The I1−P1 and C2/c – P21/c displacive phase transitions in Ca-rich plagioclase and pigeonitic pyroxenes, respectively, produce antiphase domains (APDs) in the low temperature phases. The observed APDs are either large, with sharp, ribbon-like, boundaries and can be characterized by a uniform order parameter within the domains or small without clearly defined boundaries. For such samples the order parameter changes continuously from place to place. Recent high-resolution transmission electron microscopy (HRTEM) observations of Ca-rich plagioclase suggest that these boundaries originate from fluctuations in the order parameter for the displacive lowtemperature I1 − P1 phase transition (Nèmeth et al. 2007). The fluctuations are nonperiodic, and diffuse elongated reflections occur in diffraction patterns. In contrast, sharp reflections appear for samples having large APDs. The non-periodic fluctuations in minerals can be interpreted as the results of compositional or crystallographic heterogeneities. Although compositional heterogeneities probably promote the formation of non-periodic fluctuations, such fluctuations are also observed in synthetic Na-free anorthite. Non-periodic fluctuations originate in a low-strain regime and can be observed at room temperature for compositions close to bytownite or subcalcic augite. The non-periodic structure, which occurs between the low- and high- temperature phases, significantly affects the phase transition behavior. The analysis of IR spectra in anorthite (Atkinson et al. 1999) and in situ high-temperature TEM observation of plagioclase and pigeonite (Tribaudino 2000) suggest that samples affected by non-periodic fluctuations are structurally intermediate between the high- and low symmetry phases. Major constituents of Earth’s crust. Most important minerals of igneous petrology. Chemically, they are solid solutions of albite (Ab, NaAlSi 3 O 8 ) and anorthite (An, CaAl 2 Si 2 O 8 ) Structurally, they consist of a framework of AlO 4 and SiO 4 tetrahedra that are linked by shared oxygens. Plagioclase feldspars Pigeonitic pyroxenes Major constituents in several volcanic rocks and meteoritics. Chemically, they are solid solutions of enstatite (En, Mg 2 Si 2 O 6 ), Ferrosilite (Fs, Fe 2 Si 2 O 6 ) and diopside-hedenbergite (Di-En CaMgSi 2 O 6 -CaFeSi 2 O 6 ) Structurally, they are formed by infinite chains of SiO 4 tetrahedra, with cation in two M1 and M2 sites. (1)Chemical Research Center of the Hungarian Academy of Sciences, Budapest (2) Dipartimento di Scienze della Terra, Parma, Italy (3)School of Earth and Space Exploration,Tempe, U.S.A Both show a displacive phase transition with temperature: Plagioclase feldspars: P - I Pigeonitic pyroxenes : P2 1 /c - C2/c Both show critical domains Critical domains Conclusions:  The non periodic mottled modulations are different from the antiphase domains.  Strain affects significantly the domains in anorthite and pigeonites.  Non periodic modulations are present in low strain occurences (near the high T transition, close to critical composition) Non periodic modulations and antiphase domains Anorthite (Van Tendeloo et al 1989) and anorthitic feldspars (Tribaudino et al 2000) Ca poorer and richer iron-free pigeonite (Di 40 En 60 and Di 59 En 41 ) (Tribaudino, 2000)  Larger domains are present in anorthite and Ca-poorer pigeonite;  in anorthite the size of the domains depends on Al-Si order and Na content, in pigeonite on Ca content. Both are affected by annealing T and t  Small size domains cause diffuse reflections to appear  A preferential orientation (steps arrowed in anorthite, plane in Di 40 En 60 ) is present, indicating the presence of strain.The orientation of the domains is apparent in small sized domains by the orientation of critical reflections in SAED patterns -301 Recent work (Nemeth et al 2007) has shown in anorthite that small sized antiphase domains are likely non-periodic modulations of the displacive order parameter, without a definite boundary between the domains. This situation occurs close to the transition with temperature, with switching of sharp critical reflections to diffuse ones, and the formation of small sized antiphase domains (Van Tendeloo et al 1989), now interpreted as non periodic modulations. In anorthitic feldspars, either by Sr for Ca or NaAl for CaSi substitutions diffuse reflections are present at lower temperature and for a larger T range Critical reflections, between two stronger ones, in anorthite, left and An 80 SrF 20, right. Note the presence of diffuse reflections near the transition and for a larger T range left in An 80 SrF 20 (Tribaudino et al. 2000). Transition to small sized antiphase domains near the transition in anorthite (Van Tendeloo et al 1989) In pigeonite sharp reflections are present till the transition in first order transition with low ca content and high spontaneous strain, whereas diffuse reflections appear in the transition close to the critical composition, in sample displaying low spontanous strain. Unfortunately data in anorthitic plagioclase with lower spontaneous strain (An content approximately lower than An95) are lacking. Further work is in progress spontaneous strain data by Tribaudino et al 2002, Tribaudino et al 2003 and for the HT SAED patterns by Tribaudino (2000). Non-periodic fluctuations of the displacive order parameter and phase transitions in pyroxene and plagioclase Di 15 En 85 Di 59 En