Thematic Articles

Garnet often occurs as naturally multifaceted, brightly colored, transparent, single crystals. These crystals represent chemically diverse solid solutions with a remarkable range of colors, which are largely controlled by the crystal chemistry of transition elements such as Fe, Mn, Ti, Cr, and V. These same optical properties have given garnet important cultural and historical relevance as a sought-after gemstone, from biblical times to the present day.

Silicate garnet is a key rock-forming mineral, and various synthetic nonsilicate garnets find important use in a number of technological areas. Garnet’s crystal structure provides the basis upon which many microscopic–macroscopic property relationships may be understood. Most rockforming garnets are substitutional solid solutions and, thus, mineral scientists are focusing their efforts on investigating local structural properties, lattice strain, and thermodynamic mixing properties. Nonsilicate compositions are used, or have potential use, in various scientific and industrial areas because of their magnetic, optical, lasing, and ion-conducting properties. Research on garnet is multidisciplinary and involves scientists in the materials and mineral sciences, physical and inorganic chemistry, and solid-state physics.

Garnet bears witness to the importance of kinetics during metamorphism in its microstructural features, compositional zoning, and diffusional response to thermal events. Porphyroblastic textures carry quantitative signals of protracted nucleation and sluggish intergranular diffusion, key impediments to reaction progress that may result in crystallization under conditions well removed from equilibrium. Growth zoning in garnet reveals partial chemical equilibration with matrix minerals: intergranular transport keeps pace with garnet growth for some elements but not for others, leading to variable degrees and length scales of chemical equilibration. Partial relaxation of compositional zoning by intracrystalline diffusion is a sensitive and quantitative indicator of thermal history, constraining rates and timescales of peak metamorphic heating, processes of burial and exhumation, and retrogression on cooling.

Garnet’s potential as a chronometer of tectonometamorphic processes and conditions was fi rst recognized over 30 years ago. The Sm–Nd and Lu–Hf systems have since emerged as the most effective chronometers, permitting age precision of better than ±1 My, even on tiny samples such as concentric growth zones within individual crystals. New, robust analytical methods mitigate the effects of ubiquitous mineral inclusions, improving the precision and accuracy of garnet dates. Important differences between Sm– Nd and Lu–Hf with respect to partitioning, diffusivity, contaminant phases, and isotopic analysis make these two systems powerfully complementary.

Thanks to its unique chemical and mechanical properties, garnet records evidence of rocks’ paths through the crust at tectonic plate boundaries. The compositions of garnet and coexisting mineral phases permit metamorphic pressure and temperature to be determined, while garnet’s compositional zoning allows the evolution of these parameters to be constrained. But careful study of garnet reveals far more, including the dehydration history of subducted oceanic crust, the depths reached during the earliest stages of continental collision, and the mechanisms driving heat and mass flow as orogens develop. Overall, chemical and textural characterization of garnet can be coupled with thermodynamic, thermoelastic, geochronologic, diffusion, and geodynamic models to constrain the evolution of rocks in a wide variety of settings.

Aluminous garnet, (Mg,Fe2+,Ca)3(Al,Cr)2Si3O12, is an important constituent of mantle peridotite (~10%) and of the other abundant upper mantle rock, eclogite (~50%). Its unusual crystal chemistry means that it strongly prefers some trace elements and confers a “garnet signature” on mantle melts. As depth increases from 250 to 600 km, garnet increases in abundance in mantle rocks, dissolving large fractions of the other silicates and becoming Si rich (majoritic). These compositional changes are observed in some garnets found as inclusions in diamond. Garnet disappears from mantle assemblages at about 700 km depth, where it is replaced by an even denser silicate, perovskite.

Garnet is a widespread mineral in crustal metamorphic rocks, a primary constituent of the mantle, a detrital mineral in clastic sediments, and an occasional guest in igneous rocks. Garnet occurs in ultramafic to felsic bulk-rock compositions, and its growth and stability span from