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Topic: Crystal twinning

	Twinning, Polymorphism, Polytypism, Pseudomorphism
		Twinning is important to recognize, because when it occurs, it is often one of the most diagnostic features enabling identification of the mineral.
		Growth Twins - When accidents occur during crystal growth and a new crystal is added to the face of an already existing crystal, twinning can occur if the new crystal shares lattice points on the face of the existing crystal, but has an orientation different from the original crystal.
		Because this transformation involves a drastic rearrangement of atoms on the crystal structure, as evidenced by the fact that diamond is the hardest naturally occurring substance and graphite is one of the softest) diamond is found at the T and P conditions present at the Earth's surface, where it is therefore only metastable..
	www.tulane.edu /~sanelson/eens211/twinning.htm (2863 words)

	Crystal twinning - Wikipedia, the free encyclopedia
		Growth twins are the result of an interruption or change in the lattice during formation or growth due to a possible deformation from a larger substituting ion.
		Twin boundaries are partly responsible for shock hardening and for many of the changes that occur in cold work of metals with limited slip systems or at very low temperatures.
		They also occur due to martensitic transformations: the motion of twin boundaries is responsible for the pseudoelastic and shape-memory behavior of nitinol, and their presence is partly responsible for the hardness due to quenching of steel.
	www.search.com /reference/Crystal_twinning (559 words)

	Crystal twinning: Massimo Nespolo's Research Themes - LCM3B UHP Nancy 1
		The twin operation is a symmetry operation for the twinned edifice but not for the individuals: it relates different individuals in the twin and belongs to a crystallographic point group.
		Twinning by reticular merohedry may thus occur, the twin operation being a point-symmetry operation belonging to the mmm holohedry of the sublattice but not to the 2/m holohedry of the lattice.
		Whereas for twins the twin index and the twin obliquity directly influence the probability of twin occurrences, for plesiotwins a similar lattice control is not recognized.
	www.lcm3b.uhp-nancy.fr /lcm3b/Pages_Perso/Nespolo/twinning.php?PHPSESSID=a98f638ddd5293c3cfe58a49e98772d5 (3464 words)

	NIH MBI Laboratory for Structural Genomics and Proteomics
		Twinning is a crystal growth disorder in which the specimen is composed of distinct domains whose orientations differ but are related in a particular, well-defined way.
		In epitaxial twinning, different domains are typically oriented so that their molecular spacings match at the crystal face where they meet, but their crystal lattices are not superimposable.
		Clues that diffraction data may be from a perfectly twinned specimen include: (a) a unit cell that is too small to contain the known molecule under the apparent space group symmetry, and (b) an intensity distribution that does not follow Wilson statistics.
	nihserver.mbi.ucla.edu /Twinning/intro.html (717 words)

	[No title]
		Crystals in which the centers of the constituent atoms occupy geometrically similar positions, regardless of the size of the atoms and the absolute dimensions of the structure, are said to belong to the same structure type (KandH, p.
		Mineralogical twins are analogous to monozygotic, Siamese twins.
		Twinning is defined by a twin law, which indicates whether there is a center, axis or plane of twinning, and also defines the crystallographic orientation of the twin element (usually with Miller Indices).
	www.auburn.edu /~hameswe/MinDay7.html (2029 words)

	CRYSTAL HABIT
		These crystal forms are controlled by the structure and therefore the symmetry of the crystal.
		If a dome or sphenoid is on the side of a crystal it might be confused with two of the four faces of a prism (look for similar faces on the other side of the crystal axes).
		The scalahedron is a general term for three crystal forms differentiated by the symmetry of the prominent axis.
	mineral.galleries.com /minerals/property/crystal.htm (2460 words)

	Science of Minerals
		Depending on the number of degrees through which the crystal must be rotated to produce this effect, a crystal may look the same from 2, 3, 4, or 6 different positions; such a crystal is described as having n-fold symmetry (e.g.: "2-fold symmetry") along that axis of rotation.
		Crystal habit depends not only on atomic structure and crystal symmetry, but also on on the environment in which the crystals grew, and so a given mineral species may exhibit different crystal habits at different localities.
		occurs during crystal growth, and is thought to occur as a result of nucleation errors, due to the addition of a few atoms or ions in the wrong place on the surface of a growing crystal.
	www.utexas.edu /tmm/npl/mineralogy/Science_of_Minerals/index.htm (1692 words)

	Untitled
		The laws of thermodynamics require that in order for a crystal to grow, the total free energy of the crystalline phase must be less that that of the phase from which the crystal is forming (e.g., a liquid).
		Since equilibrium crystallization requires that the crystalline phase have a lower energy than the reactant phase, energy in the form of heat must be released during crystallization.
		In the context of twinning, rational means that the two portions of the twin have a point, a line or a plane in common, and symmetrical means that the parts of the twin are related by some crystallographic symmetry operation, such as reflection, rotation or translation.
	darkwing.uoregon.edu /~jrice/geol_311/xtalgrow.html (1638 words)

	Mineral Gallery - THE TWINNED MINERALS
		The twin laws are crystallographic in nature and are caused by flaws in the crystal structure that occur during growth or during changes in phases such as from a high temperature phase to a low temperature phase.
		Twins are recognized by penetration angles or notches in the crystal, mirror planes that do not normally occur on a specific mineral and crystallographic techniques not normally available to the average collector.
		Twinning often has a dramatic effect on the outward symmetry of the mineral either by raising or lowering the symmetry.
	mineral.galleries.com /minerals/property/twins.htm (1094 words)

	Amethyst Crystal Structure @ Jewel Info 4 U
		Amethyst crystals are formed as flat hexagon with alternate purple and clear quartz sections formed by numerous crystals twinning in planes.
		One crystal may show pale pink to lavender shade near the base to a deep dark purple at the point where as the other crystal may show uniform purple color from base to top.
		This happens due to process called crystal zoning which is layers of different colors caused by changing compositions of the fluids that formed the amethyst.
	www.jewelinfo4u.com /Amethyst_Crystal_Structure.aspx (1077 words)

	Chem4Kids.com: Glossary: C
		The fresh surface is not as smooth as the naturally occurring surfaces of the crystal.
		A crystal lattice is a very exact organization of atoms that allows for a specific place for every molecule or atom in the solid.
		Crystal twinning occurs when two crystals grow in different directions and eventually intersecting.
	www.chem4kids.com /glossary/c.html (1720 words)

	Physical Detwinning of Hemihedrally Twinned Hexagonal Crystals of Bacteriorhodopsin -- Efremov et al. 87 (5): 3608 -- ...
		It is concluded that crystals of bacteriorhodopsin are composed
		TABLE 1 Comparison of crystallographic parameters and twinning fractions for two sets of data measured from the same crystal: 1), data measured on rotating anode generator (procedure of measurement is described in Materials and Methods)—partial data set, and 2), data measured on a synchrotron source — complete data set; statistics were calculated for noncentrosymmetric reflections
		Crystallization and 2.2 A resolution structure of R-phycoerythrin from Gracilaria chilensis: a case of perfect hemihedral twinning.
	www.biophysj.org /cgi/content/full/87/5/3608 (2702 words)

	ROCK CRYSTAL Quartz Factsheet & Information Page at mineralminers.com
		Twinning: Dauphine twin with c the twin axis, Brazil twin with {1120} the twin plane, Japanese twin is rare with {1122} the twin plane.
		Phantom quartz shapes are sometimes present in the interior of rock crystal, outlining an earlier stage of the rock crystal's crystallization.
		Rock crystal is considered to be the most usefull and versatile crystal for metaphysical purposes, usable in both its rough form and in a variety of sculpted items such as spheres (crystal balls), gemstones, polished crystals, obelisks, pyramids, skulls, massage tools, etc...
	www.mineralminers.com /html/rkxminfo.htm (1344 words)

	Crystallization of bacteriorhodopsin from bicelle formulations at room temperature -- Faham et al. 14 (3): 836 -- ...
		Crystallization of bacteriorhodopsin from bicelle formulations at room temperature -- Faham et al.
		The crystals diffracted to ~1.8 Å at the ALS beamline
		Thiessen, K.J. The use of two novel methods to grow protein crystals by microdialysis and vapor diffusion in an agarose gel.
	www.proteinscience.org /cgi/content/full/14/3/836 (2072 words)

	Unusual Snow Crystals
		Crystal twinning is a phenomenon whereby two crystals grow out of a single seed and have a specific crystallographic orientation relative to one another.
		The seam between the crystals can be seen when some of the ice evaporates, leaving an evaporation groove, as indicated in the picture at right.
		Another example of crystal twinning occurs when twin columns grow from a common center, one rotated 60 degrees relative to the other.
	www.its.caltech.edu /~atomic/snowcrystals/unusual/unusual.htm (384 words)

	Crystal bends light back TRN 110503
		What sets the researchers's type of crystal apart is that it has a twinning structure, which means that the orientation of the crystal's atoms on one side of the junction is the mirror image of the atomic structure on the other side.
		The permittivity and permeability of the twinning crystal structure are positive.
		The twin crystal structure could be used to channel light now, according to Zhang.
	www.trnmag.com /Stories/2003/110503/Crystal_bends_light_back_110503.html (767 words)

	[No title]
		This is a crystal twinning habit in which the two crystals are of opposite handedness, one being right and the other left,with a face of the trigonal prism of the second order as the twinning plane.
		Study suggests that this unusual form of crystallization is caused by pyroelectricity during crystal growth when positive and negative electrical charges develop simultaneously on different parts of the same crystal in response to temperature changes suitable to evoke this electrical phenomenon.
		Above 2,400 meters to 4,000 meters in altitude the quartz crystals are a smokey color, with the intensity of coloration increasing in direct proportion to the proximity of crystal location toward an elevation of 4,000 meters.
	web.tiscali.it /mmacchieraldo/artmbian.htm (4502 words)

	CCP4 Program Suite
		For this to happen two lattice repeats in the crystal must be of equal length to allow the array of unit cells to pack compactly.
		If we have two twin domains and every reflection has contributions from both, it is unlikely that both contributions will have very high or that both will have very low intensities, so the intensities will be distributed so that there are fewer extreme values.
		A crystal is a 3-dimensional translational repeat of a structural pattern which may comprise a molecule, part of a symmetric molecule, or several molecules.
	www.ccp4.ac.uk /dist/html/twinning.html (1588 words)

	List of mineral specimens available through mail order. Many classic minerals from old collections.
		The crystals are twinned on the {01 2} plane one of the rarer forms (more commonly they twin on the {01 8} plane).
		The enargite crystals have a dull luster and striated crystal surfaces.
		Plate-like crystal aggregates of beige-colored hydroxyl-herderite crystals with scattered overgrowth of colorless apatite crystals, presumably hydroxylapatite - not fluorapatite.
	www.johnbetts-fineminerals.com /jhbnyc/non-web.htm (6833 words)

	General Ontology XXIXm1a4 (Site not responding. Last check: )
		Growth twins are the result of an emplacement of atoms, or ions (or groups of atoms or ions) on the outside of a growing crystal in such a way that the regular arrangement of the original crystal structure (and, therefore, its lattice) is interrupted.
		The growth of columnar crystals is controlled by the interplay between faceting and branching (The latter is, as we saw earlier, a growth of fast-growing faces while their surface area remains very small).
		The crystal in the left image appears (to me) to reveal the presence an evaporation groove around the middle of the column, indicating that the crystal is in fact a twin.
	metafysica.nl /ontology/general_ontology_29m1a4.html (5127 words)

	CCP4 Program Suite : detwin
		Twinned data is measured when two or more copies of the reciprocal lattice overlap.
		See Cumulative distribution plot, which for twinned data becomes sigmoidal, and the moments of E (or Z) which are different for twinned data than for untwinned.
		Twinning operator, given as the indices of the reflection that is related to the reflection (h,k,l) by the twin operator.
	www.ccp4.ac.uk /html/detwin.html (1209 words)

	Crystal Habit
		The term "crystal habit" is used to identify the shape, size and appearance of a crystal's unique growth characteristics, or "Crystal Forms".
		Crystal habits are useful in communicating what specimens of a particular mineral would, or should look like.
		Most gemstone crystals are not found as perfectly formed single crystals but are found as aggregates of several crystals grown together in one mass.
	www.khulsey.com /jewelry/crystal_habit.html (274 words)

	artisan jewelry, handcrafted beaded jewelry, gemstone jewelry
		The name Cerussite is derived from the Latin "Cerussa" which means "White Lead" in referring to its color which is usually colorless or white but is also found in gray, yellow, and even a blue-green color.
		Crystal twinning is common in Cerussite, a feature that make Cerussite desirable for collectors.
		As well as the intricate structure, singular twins are also a must for those who are fascinated with twinned crystals.
	www.thebeadedgarden.com /gemstone_info/Cerussite_gemstone.php (240 words)

	Thaigem.com \| Info Center \| Gemstone Discovery \| Cerussite
		Cerussite is well known among mineral collectors for its interesting crystal structure and its fluorescence.
		Crystal twinning is common in Cerussite, a feature that make Cerussite desirable for collectors.
		As well as the intricate structure, singular twins are also a must for those who are fascinated with twinned crystals.
	www.thaigem.com /dis_cerussite.asp (258 words)

	Chinese Beryl Crystal Mimic Twinning - Brief Article Rocks & Minerals - Find Articles (Site not responding. Last check: )
		Frequently these are accompanied by tabular crystals of beryl, either alone on muscovite or sharing the muscovite matrix with transparent or milky-white twinned feldspar, presumably albite (see photo, Mineralogical Record 27:219, 1996).
		The only way these crystals could be parallel-axis twins is if they possess a lower symmetry than typical beryl, due to the systematic inclusion of impurities.
		On all of the crystals we examined, the growth hillocks are consistent with full symmetry.
	www.findarticles.com /p/articles/mi_m0GDX/is_5_74/ai_58243224 (852 words)

	Hampton Research
		Most of it has to do with data handling and not how to magically convert twinned crystals to better crystals, but nonetheless are useful resources.
		The protein crystal growth conditions of each of the micro-chambers is adjusted so that the protein crystal growth conditions in at least two of the micro-chambers differs.
		These dyes are selectively incorporated or associated with crystals of particular character thereby rendering crystals of particular character colored and improving detection of the dyed crystals.
	www.hamptonresearch.com /service/CrystalNewsArchive/3-10-2003.aspx (2308 words)

	Gemstones :: Crystal Habit
		The many terms used by mineralogists to describe crystal habits are useful in communicating what specimens of a particular mineral often look like.
		Crystal habit may mislead the inexperienced as a mineral's crystal system can be hidden or disguised.
		Factors influencing a crystal's habit include: a combination of two or more forms; trace impurities present during growth; crystal twinning and growth conditions (i.e., heat, pressure, space).
	www.bleedingedge.net /Gemstones/Crystal-Habit.aspx (240 words)

	Unusual Snow Crystals
		Crystal twinning is a phenomenon whereby two crystals grow out of a single seed and have a specific crystallographic orientation relative to one another.
		The seam between the crystals can be seen when some of the ice evaporates, leaving an evaporation groove, as indicated in the picture at right.
		Another example of crystal twinning occurs when twin columns grow from a common center, one rotated 60 degrees relative to the other.
	www.cco.caltech.edu /~atomic/snowcrystals/unusual/unusual.htm (384 words)

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