Rainbow lattice sunstone (RLS) is a rare gem material that is only found in a pegmatite mine in the Harts Range of Australia, characterized by the perfectly oriented ribbon-like inclusions that produce a spectacular aventurescence effect (Fig. 1a) (Koivula and Gunter, 1989). Magnetite and hematite have been recently identified as the inclusions creating the “rainbow-lattice” effect in RLS (Liu et al., 2018). A strong iridescence (adularescence) effect is also observed in RLS (Fig. 1b) at a slightly different angle from the aventurescence, indicating the presence of submicron, strain-controlled exsolution lamellae (cryptoperthite) in the feldspar crystal. The high potassian composition (Liu et al., 2018) of RLS is very rare, if not unique, for iridescent alkali feldspar because perthites with more potassian bulk compositions (more than Or80) are almost always subject to hydrothermal or deuteric alteration (Parsons and Brown, 1991), which dramatically coarsens the lamellae texture and erases any iridescence effect. This means the RLS is not only a valuable gemstone for its unusual appearance but also a very special mineral specimen with extraordinary thermal and chemical histories.

The terminology describing the special optical effects commonly observed in feldspars, including such terms as schiller and iridescence, is often poorly defined and confusing. Therefore, it is necessary to be clarified here. All the types of play of light effects in feldspar minerals are created by the reflection of light at a particular angle from inside the feldspar crystal. Two types of reflectors have been identified in feldspars: submicron exsolution lamellae and metallic inclusions with flat shiny surfaces (iron oxide or copper). The effect produced by exsolution lamellae with thickness close to the wavelength of visible light is generally called iridescence in the feldspar literature (Smith and Brown, 1988, p. 20) (although not all iridescent feldspars display rainbow-like colors). These lamellae (<200 nm) can only be observed using an electron microscope as they are thinner than the resolution of an optical microscope. The iridescence in feldspars may appear with different colors depending on the regularity of the exsolution textures. The periodic lamellae in labradorite produce a brilliant play of color (blue to red), known as labradorescence, through constructive interference of light with certain wavelengths. The milky white or silver glow of light (Fig. 1b) observed in moonstones is commonly called adularescence (a misnomer rarely used in the feldspar literature because most moonstones are not adularia). The iridescence in peristerite (sodic plagioclase) may appear similar to either labradorite or moonstone. The reflection from orientated iron oxide or copper inclusions, on the other hand, is called aventurescence (originally “aventurization” by Andersen, 1915). The individual inclusion can be visible under optical microscope and sometimes even to the naked eye (Fig. 1a). The color associated with aventurescence is more complicated. Colors observed can be either the shiny surface color of the opaque inclusions causing aventurescence (red for copper, silver for magnetite) or the thin-film interference color seen for transparent hematite inclusions (Fig. 1a). Gem-quality feldspars with aventurescence are called sunstones (e.g., Oregon sunstone, rainbow lattice sunstone). Aventurescence may appear similar to iridescence when dense micron-scaled inclusions create a homogeneous reflection, and labradorites displaying both iridescence and aventurescence (at different angles) are not uncommon (Jin et al., 2021). The term “schiller” has been used to describe both iridescence and aventurescence (McConnell, 1974; Hofmeister and Rossman, 1983; Ribbe, 1983a; Smith and Brown, 1988), which is a word best known to the gem and mineral collectors but should be avoided in scientific contexts.

Iron oxide inclusions are very common in plutonic feldspar crystals, and they are one of the main causes for the opaque to translucent red or dark color of the otherwise transparent and light-colored mineral. Both the dark clouded feldspars with opaque inclusions and the red-clouded feldspars with transparent hematite inclusions have been extensively studied (e.g., Smith and Brown, 1988, pp. 637–642). All the iron oxide inclusions are distinctively oriented, indicating strong crystallographic constraints from the host feldspar matrix (Copley and Gay, 1979; Nienaber-Roberts, 1986; Sobolev, 1990; Wenk et al., 2011; Ageeva et al., 2020; Bian et al., 2021). The exact mechanism of the formation of these iron oxide inclusions is still poorly understood due to the potentially complex chemical reactions leading to their precipitation and variations in the thermal history and chemistry of the rocks hosting these included feldspars. Exsolution is believed to be responsible (at least partially) for the opaque acicular inclusions, but metasomatism has been advanced as the formation mechanism for the oriented thin flaky inclusions (Smith and Brown, 1988, pp. 637–642). It is possible that the same type of inclusions in various geological settings are formed via different mechanisms despite the similarity in appearances.

Alkali feldspars have been known for their wide varieties of exsolution, twinning and domain micro-textures resulting from subsolidus processes after the initial crystal growth, which can provide a record of the thermal history and replacement reactions of the host rock. The polymorphic and subsolidus phase relations of the alkali feldspars have therefore been intensively studied over the decades given their importance to igneous and metamorphic petrology. The earlier results are best summarized by Brown and Parsons (1989), and the exsolution mechanisms and kinetics are discussed comprehensively by Parsons and Brown (1991). The review by Parsons et al. (2015) includes the more recent microscopy work. The advancement of new technology such as atom probe tomography (APT) has made nanoscaled 3D chemical analysis possible, which provided valuable information for the initial state of the exsolution process (Petrishcheva et al., 2020b). Recent computer modeling has also provided a better understanding of how the lamellae textures are controlled by the cooling history (Abart et al., 2009a, b; Petrishcheva and Abart, 2009, 2012; Petrishcheva et al., 2014, 2020a).

The unique combination of the exsolution lamellae and oriented oxide inclusions in RLS provides a valuable opportunity for understanding the formation of the iron oxide crystals inside feldspars. By combining microscopic observations, chemical analyses (both bulk and microanalysis) and single crystal X-ray diffraction (SC-XRD), the iron oxide inclusions and their relations with the exsolution textures of the host feldspars are studied in this work. A new formation process is proposed for this special case. These results have potentially wide ranging implications for the general forming mechanisms for iron oxide inclusions in feldspars.

RLS is produced from a single source in the world in a pegmatite intruded into the Irindina Gneiss in the Harts Range in the Northern Territory, Australia. The mine is known by various names including the Rainbow Caterpillar mine, Utnerrengatye or Rainbow Serpent Mine. This locality was originally known as a mica mine, under the name Kong mine or Webbs Mica Mine No. 1. The pegmatite consists of fine-grained to medium-grained quartz, orthoclase and muscovite, with some tourmaline and beryl (Rochow, 1962). Graphic intergrowth between quartz and feldspar has also been reported (Thompson, 1986). The limited information about the mining process (https://rainbowlattice.com/mining, last access: October 2021) suggests that the host rock of RLS has been seriously weathered into loose materials, from which the gemstones are hand-mined and screened with large steel sieves. The sample studied in this work is a rough crystal typical of RLS material (2 cm × 1.5 cm × 0.5 cm) acquired from a mineral shop in Alice Springs, Australia.

Pegmatites occur throughout the Harts Range Metamorphic Complex (HRMC), which were emplaced through a series of pulses during the Alice Springs Orogeny (450–300 Ma) (Buick et al., 2008; Scrimgeour, 2013). The pegmatites were not locally derived but fractionated from larger unexposed granite at depth (Buick et al., 2008). The HRMC experienced peak P–T (pressure and temperature) conditions of 8–10 kbar (0.8–1 GPa) and ∼ 800 ∘C, before the Alice Springs Orogeny and the emplacement of the pegmatites at ca. 480–460 Ma (Hand et al., 1999; Mawby et al., 1999; Buick et al., 2001, 2005, 2008). Most of the pegmatites contains perthitic microcline showing strong fluid influences, and orthoclase is very rare (Joklik, 1955a), suggesting that the rock hosting RLS (perthitic orthoclase) is unusually dry in this area. The reported bulk compositions of the perthites from different pegmatites in the HRMC are consistently ∼ An1.5Ab14Or84.5 despite the dramatically different appearances and locations ∼ 100 km apart (Joklik, 1955b).

The chemical composition of the sample was analyzed with a laser ablation inductively coupled plasma mass spectrometer (LA-ICP-MS) at the Gemological Institute of America in Carlsbad, California. A Thermo Scientific iCAP Qc ICP-MS was connected to an Elemental Scientific Lasers NWR213 laser ablation system with a frequency quintupled Nd:YAG laser operated in Q-switched (pulsed) mode at a wavelength of 213 nm and pulse duration of 4 ns. Standard GSD-1G, GSE-1G and NIST SRM 610 were used for external calibration. A 55 µm diameter laser spot size was used to ablate the sample at a 20 Hz repetition rate with a fluence (energy density) of ∼ 10 J cm-2. The composition was initially internally standardized with 29Si at a pre-set value of 248 000 ppmw (parts per million weight). The data were converted to weight percent oxides and normalized to 100 wt %. The concentrations of the major elements are normalized to eight oxygen in feldspar formula as presented in Table 1. The bulk composition of RLS is very similar to those reported for the perthites from other pegmatites in the HRMC (Joklik, 1955b). The concentrations of trace elements are converted back to parts per million weight and listed in Table S1. Some laser ablations spots contain visible iron oxide inclusions, but no obvious increase in the resulting iron concentration is observed.

Compositions of individual lamellae and the matrix in the RLS were analyzed with a JEOL JXA-8200 electron probe micro-analyzer (EPMA) equipped with five wavelength-dispersive X-ray spectrometers at the Geological and Planetary Sciences Division Analytical Facility of California Institute of Technology. The data were collected with a focused beam at 10 kV and 10 nA beam current to minimize the interaction volume with the sample. Microcline, albite, synthetic anorthite, benitoite, fayalite and Sr-silicate glass were used as standards to analyze the concentrations of K, Na, Ca, Ba, Al, Si, Fe and Sr. The results of the EPMA analyses are listed in Table 2. The concentrations of Fe and Sr are about the same as the detection limit (dl), and no obvious fractionation between the exsolution lamellae and the matrix was observed. Therefore, Fe and Sr are not listed in the table.

Oriented sections parallel to (100), (010) and (001) planes/cleavages of the host feldspar were made in order to best characterize the exsolution lamellae and their relation with the inclusions. Crystals for SC-XRD analysis were cut from the (100) thin section, and the data were collected on a Bruker Apex II Ultra CCD Mo Kα microfocus rotating anode diffractometer at the crystallography lab of the University of California San Diego. The data were collected at 100 K, with the detector set at a distance of 6 cm from the crystal. The CrysAlis Pro software was used to reconstruct undistorted sections of the reciprocal space and refine the lattice parameters from the data, and the APEX3 software was used to integrate peak intensities for structural refinement. The structures were refined using JANA2006 (Petříček et al., 2014), and the 3D structure models and crystal shapes were visualized using VESTA (Momma and Izumi, 2011).

Two different sizes of feldspar exsolution lamellae extended in (6‾01) can be observed under a scanning electron microscope (SEM; Fig. 2). The coarser lamellae (∼2 µm thick) with relatively low aspect ratios are often called “spindles” (Evangelakakis et al., 1993; Abart et al., 2009b), which are regarded as a variety of the film perthite (Parsons et al., 2015). The (010) sections (Fig. 2a, b) of the spindles are more extended with mostly parallel boundaries that taper off quickly near the end, whereas the (001) sections (Fig. 2c, d) show more typical lens shapes. Fractures along the (001) cleavage occur along the spindles as a result of the volume shrinkage of exsolution lamellae relative to the orthoclase matrix. These fractures are enlarged versions of the “pull-aparts” first described by Fitz Gerald et al. (2006). The polysynthetic Albite twins can be directly observed in some larger spindles in the (001) cleavage section (Fig. 2d). Thin films of orthoclase lamellae can sometimes be found “sandwiched” in the spindle lamellae (Fig. 2b), although the 3D topology of this intergrowth cannot be determined. Much thinner film lamellae (∼ 100 nm), matching the expected thickness for the adularescence effect (Fig. 1b) and spreading several tens of micrometers in both [010] and [106] directions, can be found either isolated between the spindles or as “tails” (Tajčmanová et al., 2012) of the spindles (magenta arrows in Fig. 2). The thin films are very similar in dimensions and density as those observed in VSL295 by Evangelakakis et al. (1993). Only the spindles are large enough to be directly observed using an optical microscope, which shows the projections of the lamellae instead of the cross sections as in SEM (Fig. 3). The thin film lamellae are below the resolution of an optical microscope but can produce linear heterogeneous features in the matrix (Fig. 3a, b) through their strain field. The projection perpendicular to (6‾01) plane (Fig. 3c, d) reveals that most of the spindles are belt-shaped (5–10 µm wide and up to >200 µm long) extending in the [106] direction (Fig. 3c), whereas irregular shapes with pointy ends and zigzag edges also sporadically occur (Fig. 3d).

The compositions of the different components in the RLS are labeled following the same notation as Evangelakakis et al. (1993): L2 for the coarse spindles, L3 for the thin films, and M2 and M3 for the matrix in between the coarse spindles (including thin films) and the matrix between thin films (excluding all exsolution lamellae) respectively. The thin films are much smaller than the interaction volume of the electron beam; therefore only the pure-phase composition of L2 (spindles) and M3 (matrix excluding all exsolution lamellae) can be accurately analyzed with EPMA. The coarse spindles (L2) are mostly oligoclase with compositions between An25 and An30 (Table 2). More calcic compositions up to ∼ An33 may occur in the center of some larger irregular blebs. Very low K (∼ Or1) and no Ba (below detection limit) are detected in L2. The orthoclase matrix (M3) is quite homogeneous with low Ca concentration (An0.3Ab11.5Or87.3Cn0.9). The M2 composition (weighted average of L3 and M3) should be only slightly more sodic than M3 given the small volume proportion of thin film lamellae (L3) in the matrix.

The iron oxide inclusions in the RLS (Fig. 4) can be easily distinguished by their body color. Most of the inclusions are dark-colored magnetite crystals, and only a small proportion of them are light-colored hematite. Hematite often appears as irregular patches on the edges or corners of the thinner magnetite crystals, indicating that they are secondary to the magnetite crystals resulting from oxidation. The thickness of the iron oxide inclusions measured from SEM images (Fig. S1 in the Supplement) are all less than 200 nm, agreeing with the colors of the hematite inclusions (both the body colors and the interference colors as shown in Fig. S2) and the slight transparency of the magnetite inclusions (Yazdi et al., 2011). Therefore, these thin oxide inclusions will be referred to as “iron oxide films” (or “magnetite films” and “hematite films”) in this paper.

Most of the iron oxide films appear as bands extended at an angle ∼ 60∘ relative to the (010) plane (Fig. 4). The film inclusions cover a wide range of sizes, from ∼ 100 µm to 5 mm in length. The widths of the bands, however, rarely exceed 500 µm (Fig. 1). The corners of these flakes form angles of either 60 or 120∘, indicating a hexagonal crystal form. It is also very common to find smaller inclusions with a perfect equilateral triangle shape with one edge parallel to the (010) plane (Fig. 4c). These shapes suggest that all the iron oxide films started growing as symmetrical triangles or rhombi until one pair of the edges got abandoned, and growth only occurred along the other direction.

The relation between the iron oxide films and the oligoclase spindles is better shown in the (010) sections (Fig. 5). The oxide films are exactly perpendicular to the (010) plane as their projections only appear as thin lines. The oxide films are viewed along their longest dimensions, which extend along the entire length of the section. By measuring the angles among the oxide films, the (001) cleavage and the oligoclase spindles, most oxide films are seen to be parallel to the (100) plane of the orthoclase matrix, and some oxide films are found to be parallel to the (102) planes of the orthoclase. All of the (102) oxide films appear to cut across the oligoclase spindles (Fig. 5a, b), and some slightly smear the oligoclase spindle along the oxide film (Fig. 5b). This cross-cutting relation indicates that the iron oxide films grew after or during the final stage of the oligoclase spindle growth. The (100) oxide films, on the other hand, appear to have formed earlier. They either have no intersections with the oligoclase spindles (Fig. 5e) or strongly affect the shapes of the oligoclase spindle that they intersect with (the blebs in the oligoclase spindles shown in Figs. 5c and d and S1d are only found at intersections with the film inclusions), suggesting that they grew before or during the early stage of the spindle growth.

A different type of magnetite inclusion (identified with SC-XRD in the next section), which will be referred to as the “magnetite seeds” in this paper, can be found in the RLS at higher magnification under the optical microscope (Fig. 6). The magnetite seeds are sparsely dispersed throughout the crystal and are hard to notice at lower magnifications. However, they are somewhat common and can be found without much difficulty when looked for intentionally. These crystals have much lower aspect ratios compared to the ribbon-like thin film inclusions. The euhedral isometric crystal form (cubic or octahedral) can be identified on some of the seeds. They all are about the same size, ∼ 10 µm in diameter. The magnetite seeds appear to be randomly oriented relative to the orthoclase host. All of these seeds are accompanied by a larger irregular-shaped oligoclase spindle similar to the one in Fig. 2d, which does not completely engulf the magnetite seeds but is always in direct contact with part of the crystal. A tiny particle (∼ 1–2 µm) of an unidentified phase can sometimes be found in the albite lamella associated with the magnetite seed (marked by blue arrows in Fig. 6b–f).

Two crystals are analyzed with SC-XRD: one containing a magnetite film (Crystal 1) and the other a magnetite seed (Crystal 2). Four different phases (three feldspars + magnetite) and up to eight domains can be identified in the SC-XRD data. Some experimental details are summarized in Table 3, and the T–O bond distances of the refined structures are listed in Table 4. The crystallographic information file (CIF) of the refined structures can be found in the Supplement, which contains the complete and detailed description of the data reduction and structure refinement process for the significantly overlapped data.

The reconstructed sections of the reciprocal space of some major orientations of Crystal 1 are shown in Fig. 7, with the reciprocal lattice of orthoclase, oligoclase, albite and magnetite marked by dashed lines. Orthoclase is evidently the dominant phase, with the diffraction peaks much stronger than the other phases. The average structure of orthoclase is of monoclinic symmetry with C2/m space group. The “star-shaped” streaking around Bragg peaks (Fig. 7c) indicates a typical “tweed” structure containing nanoscaled twin domains with short range Al–Si ordering in the T1 sites (Brown and Parsons, 1989). Very diffuse scattering centered on the h+k=2n+1, violating the C-centering condition, can be observed in the h2l section as shown in Fig. 8a, which is evidence for local K–Na ordering resulting from slow cooling at low temperature (Xu et al., 2019). The lattice parameters of the orthoclase structure plot are very close to the unstrained band in the diagram proposed by W. H. Bernotat (Kroll and Ribbe, 1983) as shown in Fig. 9, indicating no detectable strain is imposed on the orthoclase phase by the exsolution lamellae, which is expected given the dominance of the orthoclase phase in the RLS. The orthoclase phase shows a structure similar to the Himalaya Orthoclase from California (Prince et al., 1973), which has reached maximum ordering for a macroscopically monoclinic structure with ∼ 80 % of Al ordering into T1 sites (Smith and Brown, 1988, p. 44). The composition of the orthoclase phase from structure refinement is An0.2Ab12Or87Cn0.8, agreeing nicely with the EPMA analysis results. A second orthoclase domain, rotated ∼ 1.9∘ around an irrational axis 0.006210.011‾ relative to the main domain with significantly weaker reflections (∼ 1/300 of the main domain), can be identified in Crystal 1 (green arrows in Fig. 7b), which could be from an orthoclase film sandwiched by an oligoclase spindle similar to that shown in Fig. 2b.

The two oligoclase domains are related by the Albite twin law (marked with green lattices in Fig. 7), both showing sharp diffraction peaks with no obvious elongation or distortion along any direction, matching the micron-scaled twin lamellae observed with SEM (Fig. 2b). The refined structural composition of An28Ab72 matches the EPMA analysis. The oligoclase structure is very similar to those reported by Jin and Xu (2017) with split M sites and maximum Al–Si ordering (Table 4) even though the lattice parameters are slightly strained from typical oligoclase (Fig. 9). The thin films, on the other hand, are twinned following the Pericline law (marked with blue lattices in Fig. 7), consisting of nanoscaled (<100 nm) twin lamellae subparallel to the (001) plane, as indicated by the strongly elongated diffraction peaks along the 001∗ direction (Fig. 8b), consistent with the transmission electron microscope (TEM) images of VSL295 from Evangelakakis et al. (1993). The composition of the thin films are shown to be almost pure albite from the structure refinement with no excess electron density on the M site relative to Na. The structure is intensely strained due to the interface with the orthoclase structure (Fig. 9). The b and c axes of the albite film are almost identical to the orthoclase matrix, suggesting perfect coherency at the interface. The 〈T–O〉 bond distances of the albite film shows a monoclinic topochemistry (Table 4) similar to the orthoclase matrix, with ∼ 75 % of the Al equally split between T1o and T1m sites. Arc-shaped streaks in the reconstructed sections of the reciprocal lattice, connecting the reflections from oligoclase and albite (arrow in Fig. 8c), are most likely from the transition between the oligoclase spindles and their albite film “tails” (arrows in Fig. 2).

The magnetite film (in Crystal 1) is Spinel-twinned, with a 180∘ rotation around the [111]* axis. The magnetite inclusion follows a specific crystallographic orientation relationship (COR) (Griffiths et al., 2016) with the host orthoclase: 111Mt∥100Or and 11‾0Mt∥001Or. No obvious elongation of the diffraction peaks of magnetite along the [111]* direction is observed despite the thin-film nature of the inclusion. This indicates the Spinel twin is not polysynthetic with multiple lamellae parallel to the (111) plane but is most likely comprised of only two continuous twin domains separated by one composition plane roughly in the middle of the magnetite film. The magnetite seed (in Crystal 2), although appearing randomly orientated, is also crystallographically constrained by the host orthoclase, with 111Mt∥110Or and 11‾0Mt∥11‾0Or (see Fig. S3). No twinning is detected for the magnetite seed. Other CORs between the magnetite seeds and the orthoclase host probably also exist given the variety of crystal forms and apparent orientations (Fig. 6). No diffraction signal is detected for the particle of unidentified phase next to the magnetite seed (Fig. 6), suggesting it is probably not a single crystal but amorphous or polycrystalline material.

The lattice parameter of the magnetite film structure is obviously distorted from the cubic symmetry. Instead of the 90∘ angle between axes for the cubic crystal system, the magnetite film structure has α=β=γ=90.53∘, which is a significant deviation from 90∘, much larger than the measurement error. This means the symmetry of the magnetite film is reduced from cubic (Fd3‾m) to trigonal (R3‾2/m). The structure is still reported in a face-centered pseudo-cubic unit cell for comparison with regular magnetite structures. Other than the distorted lattice, the magnetite film structure shows no obvious difference from a regular cubic magnetite structure. Even the extinction condition of the pseudo-d-glide plane is obeyed in the diffraction pattern (Fig. 7b, c). The deviation from cubic symmetry is most likely caused by surface strain from the extremely anisotropic crystal shape and the interface with feldspar. It has been reported that epitaxic magnetite films grown on BaTiO3 substrate would change their magnetic properties at low temperature in a way similar to the Verwey transition, resulting from the changes in the interfacial stress (Bohra et al., 2019). A similar effect may exist in the magnetite films in RLS, which would require further magnetic analyses to confirm. Relaxing the Fe occupancy in the refinement results in a composition of Fe2.85O4 with each Fe site only 95 % occupied. Twin fractions of 60 % and 40 % were calculated for the obverse and reverse twin domains in the refinement. The magnetite seed appears to have a regular cubic lattice and structure with space group symmetry of Fd3‾m. The Fe occupancies for the octahedral and tetrahedral sites are also 95 % when relaxed in the refinement, the same as in the magnetite film structure. This apparent Fe deficiency in the structure refinement could be some artifact from the data reduction affected by peak overlapping between different phases, considering the lattice parameters are not obviously smaller than the standard magnetite structure (even though the data are collected at 100 K). It is also possible that Al and Ti substitution in the magnetite structure is the reason for the apparent vacancies in the structure, which would require nanoSIMS (nanoscale secondary ion mass spectrometry) or TEM analysis to confirm.

The strain-controlled exsolution textures in the RLS are very unusual, especially for its composition and scale. Elongated belt-shaped pristine lamellae that are several micrometers thick have not been reported before. The “pull-aparts” in the RLS accommodating the lattice strain on the interfaces are not evenly distributed in every oligoclase spindle, and they only appear in (010) sections but not in (001) sections. The wide variety of different sizes and shapes of the film perthite, from nano-platelets to thin films to spindles, is likely due to the combined effect of chemical composition, pressure and cooling rate. Several aspects of the strain-controlled exsolution textures in alkali feldspars may be explored in future studies, including the effect of pressure on the composition of the plagioclase lamellae, as well as the chemical zoning in the plagioclase lamellae in alkali feldspars as a function of cooling rate. The structural transition from oligoclase spindles to albite films may also reveal some details about the nucleation and growth process.

SC-XRD has been shown to be a powerful tool for studying the submicron inclusions and exsolution lamellae in minerals and is complementary to electron microscopy. It is especially well-suited for strain quantification and structure analysis in strain-controlled exsolution textures (Jayaraman, 1959; MacKenzie and Smith, 1962; Ribbe, 1979). Unlike intergrowths formed by replacement (Balić-Žunić et al., 2013), different lamellae of the same phase in diffusion-formed intergrowth have exactly the same orientation and therefore produce the same diffraction pattern. The strained structure of the albite lamellae in film perthite is refined for the first time, revealing a monoclinic topochemistry inheriting the framework from the orthoclase host. This is further evidence supporting the framework distortion or collapsing as being the main driving force for the Al–Si ordering in plagioclase feldspar (Jin et al., 2019). It also demonstrates that the ordering state may provide additional constraints on the nucleation temperature of the exsolution lamellae. Some previously studied film perthite (such as the Shap granite) should be revisited with SC-XRD to see if there are any structure variations in the albite films due to different composition and cooling rate.

The unusual relation between the magnetite films and the exsolution textures in the RLS reveals a new forming mechanism for the enigmatic Fe-bearing inclusions commonly found in feldspar minerals. Feldspar samples with different forms of iron oxide inclusions and exsolution textures from other localities, such as the popular peach moonstone or black moonstone on the gem market, may provide additional clues for a more complete picture. The exact atomic configuration at the magnetite-film surface may provide more details about the nucleation and growth process in included feldspars. The detailed chemical composition profiles across these boundaries can be analyzed using atom probe tomography (APT) or analytical TEM, and the trace element concentrations in each individual phase can be analyzed with nanoSIMS, which may help us to understand the chemical diffusion process during the nucleation and growth.