An exsolution silica-pump model for the origin of myrmekite

Myrmekite, as defined here, is the microscopic intergrowth between vermicular quartz and modestly anorthitic plagioclase (calcic albite-oligoclase), intimately associated with potassium feldspar in plutonic rocks of granitic composition. Hypotheses previously invoked in explanation of myrmekite include: (1) direct crystallization; (2) replacement; (3) exsolution. The occurrence of myrmekite in paragneisses and its absence in rocks devold of discrete grains of potassium feldspar challenge those hypotheses based on direct crystallization or replacement. However, several lines of evidence indicate that myrmekite may in fact originate in response to kinetic effects associated with the exsolution of calcic alkali feldspar into discrete potassium feldspar and plagioclase phases. Exsolution of potassium feldspar system projected from [AlSi₂O₈] involves the exchange CaAlK_-1Si_-1, in which the AlSi_-1 tetrahedral couple is resistant to intracrystalline diffusion. By contrast, diffusion of octahedral K proceeds relatively easily where it remains uncoupled to the tetrahedral exchange. We suggest here that where the ternary feldspar system is open to excess silica, the exchange reaction that produces potassium feldspar in the ternary plane is aided by the net-transfer reaction K+Si=Orthoclase, leaving behind indigenous Si that reports as modal quartz in the evolving plagioclase as the CaAl component is concomitantly incorporated in this same phase. Thus silica is pumped into the reaction volume from a silica reservoir, a process that enhances redistribution of both Si and Al through the exsolving ternary feldspar.     

Exsolution in ternary feldspars

The equilibrium precipitate formed by exsolution in alkali feldspar containing calcium in solid solution is a two-phase intergrowth of plagioclase and quartz. It is the kinetics of the precipitation, however, which decides whether or not this reaction will take place. It is shown that lamellar growth of plagioclase and quartz is only accomplished by the advancement of an incoherent boundary originating at an alkali feldspar — alkali feldspar boundary or at an interface between alkali feldspar and plagioclase.     

Two-feldspar geothermometry,geobarometry in mesozonal granitic intrusions: Three examples from the Piedmont of Georgia

The equilibrium temperatures for coexisting plagioclase and potassium feldspar pairs have been calculated for various textural varieties of feldspar from 3 post-metamorphic granites from the Georgia Piedmont; the Danburg, Siloam, and Stone Mountain plutons. Assuming an intermediate structural state for the feldspars at time of equilibration, crystallization temperatures match those expected from experimental data for quartz monzonite magmas (650 to 780° C). The variations in solidus temperature, recorded in the feldspars, may be used to estimate relative differences in depth of intrusion. Sharp reversals in plagioclase compositional trends may be caused by isothermal decreases in confining pressure associated with upward migration through the crust. In fine grained and slowly cooled intrusions, albite tends to be lost from the alkali feldspar grains, and recrystallizes as separate unzoned grains of oligoclase, thus erasing the previous thermal history. Perthite exsolution and re-equilibration within the alkali feldspar grains appears to continue down to temperatures of 400° C or so, although the zoned plagioclase does not homogenize. The recrystallization associated with changes in structural state may facilitate exsolution within alkali feldspar grains.     

Myrmekite — one hundred years later

Myrmekite, first detected by Michel-Lévy in 1875 and named by Sederholm in 1899, is an intergrowth between vermicular quartz and (sodic) plagioclase situated next to potash feldspar. In felsic plutonic rocks it occurs as: rims bordering granular plagioclase, intergranular blebs set between adjacent microperthite crystals, lobes associated with muscovite in deformed alkali feldspar megacrysts or as bulbous growths at their margins, and rims on plagioclase inclusions held within orthocalse megacrysts. A literature review based largely on papers published in the past quarter century shows that hypotheses for myrmekite genesis fall mainly into five categories: simultaneous or direct crystallization, replacement of potash feldspar by plagioclase, replacement of plagioclase by potash feldspar, solid-state exsolution, and recrystallizing quartz involved with blastic plagioclase.     

Crystallographic investigation of the huttenlocher exsolution at high temperature

Three plagioclases in the composition range An_66–70 from the different geological environments were selected for investigation of the Huttenlocher exsolution by Laue and Weissenberg methods with monochromatic X-ray radiation. In order to find the characteristics of the Huttenlocher exsolution at high temperatures heating experiments were carried out. The reversible phase transition between body centered plagioclase and high plagioclase in the anorthite-rich composition of the plagioclase and the irreversible transition from intermediate plagioclase to high plagioclase in the anorthite-poor composition were observed.     

Exsolution in ternary feldspars

It is shown that the microstructures of the antiperthites of the South Norwegian anorthosites strongly suggest an exsolution origin. In addition to the normal exsolution reactions responsible for most antiperthites, exsolution of potash feldspar in plagioclase may occur by a discontinuous mode of precipitation.     

Retrograde metamorphic reactions in deforming granites and the origin of flame perthite

Abstract Microprobe analyses of feldspars in granite mylonites containing flame perthite give compositions that invariably plot as three distinct clusters on a ternary feldspar diagram: orthoclase (Or_92–97), albite and oligoclase-andesine. The albite occurs as grains in the matrix, as flame-shaped lamellae in orthoclase, and in patches within plagioclase grains. We present a metamorphic model for albite flame growth in the K-feldspar in these rocks that is related to reactions in plagioclase, rather than alkali feldspar exsolution. Flame growth is attributed to replacement and results from a combination of two retrograde reactions and one exchange reaction under greenschist facies conditions. Reaction 1 is a continuous or discontinuous (across the peristerite solvus) reaction in plagioclase, in which the An component forms epidote or zoisite. Most of the albite component liberated by Reaction 1 stays to form albite in the host plagioclase, but some Na migrates to form the flames within the K-feldspar. Reaction 2 is the exchange of K for Na in K-feldspar. Reaction 3 is the retrograde formation of muscovite (as ‘sericite’) and has all of the chemical components of a hydration reaction of K-feldspar. The Si and Al made available in the plagioclase from Reaction 1 are combined with the K liberated from the K-feldspar, to produce muscovite in Reaction 3. The muscovite forms in the plagioclase, rather than the K-feldspar, as a result of the greater mobility of K relative to Al. The composition of the albite flames is controlled by both the peristerite and the alkali feldspar miscibility gaps and depends on the position of these solvi at the pressure and temperature that existed during the reaction. Using an initial plagioclase composition of An₂₀, the total reaction can be summarized as: 20 oligoclase + 1 K-feldspar + 2 H₂O = 2 zoisite + muscovite + 2 quartz + 15 albite_plagioclase+ 1 albite_flame. This model does not require that any additional feldspar framework be accreted at replacement sites: Na and K are the only components that must migrate a significant distance (e.g. from one grain to the next), allowing Al to remain within the altering plagioclase grain. The resulting saussuritization is isovolumetric. The temperature and extent of replacement depends on when, and how much, water infiltrates the rock. The fugacity of the water, and therefore the pressure of the fluid, may have been significantly lower than lithostatic during flame growth.     

High pressure crystallization of the Ewarara,Kalka and Gosse Pile intrusions,Giles Complex,central Australia

Evidence is presented for the primary high pressure crystallization of the Ewarara, Kalka and Gosse Pile layered intrusions which form part of the Giles Complex in central Australia. These pressures are estimated at 10 to 12 kb. The high pressure characteristics include subsolidus reactions between olivine and plagioclase, orthopyroxene and plagioclase, and orthopyroxene and spinel; spinel and rutile exsolution in both ortho- and clino-pyroxene; spinel exsolution in plagioclase; high Al₂O₃ and Cr₂O₃ contents of both ortho- and clinopyroxene; high Al^VI in clinopyroxene; dominance of orthopyroxene as an early crystallizing phase; high distribution coefficients for co-existing pyroxene pairs; and thin chilled margins. Such phenomena are rare in documented layered basic intrusions.     

Mantled feldspars from the Golden Horn batholith,Washington

Mantled feldspars that formed by resorption, development of skeletal plagioclase crystals, and filling with alkali feldspar are common in the Golden Horn batholith, Washington. Subhedral plagioclase mantles have weak normal zoning from An₁₇ to An₁₀. Plagioclase zoning and twinning are crosscut by resorption channels. Resorption cavities and channels are coated with albite (An₁₀). Anhedral, perthitic orthoclase within the plagioclase is optically continuous with orthoclase in channels and on the mantle exterior.This texture resulted from resorption of calcic cores of plagioclase as pressure decreased when water-undersaturated granite magma intruded to a shallow crustal level. At shallow level, only alkali feldspar and quartz crystallized and were available to fill the skeletal plagioclase.     

Activity-composition relations in feldspars

Seck's (1971a) compositional data on coexisting feldspars in the Or-Ab-An ternary at 650° C and 1 kb were used to calculate the activity-composition relations in binary alkali feldspar and binary plagioclase. The energy constants in Guggenheim's expression for excess free energy of mixing are A ₀=3920 and A ₁=657 cal/mole for alkali feldspar, in excellent agreement with values obtained by Thompson and Waldbaum (1969), and 1320 and 373 cal/ mole for plagioclase. Using Orville's (1972) data from ion-exchange experiments between plagioclase and Na—Ca chloride solutions at 700° C and 2 kb, we obtained 967 cal/mole for A ₀ and 715 cal/mole for A ₁ in the plagioclase crystalline solution.Activity-composition relations for plagioclase are interpreted in terms of a continuous, random substitution of CaAl for NaSi across the high structural state plagioclase series. This interpretation is consistent with that obtained from a consideration of lattice parameters.     

Plagioclase-spinel intergrowths in alkali basaltic rocks from the Southern Highlands,N.S.W.

Dominant plagioclase (An₅₉) and minor spinel form possible cumulates in alkali basaltic flows from the Southern Highlands, N.S.W. The mode of occurrence of the spinel, and the chemical composition of both the spinel and the plagioclase suggest the spinel originated by exsolution from plagioclase formed at high pressure.     

Textural evolution of the rapakivi granites,south Greenland —Sr,O and H isotopic investigations

The development of rapakivi texture in feldspars from the Ketilidian granitoids of south Greenland has been investigated using Sr, O and H isotopes. A low temperature signature is found in the Sr and O data which seemingly contradicts some textural features that point to a magmatic origin of the plagioclase mantles around the K-feldspar ovoids. An origin for these mantles involving exsolution from an original alkali feldspar solid solution is proposed, which involves growth of mantles over a range of conditions determined by the mobility of the exsolving sodic feldspar. This mobility may be enhanced at high temperatures in the presence of melts or increased fluid pressures and at lower temperatures by the processes responsible for the transformation of K-feldspar to microcline. Rapakivi granites with both white and dark green feldspar occur in south Greenland but show no major isotopic differences, although the dark alkali feldspars contain significantly more fluid. Equivalent fluids in the white alkali feldspars may have escaped during plagioclase exsolution.     

Growth of plagioclase rims around metastable kyanite during decompression of high‐pressure felsic granulites (Bohemian Massif)

Samples of high‐pressure felsic granulites from the Bohemian Massif (Variscan belt of Central Europe) characterized by a peak metamorphic (high‐pressure) mineral assemblage of garnet – kyanite – plagioclase – K‐feldspar – quartz ± biotite show well‐developed plagioclase reaction rims around kyanite grains in two microstructural settings. In one setting, kyanite is randomly distributed in the polyphase matrix, whereas in the other setting, it is enclosed within large perthitic K‐feldspar. Kyanite is regarded as a relict of the high‐pressure metamorphic assemblage that became metastable during transition to a low‐pressure overprint. Plagioclase rims from both microstructural settings show continuous outwards decrease of the anorthite content from An_32–25 at the contact with kyanite to An_20–19 at the contact with the matrix or to the perthitic K‐feldspar respectively. Based on mass balance considerations, it is shown that in some cases, a small amount of kyanite was consumed in the rim‐forming reaction to provide the Al₂O₃ component for the growth of plagioclase, whereas in other cases no Al₂O₃ from kyanite was necessary. In a majority of examples, the necessary Al₂O₃ was supplied with CaO and Na₂O from the surrounding matrix material. For kyanite in perthite, a thermodynamic analysis reveals that the kyanite became metastable at the interface with the host perthite at the peak metamorphic pressure, and therefore the plagioclase rim started to grow at ～ 18 kbar. In contrast, kyanite in the polyphase matrix remained stable down to pressures of ～ 16 kbar, and the plagioclase rim only started to grow at a later stage during the decompression. Plagioclase rims around kyanite inclusions within large perthite have a radial thickness of up to 50 μm. In contrast, the radial thickness of plagioclase rims around kyanite in the polycrystalline matrix is significantly larger, up to 200 μm. Another peculiarity is that the plagioclase rims around kyanite in the matrix are polycrystalline, whereas the plagioclase rims around kyanite inclusions in perthitic hosts are single crystals with the same crystallographic orientation as the host perthite. The difference in rim thickness for the two microstructural settings is ascribed to the differences in the efficiency of chemical mass transfer next to the reaction site. The comparatively large thickness of the plagioclase rims grown around kyanite in the matrix is probably due to efficient material transport along the grain and phase boundaries in the matrix. In contrast, chemical mass transfer was comparatively slow in the large perthitic K‐feldspar grains.     

Liquidas phase relationships in the system CaAl2Si2O8-NaAlSi3O8-KAlSi3O8-NaAlSiO4-KAlSiO4 atP(H2O)=5 kb

Liquidus phase equilibria have been determined in the system CaAl₂Si₂O₈-NaAlSi₃O₈-KAlSi₃O₈-NaAlSiO₄-KAlSiO₄ (An-Ab-Or-Ne-Ks) at a pressure of water of 5 kb, for low anorthite contents. The main effects of increasing anorthite content on phase relationships in the system Ab-Or-Ne-Ks include the expansion of the plagioclase stability field towards the potassium-rich part of the system, and an accompanying contraction of the alkali feldspar, leucite, nepheline and kalsilite stability fields; and an increase in liquidus temperatures throughout most of the compositional range. Two quaternary invariant points have been identified in the system, one a reaction point between the fields of alkali feldspar, plagioclase, nepheline and kalsilite at approximately An₄, and the other probably a quaternary eutectic between the fields of alkali feldspar, plagioclase, leucite and kalsilite at approximately An₆. A shallow minimum trough in liquidus temperatures occurs on the two-feldspar surface, and this would be expected to control the paths of liquids cooling under equilibrium conditions. Phase relationships in this quaternary system have been applied to the interpretation of the histories of the potassium-rich rocks of the Roman Volcanic Region, Italy. Differentiation of the phonolitic series in this region may have occurred by two-feldspar fractionation.     

Structural characterization of labradorite-bytownite plagioclase from volcanic,plutonic and metamorphic environments

The transmission electron microscope and the electron microprobe are used to characterize calcic plagioclase (An₆₅ to An₈₅) from a variety of geological environments. The cooling histories of samples from volcanic, plutonic and metamorphic environments are estimated and the transformation and exsolution sequence is inferred from observations in the transmission electron microscope. Several distinctive textural modifications occur depending both on bulk composition and cooling history. (1) Exsolution occurs in increasingly calcic bulk compositions upon slower cooling, and the coexisting phases are An₆₆ intermediate plagioclase and An_85–90 P¯1, c=14 Å plagioclase in the sample from the metamorphic environment, (2) the morphology of b antiphase boundaries (APBs) in An₇₅ to An₈₅ plagioclase changes from smoothly curving (rapid cooling and calcic compositions) to zig-zag (slower cooling or sodic compositions). (3) The concentration of defects in the intermediate plagioclase superstructure changes from a high density in rapidly cooled plagioclase to a lower density in slowly cooled plagioclase. In all plagioclases except for the rapidly cooled, volcanic specimens there is evidence in images and diffraction patterns for short-range ordered domains with P¯1 symmetry. The observations allow the microstructure of a single zoned plagioclase to be used as an indication of the geologic environment under which it cooled.     

Nanoscale chemical characterisation of phase separation,solid state transformation,and recrystallization in feldspar and maskelynite using atom probe tomography

The feldspar minerals occur in a wide variety of lithologies throughout the Solar System, often containing a variety of chemical and structural features indicative of the crystallization conditions, cooling history and deformational state of the crystal. Such phenomena are often poorly resolved in micrometre-scale analyses. Here, atom probe tomography (APT) is conducted on Ca-rich (bytownite) and Na-rich (albite) plagioclase reference materials, experimentally exsolved K-feldspar (sanidine), shock-induced plagioclase glass (labradorite-composition), and shocked and recrystallized plagioclase to directly test the application of APT to feldspar and yield new insights into crystallographic features such as amorphisation and exsolution. Undeformed plagioclase reference materials (Amelia albite and Stillwater bytownite) appear chemically homogenous, and yield compositions largely within uncertainty of published data. Within microstructurally complex materials, APT can resolve chemical variations across a ~?20 nm wide exsolution lamella and define major element (Na, K) diffusion profiles across the lamella boundaries, which appear gradational over a ~?10 nm length scale in experimentally exsolved K-feldspar NNPP-04b. The plagioclase glass within the Zagami shergottite shows no heterogeneity in the distribution of major elements, although the enrichment of Fe, Mg and Sr in the bulk microtip points to at least minor incorporation of surrounding phases (pyroxene), and with that supports a shock-melt origin for the glass (maskelynite). The recrystallization of feldspar during post-shock annealing, such as in poikilitic shergottite NWA 6342, appears to induce a range of chemical nanostructures that locally effect the composition of the material. These findings demonstrate the ability of APT to yield new insights into nanoscale composition and chemical structures of alumniosilicate phases, highlighting an exciting new avenue with which to analyse these key rock-forming minerals.     

Coupled CaAl-NaSi diffusion in plagioclase feldspar: Experiments and applications to cooling rate speedometry

The rate of CaAl-NaSi interdiffusion in plagioclase feldspar was determined under 1 atm anhydrous conditions over the temperature range 1400° to 1000°C in calcic plagioclase (An_80?81) by homogenizing coherent exsolution lamellae. The dependence of the average interdiffusion coefficient on temperature is given by the expression: $\text{D}\text{?}\text{= 10.99 (}\text{cm}{}^2\text{/sec) exp}\text{(?123.4(}\text{kcal/mol}\text{)/RT), (T}$ in °K). This value is for diffusion perpendicular to the (03 1?) interface of the lamellae. CaAl-NaSi interdiffusion is 4 to 5 orders of magnitude slower than oxygen diffusion in the temperature range 1400° to 1200°C and possibly 10 orders of magnitude slower at subsolidus temperatures.The large differences in diffusion rates explain the apparent contradiction posed by the plagioclases of large layered intrusions (e.g., the Skaergaard), which retain delicate Ca, Na compositional zoning profiles on the micron scale, but have undergone complete oxygen isotopic exchange with heated meteoric groundwater from the surrounding wall rocks. CaAl-NaSi diffusion is slow, the closure temperature is high (within the solidus-liquidus interval), and Ca-Na zoning is preserved. Oxygen diffusion is faster, the closure temperature is lower (350°-400°C) and the feldspars exchange oxygen with the low-temperature hydrothermal fluids.The complex micron-scale oscillatory zones in plagioclase can also be used as cooling rate speedometers for volcanic and plutonic plagioclase. Cooling histories typical of large mafic intrusions (e.g. the Stillwater) are slow, begin at high initial temperatures (1200°C) and result in homogenization of oscillatory zones on the scale of 10 microns. The oscillatory zones found in the plagioclase of granodioritic plutons are preserved because cooling is initiated at a lower temperature (1000°C) limiting diffusion to submicron length scales despite the slow cooling rate of the intrusion.     

Hydrothermal alteration and mass exchange in the hornblende latite porphyry,Rico, Colorado

The Rico paleothermal anomaly, southwestern Colorado, records the effects of a large hydrothermal system that was active at 4 Ma. This hydrothermal system produced the deep Silver Creek stockwork Mo deposit, which formed above the anomaly's heat source, and shallower base and precious-metal vein and replacement deposits. A 65 Ma hornblende latite porphyry is present as widespread sills throughout the area and provided a homogenous material that recorded the effects of the hydrothermal system up to 8 km from the center. Hydrothermal alteration in the latite can be divided into a proximal facies which consists of two assemblages, quartz-illite-calcite and chlorite-epidote, and a distal facies which consists of a distinct propylitic assemblage. Temperatures were gradational vertically and laterally in the anomaly, and decreased away from the centra heat source. A convective hydrothermal plume, 3 km wide and at least 2 km high, was present above the stock-work molybdenum deposit and consisted of upwelling, high-temperature fluids that produced the proximal alteration facies. Distal facies alteration was produced by shallower cooler fluids. The most important shallow base and precious-metal vein deposits in the Rico district are at or close to the boundary of the thermal plume. Latite within the plume had a large loss of Na₂O, large addition of CaO, and variable SiO₂ exchante. Distal propylitized latite samples lost small amounts of Na₂O and CaO and exchanged minor variable amounts of SiO₂. The edge of the plume is marked by steep Na₂O exchange gradients. Na₂O exchange throughout the paleothermal anomaly was controlled by the reaction of the albite components in primary plagioclase and alkali feldspars. Initial feldspar alteration in the distal facies was dominated by reaction of the plagioclase, and the initial molar ratio of reactants (alkali feldspar albite component to plagioclase albite component) was 0.35. This ratio of the moles of plagioclase to alkali feldspar albite components that reacted evolved to 0.92 as the reaction progressed. Much of the alkali feldspar albite component in the proximal facies reacted while the, primary plagioclase was still unreacted, but the ratio for these assemblages increased to 1.51 when the plagioclase entered the reaction paragenesis. Plagioclase reaction during distal propylitic alteration resulted in pseudomorphic albite mixed with illite and a loss of Na₂O. CaO is lost in the distal facies as hornblende reacts to chlorite, although some calcium may be fixed in calcite. CaO is added to the proximal facies as the quantity of chlorite replacing hornblende increases and epidote and calcite are produced.     

Feldspar evolution in the Roxby Downs Granite,host to Fe-oxide Cu-Au-(U) mineralisation at Olympic Dam,South Australia

The textural relationships and geochemistry of feldspars from least-altered to sericite-hematite altered and mineralised ~ 1.595 Ga Roxby Downs Granite (RDG) at Olympic Dam, South Australia, were examined. The sample suite is representative of RDG both distal (> 5 km) and proximal (< 1 km) to the hydrothermal breccias of the Olympic Dam Breccia Complex (ODBC), which host Fe-oxide Cu-Au-(U) mineralisation at Olympic Dam. Microscopic observations and quantitative analyses indicate that a range of feldspar reactions have taken place within the RDG hosting the Olympic Dam deposit. An early phase of igneous plagioclase (~ An_27–34) is recognised, along with a more abundant, less-calcic plagioclase (~ An_12–20) both displaying rapakivi and anti-rapakivi textures with alkali feldspar. Alkali feldspars (~ Or₅₅Ab₄₃An₂) record post-magmatic evolution from cryptoperthite to patch perthite. Subsequent patch perthite is overprinted by highly porous, near end-member albite and K-feldspar, while plagioclase undergoes replacement by albite + sericite ± Ba-rich K-feldspar. In sericite-hematite altered and mineralised RDG along the margin of the ODBC, sericite replaces all plagioclase, whereas red-stained, Fe-rich K-feldspar persists. Sulphide-uranium-rare earth element mineralisation is observed in association with hydrothermal feldspars, and increases in abundance with proximity to the orebody. Petrographic observations and whole-rock geochemistry illustrate the transformation of plagioclase and alkali feldspar from igneous to hydrothermal processes, and indicate that hydrothermal albite and K-feldspar formed within the RDG without the need for an external source of alkalis. Feldspar geothermometry indicates a minimum crystallisation temperature of 765 °C at 2.2 kbar for alkali feldspar (pressure estimate obtained using plagioclase-amphibole geobarometry) followed by a range of lower temperature transformations. Late-stage magma mixing/contamination is postulated from supportive temperature and pressure estimates along with feldspar and mafic mineral relationships.     

Structure,formation, and decomposition of APB's in calcic plagioclase

In calcic plagioclase (Ca, Na) [(Al, Si) AlSi₂O₈] Al-Si ordering produces superstructures with periodic and non periodic antiphase boundaries (APB's). Crystals growing at high temperature close to the melting point and cooling fairly rapidly order by nucleation of ordered domains which grow, resulting in an irregular pattern of curved APB's (b plagioclase). A modulated structure with periodic APB's forms by continuous ordering at large undercooling below the critical ordering temperature (e plagioclase). During annealing APB's are eliminated by pairwise recombination of adjacent APB's to reduce strain energy along the boundaries thereby transforming nonstable e plagioclase into stable b plagioclase without change in chemical composition. This process is often accompanied by a chemical phase separation with APB's providing favorable surfaces for diffusion. Transformations are documented by transmission electron microscopy (TEM) micrographs illustrating the variation in morphology of APB patterns in igneous and metamorphic plagioclase. They are in agreement with Korekawa et al.'s (1978) model of intermediate plagioclase which relies on periodic stacking of basic units rather than wavelike modulations. The paper includes observations of a new type of satellite in Stillwater bytownite (‘h’ satellites) which are due to fine lamellar exsolution.