December 2016 Issue Table of Contents
DOI: 10.2113/gselements.12.6.379Dear friends. This final issue of Elements for 2016 marks the end of my term as a Principal Editor.
Looking back, my tenure finishes near to where it began—with a focus on mineral–water interfaces (Elements, “Mineral–Water Interactions,” v9n3, 2013). A recurring theme throughout the current issue is that chemical reactions at mineral surfaces likely had a role in sparking what we now experience as complex living systems. Indeed, mineral−water interfaces may reside at the heart of the ultimate scientific question—“What is the origin of life on Earth?”
We may never fully know the answer. However, advances in our knowledge of crystalline and amorphous solid−water interfaces and the molecular properties of solutions give reasons to be optimistic that we can gain a much deeper understanding.
Why make such a claim? High-resolution imaging techniques, such as in situ TEM, high-energy X-rays, and PEEM are producing an explosion of novel insights into mineral and solution structures and reactions that occur at their interface. Of particular interest is the prevalence of diverse small particles. From molecules to oligomers and nanocrystals, particles are present … well … everywhere. Our colleagues, Jillian Banfield (University of California, Berkeley, USA), Michael Hochella Jr. (Virginia Tech, USA), Lia Addadi (Weizmann Institute of Science, Israel), and many others, have demonstrated the abundance of small particles in every environmental compartment on Earth. Their ubiquity raises the question, touched upon in this issue, of whether the surface energy contributions of these smallest particles were a driver in the onset of prebiotic organization. Given the nonequilibrium interactions of particles with water, ions, and organic molecules, the answer could be a game-changer.
Other fields are showing that insights from high-resolution methods are already providing a broader understanding of mineral−water interfaces and low-temperature crystal growth. To put this into context, a crowning achievement of the mineralogical community over the past century was to establish a deep understanding of crystalline material structures and properties through detailed characterizations of “finished products.” This expertise is why the synthetic materials disciplines continue to rely upon crystallographers and crystal chemists. Indeed, the connections between the mineralogical and materials disciplines are seen in the original vision statement of Elements magazine.
Lagging behind these advances in crystallography was an understanding of the processes by which minerals form, particularly from solutions. This knowledge required experimental and theoretical capabilities that were previously unavailable. The late Robert Berner (1935−2015) is often credited with attempting to bridge this chasm by introducing the concepts of classical crystal growth physics to the geochemical community. The classical theories are rooted in studies by Burton, Cabrera, and Frank and refer to the attachment of individual ions or atoms to the terraces, ledges, or kinks of mineral surfaces (Burton et al. 1951). This thermodynamic model provided a process-based framework for understanding crystal growth in natural and synthetic systems but was based upon assumptions that apply to a relatively narrow range of conditions. It was never intended to describe all types of mineralization.
In the meantime, nonclassical crystal growth processes were recognized but remained unappreciated beyond the domain of colloid chemistry. Amorphous and crystalline particles were known to interact by a multistep pathway to form crystals, but a mechanistic understanding was limited by inferences from measurements of indirect properties, such as solution composition and turbidity.
We have now come full circle. With the advent of the high-resolution experimental and theoretical methods mentioned previously, the field of colloid chemistry was transformed seemingly overnight into the nanomaterial and biomaterial disciplines that we know today. At first glance, it would also seem that these disciplines had uncovered a novel idea: diverse materials and minerals can form by particle assembly. It captures the imagination to consider that crystals, including those with faceted habits, can grow by the aggregation and sometimes-oriented assembly of nanoparticles to form synthetic and biological minerals.
But not so fast. We should pause and be humbled by the fact that crystal growth by particle attachment was first proposed during the 18th and 19th centuries. Unbeknownst to much of the world, Russian crystallographers were publishing penciled illustrations of crystal growth by the coalignment of submicron particles into macroscopic crystalline structures (see Ivanov et al. 2014). Moreover, these pioneering Russians understood, at least conceptually, that the surface charge distribution on particle faces must play a role in driving oriented particle−particle interactions.
With the secret life of particles finally being revealed, one might ask, “Are new insights into mineral−water interfaces almost finished?” I would argue, “No!” Right before us, something old is giving rise to something very new. We may be witnessing an advance in our understanding of how the Earth works that will rival the way history now views plate tectonics.
As the molecular details of particle interactions emerge—with solutes, organics, and each other—I expect that we will see these concepts explain some of the long-standing enigmas in the Earth and planetary sciences. The origin of life? Possibly. You will see connections as you read this issue. The early Earth? Likely. For example, we may finally resolve enigmas of the Proterozoic—that somewhat unusual interval of Earth history characterized by massive deposits of carbonate, sulfate, and iron sediments having textures and compositions that are rarely, if ever, found again in the geological record. And the modern Earth? Definitely. A mechanistic picture of particle-based processes will improve our ability to interpret and manage urgent environmental challenges.
The coming decade promises to be ever more exciting as scientific discovery marches forward. I wish you godspeed in being part of all that lies ahead.
Burton WK, Cabrera N, Frank FC (1951) The growth of crystals and the equilibrium structure of their surfaces. Philosophical Transactions of the Royal Society of London Series A 243: 299-358
Ivanov VK, Fedorov PP, Baranchikov AY, Osiko VV (2014) Oriented attachment of particles: 100 years of investigations of non-classical crystal growth. Russian Chemical Reviews, 83: 1204-1222.