Lamellar twinning

Finally, one should be cautioned that, occasionally, substances form chiral single crystals of nearly racemic composition. For example, hexahelicene crystals grown from racemic solutions apparently undergo spontaneous resolution, displaying the enantiomorphic space group P2[2,2, however, the e.e. in the crystal is only —2%. This material (and probably others as well) has a lamellar, twinned structure in which alternating layers, 20 p,m thick, of optically pure (/ )-( + )-and (M)-( — )-hexahelicene are perfectly aligned to build up the observed crystal (266). 

Figure 7.1. Various growth twins, (a) Contact twin (albite) (b) inclined twin (quartz) (c) elbow twin (rutile) (d) cyclic twin (chrysoberyl) (e) lamellar twin (albite) ...

Green, B. S. and Knossow, M. (1981). Lamellar twinning explains the nearly racemic composition of chiral, single crystals of hexahelicene. Science, 214, 795-7. [30]... 

Moooclinic needles (lamellar twinning on 001) from water mp 357", Sublimes 290-295 . Kb at 18 L3 X IQ"1 Ka 0,9 X JO-10, Absorption spectrum Hartley, J. Chem-Soc- 87, 1803, 1810 (1905), One gram dissolves in about 2000 ml water, 150 ml boiling water, 2220 ml 95% alcohol sol in the fixed alkali hydroxides, coned acids, in about 22 parts of 20% aq tribasic sodium phosphate soln moderately sol in ammonia. Almost insol in benzene, ether, chloroform, carbon tetrachloride. Forms salts which are dec by water, and compds with bases which are more stable. 

Calcium sulfate inverts at 1195°C to alpha anhydrite, which is monoclinic with lamellar twinning like plagioclase and has an n of 1.50, n of 1.56, and birefringence of 0.060. Beta anhydrite is obtained by heating... 

The typical Pb granules obtained in the potentiostatic regime of electrolysis are shown in Fig. 2.34. Similar to silver granules, granules of various shapes were produced by the electrodeposition processes. The granules, such as octahedrons and hexagons, as weU as many various types of twinned particles single-twinned, multiply-twinned (MTPs), lamellar-twinned (LTPs), and many other complicated... 

Schreyerite (60430-06-0) (Named after German mineralogist Werner Sdireyer) V,Ti,0, M s 389.4786 26.16 wt.% V 36.87 wt.% Ti 36.97 wt.% 0 Coordinence Ti(6) (Oxides, and hydroxides) Monoclinic a= 706 pm b= 501 pm c= 1874 pm 119.4 (Z=6) Biaxial ( ) = 2.7 HV 1150 4480 Habik tiny crystals with lamellar twinning embedded in rutile crystals. Color black. Luster vitreous to resinous. Insoluble in adds. Mclting poink 1740 C Occurrence in vanadium-rich gneisses associated with kyanite and rutile. 

Figure 2-12 Microstructure of enstatite- 3-spodumene glass-ceramics showing lamellar twinning in enstatite.

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