Garnet, the archetypal cubic mineral, grows tetragonal

Abstract

Garnet is the archetypal cubic mineral, occurring in a wide variety of rock types in Earth's crust and upper mantle. Owing to its prevalence, durability and compositional diversity, garnet is used to investigate a broad range of geological processes. Although birefringence is a characteristic feature of rare Ca-Fe³⁺ garnet and Ca-rich hydrous garnet, the optical anisotropy that has occasionally been documented in common (that is, anhydrous Ca-Fe²⁺-Mg-Mn) garnet is generally attributed to internal strain of the cubic structure. Here we show that common garnet with a non-cubic (tetragonal) crystal structure is much more widespread than previously thought, occurring in low-temperature, high-pressure metamorphosed basalts (blueschists) from subduction zones and in low-grade metamorphosed mudstones (phyllites and schists) from orogenic belts. Indeed, a non-cubic symmetry appears to be typical of common garnet that forms at low temperatures (<450 °C), where it has a characteristic Fe-Ca-rich composition with very low Mg contents. We propose that, in most cases, garnet does not initially grow cubic. Our discovery indicates that the crystal chemistry and thermodynamic properties of garnet at low-temperature need to be re-assessed, with potential consequences for the application of garnet as an investigative tool in a broad range of geological environments.

Figure 1

Macroscopic views and petrographic features of the studied blueschists, with emphasis on optical anisotropy of garnets. (a) Jenner; (b) Cazadero; (c) Farinole.

Figure 2

Typical examples of birefringence in the studied garnets. (a) 200-μm-thick wafer of an isolated euhedral crystal from Cazadero, California, with sector zoning defined by three pairs of opposite sectors. Arrows point to inclusions of riebeckite. Crossed polarizers (XP) and lambda plate (λ). (b) Regular 30-μm thin section view of a partly resorbed garnet set in a fine-grained matrix of Na-amphibole. Arrows point to the subtle concentric oscillatory zoning. Cazadero, XP. (c) Optically sector-zoned garnet porphyroblast in a biotite-graphite schist from Pfitscher Joch. 100-μm-thick section, XP. (d) Optically sector-zoned garnet in a chlorite-muscovite phyllite from the eastern Alps. 100-μm-thick section, XP, λ. (e) 30-μm thin section view of a sector-zoned garnet in a blueschist from Farinole, Corsica. Arrows indicate the boundaries between sectors. White square indicates area enlarged in (f). XP, λ. (f) Detail of (e) showing the mottled pattern of birefringence within sectors. Arrow indicates the sharp line, that is not a microfracture, corresponding to the sector boundary. XP. The B-B’ line corresponds to the EMP transect reported in Fig. 3c. (g) Detail of a garnet in a blueschist from Jenner, California showing a well-developed mottled pattern. Arrows indicate a poorly defined sector boundary. XP, λ.

Figure 3

Compositional patterns and inhomogeneities in the studied garnets. (a) Series of images showing, from left to right, optical and BSEM view of a sector-zoned garnet from Cazadero, followed by the X-ray maps with the distribution of Mn, Ca and Fe. White arrows point to Ca-high, Fe-low layers, black arrows to Ca-low, Fe high layers in the oscillatory-zoned part of the crystal. The A-A’ line corresponds to the EMP transect reported in (c). (b) Details of a zone with marked mottled birefringence in a garnet from Jenner. From left to right optical and BSEM views, Ca and Fe X-ray maps of the same area. Arrows like in (a). The C-C’ line corresponds to the EMP transect reported in (c). (c) Major element compositional features in crystals of blueschists along transects in Figs 1f and 3a,b. A-A’: Cazadero: transect from rim (A) to core (A’) of crystal. The features related to oscillatory zoning are apparent on the left part of profile. B-B’: Farinole and C-C’: Jenner. Details of the bands and patches of Ca-Fe variations. Peaks and troughs have maximum widths of 10–20 μm. For all transects the vertical arrows locate the counterbalancing variations of Ca and Fe mirroring each other.

Figure 4

Triangular compositional plot of all EMP garnet analyses. Dashed ellipse marks the compositions of the outermost rims of Pfitscher Joch micaschist and of Maniva Pass phyllite, characterized by lower Ca and higher Mg contents.

Figure 5

FTIR imaging and the distribution of the hydrous components in a garnet from Cazadero. (a) Optical image of the examined sample. (b) Distribution of the hydrous components resulting from a grid of 40 × 40 μm² single spots. (c) High-resolution image collected as a grid (reported on the image) of 15 FPA spots, each covering 170 × 170 μm²; the image clearly shows that the hydrous components are strictly related to the included fibrous minerals, while the garnet host is anhydrous. Both images were obtained by integrating the signal in the OH-stretching 3700–3400 cm⁻¹ range. The intensity of the absorption is proportional to the colour scale on the left, where blue = zero and red = maximum. (d) Selected single spectra (plotted with the same absorbance scale), collected with a 40 × 40 μm beam in a fibrous-rich area toward the crystal core (red line), and in a clean area in the garnet host (black line). The spectrum collected in the garnet host is totally flat, indicating the sample to be totally anhydrous; the spectrum collected in the hydrous zone shows a convoluted absorption (see Methods); the peaks of amphibole (Amp) and phyllosilicate (Chl, a phase close in composition to a chlorite) are evidenced. Spectra collected in the NIR (6000–4000 cm⁻¹) range in the hydrated core (not shown) display only a weak band at 4170 cm⁻¹ while no absorption occurs at wavenumbers >5000 cm⁻¹; this indicates the presence of OH-groups only as hydrous component in the sample.

Figure 6

Structural TEM features of garnets from Cazadero. (a) Section of the 3D diffraction volume obtained by EDT data: 0kl plane, showing reflections 0kl: k, l ≠ 2n, not consistent with a Ia-3d and I4₁/acd symmetry, marked by red arrows. (b) HRTEM: HR imaging of a garnet from Cazadero (corresponding [111] SAED pattern in the inset). The planar discontinuity is consistent with the occurrence of a twinning plane. Dark contrast around the twinning plane is due to local crystal structural strain.

Figure 7

Petrographic and electron microscopic images of an anisotropic garnet from Cazadero. (a) Crossed-polarized optical photomicrograph of garnet showing well-developed sector zoning. (b) Phase recognition map from same field of view as (a) indexed with electron backscatter diffraction. Red = garnet; blue = glaucophane; green = quartz. (c) Texture component map (0–2.5°) with a rainbow color scheme reveals misorientation from the mean crystallographic orientation of the grain. Blue-green domains are close to the average orientation, while warm colors represent higher degrees of misorientation from the grain average. Apart from misorientation related to a fracture on top left, apparent misorientation from the grain average is an artefact due to changes in the unit cell parameters related to intracrystalline chemical variation.

Figure 8

Thermodynamic calculation of the stability and composition of garnet. The model used coincides with an Alm₆₂Grs₂₅Sps₁₀Pyp₃ garnet composition in the SiO₂–Al₂O₃–FeO–MnO–MgO–CaO–H₂O system and assumes the presence of a saturating pure H₂O volatile phase. Background color ranges from dark blue (low concentrations) to red (high concentrations). Thin red lines are isopleths, labelled in mol%, of the garnet components considered in each panel (from top left counterclockwise pyrope, almandine, grossular and spessartine). Thick black lines are isopleths of the target (input) composition, reported within black labels. Thin black lines are isopleths of composition corresponding to ±1 mol% of the input composition. Central inset reports the P-T location of the target isopleths.