, boron-enriched skarns, and evaporite deposits. Despite the loss of boron minerals from the geologic record due to their ready solubility in water and breakdown at low temperatures, the increase in boron-mineral diversity with time is real, and is accelerated during supercontinent assembly.

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Boron Behavior During the Evolution of the Early Solar System: The First 180 Million Years

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The behavior of boron during the early evolution of the Solar System provides the foundation for how boron reservoirs become established in terrestrial planets. The abundance of boron in the Sun is depleted relative to adjacent light elements, a result of thermal nuclear reactions that destroy boron atoms. Extant boron was primarily generated by spallation reactions. In the initial materials condensing from the solar nebula, boron was predominantly incorporated into plagioclase. Boron abundances in the terrestrial planets exhibit variability, as illustrated by B/Be. During planetary formation and differentiation, boron is redistributed by fluids at low temperature and during crystallization of magma oceans at high temperature.

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Boron Cycling in Subduction Zones

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Subduction zones are geologically dramatic features, with much of the drama being driven by the movement of water. The “light and lively” nature of boron, coupled with its wide variations in isotopic composition shown by the different geo-players in this drama, make it an ideal tracer for the role and movement of water during subduction. The utility of boron ranges from monitoring how the fluids that are expelled from the accretionary prism influence seawater chemistry, to the subduction of crustal material deep into the mantle and its later recycling in ocean island basalts.

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Boron Isotopes: A “Paleo-pH Meter” for Tracking Ancient Atmospheric CO2

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The boron isotope composition of calcium carbonate shells of marine organisms has the unique potential to record surface ocean pH, allowing the calculation of atmospheric pCO2 due to the established relationship between pH and the partial pressure of (atmospheric) CO2 (pCO2). This “paleo-pH meter” allows scientists to produce a record of the natural fluctuations of atmospheric pCO2 over geologic time, which will help us better understand the impacts of the recent anthropogenic addition of CO2 to Earth’s atmosphere. Towards this end, a tremendous effort to understand the systematics of boron uptake in marine carbonates is underway. Here, we review the potential of boron isotopes to constrain ocean pH and, thus, atmospheric pCO2.

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Origin and Distribution of Evaporite Borates: The Primary Economic Sources of Boron

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Naturally occurring borates are the major economic source of boron. Borates were first used over 4,000 years ago in precious-metal working and are now essential components of modern industry. Although borates have been exploited from other sources, three minerals from non-marine evaporites now form the major commercial sources of borate – borax, colemanite and ulexite. These major commercial deposits are associated with Neogene volcanism in tectonically active extensional regions at plate boundaries. The most important continental borate provinces are located in the USA, Argentina, Chile, Peru, and China, with the largest borate reserves in the world being found in western Anatolia (Turkey).

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Tripping the Light Fantastic: Organoboron Compounds

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Small molecules containing boron can play all sorts of roles in chemistry, biology and materials science. Molecular boron compounds display a wide range of unusual and fascinating structures, and their chemical reactivity can be very different from that of boron’s next-door neighbour carbon. Some of the reasons for this will be considered and illustrated through applications in energy, medicine and new materials. The boron dipyrrins, also known as BODIPYs, are a prime example. They are strongly fluorescent when excited by illumination and are widely used as fluorescent tags in biology and as biosensors. More recently, they have been studied for their energy transfer properties in light-harvesting applications.

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Borate and the Origin of RNA: A Model for the Precursors to Life

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According to the RNA World hypothesis, ribonucleic acid (RNA) played a critical role in the origin of life. However, ribose, an essential component of RNA, is easily degraded: finding a way to stabilize it is critical to the viability of the hypothesis. Borate has been experimentally shown to have a strong affinity for ribose, and, thus, could have protected ribose from degradation in the formose reaction, a potential process for prebiotic ribose formation. Accumulation of borate on Hadean Earth (prior to ~4,000 Ma) might have been a key step in the chemical evolution of the biotic sugar. Proto-arcs are suggested as a geological setting sufficiently rich in borate to stabilize ribose during the Hadean.

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