Exsolution lamellae

Figure 12 shows daubreelite exsolution lamellae in troilite the sample is from the Mundrabilla meteorite described by Ramdohr42. ... 

For normal cements, the effects of reducing conditions are all undesirable, but for white cements, which contain very little iron, reducing conditions are preferred because they yield a whiter product. Locher (LI2) concluded that the bad effects of reducing conditions were avoided so long as the clinker left the kiln at a temperature of at least 1250 C and thereafter cooled rapidly in air however, reoxidation of Fe in alite below 1300°C has been observed to cause the formation of exsolution lamellae consisting of CjF and belite (Lll). 

Studies on the comparative reactivities of ( -, o - and a-C,S, reviewed by Skalny and Young (S34), have given contradictory results. As these authors conclude, reactivity probably depends on specimen-specific factors other than the nature of the polymorph, and in the lower temperature polymorphs may be affected by the exsolution of impurities. This latter hypothesis receives strong support from subsequent observations that attack begins at exsolution lamellae and grain boundaries, and that these may contain phases, such as CjA, that are much more reactive than the CjS (Cl). Without these lamellae, P-C2S might be much less reactive than it is. 

The kinetics and mechanism of P-C2S hydration are similar to those for C3S, apart from the much slower rate of reaction (M53,T29,F11,014,F22), and, as noted earlier, the products are similar apart from the much smaller content of CH. Preparations appear to be more variable in reactivity than those of C3S this is partly attributable to differing stabilizers (F23), but could also be due to differing amounts or natures of phases in intergranular spaces or exsolution lamellae. Preparations made at low temperatures are... 

Figure 8.39. DF images of exsolution lamellae experimentally induced in an alkali feldspar (<>33). (a) Sample annealed at 600°C for 4 hours, g=400.

Figure 8.40. (a) BF micrograph showing albite-twinned albite exsolution lamellae in a cryptoperthite (Or72) and (b) its associated SAD pattern. (From McLaren 1974.)... 

Figure 8.44. BF micrograph of exsolution lamellae in a peristerite (An . ) with the electron beam approximately normal to (001). The overlapping interfaces give rise to complex fringe patterns. (From McLaren 1974.)...

Cliff, G., Champness, P. E., Nissen, H.-U., Lorimer, G. W. (1976). Analytical electron microscopy of exsolution lamellae in plagioclase feldspars. In Electron Microscopy in Mineralogy, edited by H.-R. Wenk, pp. 258-65. Berlin Springer-Verlag. 

Noncumulate eucrites originally formed as quickly cooled surface lava flows (unequilibrated noncumulate eucrites), but most were subsequently metamorphosed (metamorphosed noncumulate eucrites). Unequilibrated noncumulate eucrites, also referred to as the unmetamorphosed or least-metamorphosed noncumulate eucrites or Pasamonte-type eucrites, are surface lava flows (Figure 22(c)) that cooled quickly (Walker et al., 1978). As a result of their fast cooling, their pyroxenes (pigeonite of Mg —70-20) are zoned, and exsolution lamellae are only visible by TEM. These rocks have experienced only minor metamorphism (e.g., Takeda and Graham, 1991). 

Metamorphosed (equilibrated) noncumulate eucrites are also collectively referred to as the ordinary eucrites. They include the Juvinas type (main group) and the Stannem and Nuevo Laredo types. They are unbrecciated or monomict-brecciated, metamorphosed basalts (Figure 22(d)) and contain homogeneous low-calcium pigeonite (Mg —42-30) with fine exsolution lamellae of high-calcium pyroxene. The high abundance... 

Mikouchi T., Takeda H., Miyamoto M., Ohsumi K., and McKay G. A. (1995) Exsolution lamellae of kirschsteinite in magnesium-iron olivine from an angrite meteorite. Am. Mineral. 80, 585-592. 

Zhang R. Y. and Liou J. G. (2000) Exsolution lamellae in minerals from ultrahigh-pressure rocks. 

Minerals are often riddled with microstructures when observed under the electron microscope. Although the observation and classification of microstructures such as twin boundaries, anti-phase boundaries, exsolution lamellae etc., has been a longstanding activity of mineralogists and crystallographers it has only been very recently that we started to understand the enormous importance of microstructures for the physical and chemical behaviour of minerals. 

There are a number of processes that create fast pathways of exchange and effectively short-circuit volume diffusion into a crystal. Thus, the real world may be influenced by crystal defects and dislocations, mineral inclusions, exsolution lamellae, kink bands, microcracks, and other cryptic features (Fig. 12C). Diffusion is always active on a scale that can be modeled (Fig. 12B) and thus a world-view where all minerals are perfectly equilibrated and homogeneous (Fig. 12A) is generally a figment of imagination. In thermometry, these factors all potentially contribute to the compositions that are measured. Major advances have been made in determining when the macroscopic model world accurately predicts the microscopic real world situation. However, more work may be necessary to accurately deconvolute complex cases and tests should always be applied to evaluate thermometry. 

The interfaces between finescale exsolution lamellae of ilmenite in hematite have been investigated and most were shown to be coherent and dislocation free [251]. Due to differences in lattice parameters, dislocation arrays decorate the interface [Fig. 8(b)]. Contrast analysis revealed a hexagonal grid of rhombohedral dislocations [250] rather than dislocations with basal Burgers vectors [252]. The ordering phase transition in ferrian ilmenite from the high-temperature R3c disordered structure to a R3 lower-temperature ordered structure results in the creation of twin domains [253,254]. No dislocations were involved. 

---