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Topic: Rhombohedron

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	CRYSTAL HABIT
		The steepness of the rhombohedron is variable for different minerals and even within the same mineral a crystal may have several different rhombohedrons of different inclines.
		The rhombohedron is the only non-isometric closed form with parallel faces.
		The dipyramids are distinguished from rhombohedrons by a lack of any parallel faces.
	www.galleries.com /minerals/property/crystal.htm (2461 words)

	CALCITE - LoveToKnow Article on CALCITE (Site not responding. Last check: 2007-11-05)
		A flatter rhombohedron, e { I 10}, is shown in fig.
		5 is a combination of the scalenohedron v {2oI~ and the rhombohedron r {IooI; and the crystal of tabular habit represented in fig.
		Iceland-spar is extensively used in the construction of Nicols prisms for polariscopes, polarizing microscopes and saccharimeters, and of dichroscopesfor testing the pleochroism of gem-stones.
	www.1911encyclopedia.org /C/CA/CALCITE.htm (1735 words)

	DOLOMITE - LoveToKnow Article on DOLOMITE (Site not responding. Last check: 2007-11-05)
		Normal dolomite is thus not an isomorphous mixture of calcium and magnesium carbonates, but a double salt; and any variations in composition are to be explained by the isomorphous mixing of this double salt with carbonates of calcium, iron, magnesium, manganese, and rarely of zinc and cobalt.
		In the degree of symmetry possessed by the crystals there is, however, an important difference between calcite and dolomite; the former has the full number of planes and axes of symmetry of a rhombohedral crystal, whilst the latter is hemihedral with FIG.
		This lower degree of symmetry, which is the same as that of dioptase and phenacite, is occasionally shown by the presence of an obliquely placed rhombohedron, and also by the want of symmetry in the etching and elasticity figures on the faces of the primitive rhombohedron.
	www.1911encyclopedia.org /D/DO/DOLOMITE.htm (1661 words)

	GZ Journal - Translation March Sapphires part three
		17: Characteristic growth structure in a sapphire from Antsiranana with dominant positive rhombohedron r and basic pinacoid c; the growth structure is associated with an intensive colour zoning.
		In general, one of the hexagonal dipyramids is v {14 14 28 3} or z {2241} combined with the basic pinacoid c {0001}.
		Smaller surfaces of the positive rhombohedron r {1011} and an additional hexagonal dipyramid n {2243} were also often identified.
	www.gz-journal.de /en/translations/Translation_March_Sapphirespartthree.cfm (1014 words)

	Anaglyphs
		golden oblate rhombohedron, golden prolate rhombohedron, triacontrahedron, small stellated dodecahedron, great dodecahedron, pentagonal antiprism, pentagonal pyramid, rhombic dodecahedron,
		tetrahedron, octahedron, icosahedron, dodecahedron, golden oblate rhombohedron, golden prolate rhombohedron, triacontrahedron, small stellated dodecahedron, great dodecahedron, pentagonal antiprism, pentagonal pyramid, rhombic dodecahedron,
		Try out my Virtual Flower program - you can make anaglyphs of many polyhedra, and even anaglyphs of four and higher dimensional simplexes and tesseracts - and rotate them in the higher dimensions with the mouse.
	robertinventor.com /anaglyphs (499 words)

	CALCITE - Encyclopedia Britannica - CALCITE - JCSM's Study Center (Site not responding. Last check: 2007-11-05)
		are parallel to the faces of the primitive rhombohedron, and the angle between them was determined by W. Wollaston in r8i2, with the aid of his newly invented reflective goniometer, to be 74 55'.
		A flatter rhombohedron, e { I to}, is shown in fig.
		5 is a combination of the scalenohedron v {201} and the rhombohedron r {roc) ; and the crystal of tabular habit represented in fig.
	www.jcsm.org /StudyCenter/Encyclopedia_Britannica/BUN_CAL/CALCITE.html (1942 words)

	40. (Site not responding. Last check: 2007-11-05)
		The centre of a rhombohedron (point of intersection of the six body diagonals) has the same distance to the three planes, which meet in a main corner; but besides, as centre of a parallelepiped, it has the same distance to two opposite planes.
		It is thus equally far away from all six sides of the rhombohedron.
		As a result, the heights of the rhombohedron are equal and bisected at the centre, whence follows the
	mpec.sc.mahidol.ac.th /radok/physmath/MAT2/CHAP40.HTM (379 words)

	Birefringence (Site not responding. Last check: 2007-11-05)
		The oft cited example of birefringence is rhombohedron calcite.
		The incoming ray of light is broken into two rays (whose polarization is at 90 degrees to each other and whose velocities through the material is different--hence birefringence) that travel through and exit the crystal.
		As the crystal is rotated, one ray remains stationary (the ordinary ray), and one traces a circle about the other (the extraordinary ray).
	webphysics.davidson.edu /alumni/milee/JLab/Crystallography_WWW/birefringence.htm (115 words)

	Calcite -- Facts, Info, and Encyclopedia article (Site not responding. Last check: 2007-11-05)
		Calcite (A crystalline element used as a component in various electronic devices) crystals are hexagonal-rhombohedral, though actual calcite rhombohedrons are rare as natural crystals.
		However, they show a remarkable variety of habit including acute to obtuse rhombohedrons, tabular forms, prisms, or various scalenohedrons.
		Cleavage is usually in three directions parallel to the rhombohedron form.
	www.absoluteastronomy.com /encyclopedia/c/ca/calcite.htm (425 words)

	Quasilattice diamond at UM-StL (Site not responding. Last check: 2007-11-05)
		A periodic structure made from this A6 rhombohedron would have a density of approximately 92% that of a diamond face-centered structure (like diamond or semiconductor silicon).
		These include 12 atoms (24 half atoms) with a 120 degree bond pair opposite a pentagonal 108 degree pair, 6 atoms (12 half atoms) with a square 90 degree bond pair opposite a pentagonal 108 degree pair, and 6 atoms on a central ring with opposing bond pairs pulled to 90 degrees.
		A periodic structure made from this O6 rhombohedron would have a density of approximately 89% that of a diamond face-centered structure with the same average tetrahedral bond length.
	newton.umsl.edu /~philf/qdiamond.html (565 words)

	Hexagonal Crystal System XII
		The trapezohedric tetartohedric Rhombohedron, derived from the (rhombohedric hemihedric) Rhombohedron.
		The position of the face and the generation of the rhombohedron is depicted stereographically in Figure 8.
		Such a trapezohedron consists, like the rhombohedron (Figure 1), of three upper and three lower faces in such a way that there is no reflectional symmetry with respect to the equatorial plane (the plane of the horizontal crystallographic axes).
	metafysica.nl /hexagonal_12.html (2438 words)

	Hexagonal Crystal System VII
		A second rhombohedron can be derived by establishing the red triangle as representing the three upper faces of the rhombohedron, while the blue triangle then represents the three lower faces of the rhombohedron.
		This means that when we subject a type I rhombohedron (obtained from a holohedric protopyramid) to pyramidal hemihedric its external shape will not change, but its symmetry is lowered accordingly (by the loss of the mirror planes mentioned).
		The second type II rhombohedron can be found in the same way as we did with respect to the type I rhombohedron.
	home.hetnet.nl /~heackel/hexagonal_7.html (3339 words)

	Bob's Rock Shop: Introduction to Crystallography and Mineral Crystal Systems
		In this class, the 3-fold rotoinversion axis is the vertical axis (c) and the three 2-fold rotation axes correspond to the three horizontal axes (a1, a2, a3).
		The primary difference in the rhombohedron and this form is that with a rhombohedral form, you have 3 rhombohedral faces above and 3 rhombohedral faces below the center of the crystal.
		To further complicate matters, the rhombohedron and scalenohedron, as forms, often combine with forms present in higher hexagonal symmetry classes.
	www.rockhounds.com /rockshop/xtal/part6.html (2445 words)

	Read about Rhombohedron at WorldVillage Encyclopedia. Research Rhombohedron and learn about Rhombohedron here! (Site not responding. Last check: 2007-11-05)
		Look for Rhombohedron in Wiktionary, our sister dictionary project.
		Look for Rhombohedron in the Commons, our repository for free images, music, sound, and video.
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	encyclopedia.worldvillage.com /s/b/Rhombohedron (125 words)

	CRYSTAL HABIT
		The steepness of the rhombohedron is variable for different minerals and even within the same mineral a crystal may have several different rhombohedrons of different inclines.
		The rhombohedron is the only non-isometric closed form with parallel faces.
		A disphenoid is a composition of two sphenoids with their open ends turned toward each other and one sphenoid twisted 90 degrees to the other.
	mineral.galleries.com /minerals/crystal.htm (2462 words)

	Dioptase, rock, crystal, mineral, gem stone, birth stones - mineralgallery
		The faces of the rhombohedrons, and even the prism faces, are very reflective due to a fairly high luster.
		Simple rhombohedrons are also common, at times a second rhombohedron will modify the primary rhombohedron faces producing a second set of three smaller faces.
		Dioptase is a very beautiful mineral and it is one of the few minerals that can challenge the peerlessness of emerald's deep green.
	www.mineralgallery.co.za /dioptase.htm (543 words)

	BIO.COM: Biotechnology Pharmaceutical Therapeutics, Vaccines, Diagnostics, Discovery - Biotech, Pharma, Biomedical (Site not responding. Last check: 2007-11-05)
		The main focus of the work was on the mineral calcite, which has more than 300 identified crystal forms that can combine to produce at least a thousand different crystal variations.
		Crystals can form a thousand different shapes by combining the basic forms of the positive rhombohedron (a prism with six faces, each a rhombus), negative rhombohedron, steeply, moderately and slightly inclined rhombohedrons, various scalahedrons, prism and pinacoid.
		De Yoreo and Dove first determined that when combined with magnesium, the corners formed by the intersection of atomic steps flatten and roughen, leading to flattening of the crystal's corners and elongation and roughening of the crystal shape.
	www.bio.com /newsfeatures/newsfeatures_research.jhtml;jsessionid=L2X1MLPW5GS5XR3FQLMSFEWHUWBNQIV0?cid=6400015 (411 words)

	Friedelite
		All the crystals are implanted by an end, and the free end was taken as the positive pole.
		In no crystal was any measurable face except the base and the rhombohedron q seen on the free end.
		The crystals are similar to those of figure 134 but lack the positive rhombohedron q.
	franklin-sterlinghill.com /palache/friedelite.stm (981 words)

	CHABAZITE (Hydrated Calcium Aluminum Silicate)
		The typical crystal of chabazite is a rhombohedron.
		However, in chabazite the rhombohedrons are not "squashed" that severely and the angles between the crystal faces are nearly 90 degrees.
		Cleavage is poor in three directions parallel to the faces of the rhombohedron.
	mineral.galleries.com /minerals/silicate/chabazit/chabazit.htm (409 words)

	Right and Left hand quartz crystals, Rockhounding Arkansas
		It is easy to confuse which set is the positive rhombohedron, but usually, aside from being larger, the positive rhombohedron presents a higher reflection of light from the surface than does the negative form.
		When present, it is seen at the junction of 2 prism faces m and the positive rhombohedron r and it displays a trapezohedral planar shape.
		The trigonal pyramid, the last form to discuss, is designated as s and is at the junction of the positive rhombohedron r and the prism m which is in line vertically with the negative rhombohedron z.
	rockhoundingar.com /quartz/rt.left.html (1519 words)

	Gunther Schaaf's Homepage (Site not responding. Last check: 2007-11-05)
		The Ammann-Kramer-Penrose-Tiling consists of two tiles: a prolate (thick) and an oblate (thin) rhombohedron.
		While the oblate rhombohedron is packed densely within the prolate one some space remains.
		Two prolate and two oblate rhombohedrons form a rhombic dodecahedron.
	www.itap.physik.uni-stuttgart.de /~gunther/tidecoration.html (145 words)

	Intro to Minerals: Crystal Class and System
		The crystal forms of the five non-isometric crystal systems are the monohedron or pedion, parallelohedron or pinacoid, dihedron, or dome and sphenoid, disphenoid, prism, pyramid, dipyramid, trapezohedron, scalenohedron, rhombohedron and tetrahedron.
		The crystal forms of the remaining five crystal systems are the monohedron or pedion, parallelohedron or pinacoid, dihedron, or dome and sphenoid, disphenoid, prism, pyramid, dipyramid, trapezohedron, scalenohedron, rhombohedron, and tetrahedron.
		A rhombohedron resembles in appearance a cube which is poised upright upon one corner and has been either flattened or elongated along an axis which runs diagonally from corner to corner through the center.
	dave.ucsc.edu /myrtreia/crystal.html (4278 words)

	DIOPTASE - Online Information article about DIOPTASE
		prism m terminated by a rhombohedron r, the alternate edges between these being sometimes re-placed by the faces of a rhombohedron s.
		The faces are striated parallel to the edges between r, s and in.
		There are perfect cleavages parallel to the faces of a rhombohedron which truncate the polar edges of r: from the cleavage cracks See also:
	encyclopedia.jrank.org /DIO_DRO/DIOPTASE.html (494 words)

	Kueppers' Theory of Color: Three-dimensional Order Systems
		The three sensory forces of the visual organ - the 3 Fundamental Colors - are the three vectors which are positioned in solid angles of 60 degrees with the achromatic Basic Color Black positioned in the lower edge of the rhombohedron.
		The LH picture shows the rhombohedron from the front (A) and in a 180° turn position (B).
		The RH picture is a diagrammatic view of the 8 Basic Colors located in the edges of the rhombohedron.
	www.ipsi.fraunhofer.de /Kueppersfarbe/en/theorie33.html (269 words)

	GemResearch Swisslab (GRS) Specializing in Origin Determination of Fine Rubies and Sapphires (Site not responding. Last check: 2007-11-05)
		The basic habit of corundum consists of a small number of significant crystal forms: the basal pinacoid, the hexagonal prism, the positive rhombohedron, and the negative rhombohedron, as well as different hexagonal dipyramids.
		The crystal habit of Mong Hsu rubies consists of two hexagonal dipyramids, "n" (2 2 -4 3) and "omega" (14 14 -28 3), the basal pinacoid "c" (0 0 0 1), and the positive rhombohedron "r" (1 0 -1 1).
		The crystal drawn on the left is dominated by "omega" and "c" faces, with a subordinate "r" plane.
	www.gemresearch.ch /monghsu/crystal1.htm (146 words)

	Ceepusslike.nb (Site not responding. Last check: 2007-11-05)
		Then we fill niches at points of 2-fold symmetry with 2 prolate rhombohedra and then put one prolate rhombohedron at each point of 3-fold symmetry.
		At this stage we put 3 dodecahedra around each point of 3-fold symmetry and then fill niches at points of 5-fold symmetry with two layers of prolate rhombohedra.
		We then put two dodecahedra at points of 2-fold symmetry, a oblate rhombohedron at each point of 3-fold symmetry and an icosahedron at each point of 5-fold symmetry.
	torina.fe.uni-lj.si /~izidor/RhombicPolyhedra/new/part2/part2.html (153 words)

	Ruby & Sapphire: Inclusions in corundum
		Within corundum, polysynthetic twinning occurs parallel to the faces of the rhombohedron {1011}, which intersect each other at angles of 86.1 and 93.9° and meet the c axis at 32.4/57.6°.
		Since polysynthetic twinning takes place on the rhombohedron faces, it is located by looking in directions at about 33/57° to the c axis.
		As most natural rubies and sapphires are cut with the c axis at 90¡ to the table facet, this means that the twinning will usually be found at about 33/57¡ to the table.
	www.ruby-sapphire.com /r-s-bk-ch5.htm (7648 words)

	[No title]
		A center of symmetry exists, when a face, edge or point on a solid can be projected through the center of the solid and appear on the other side in a reversed position.
		There are 9, for example in an octahedron, 6 in the tetrahedron and 3 in the rhombohedron.
		In terms of centers of symmetry, an octahedron and a rhombohedron have a center, a tetrahedron does not.
	www.d.umn.edu /~pmorton/geol2300/classnotes/symmetry.doc (672 words)

	The Hindu : Sci Tech : Sea urchins reveal spiky secret
		Each sea-urchin spine is made from a single crystal of calcite, a mineral mostly consisting of calcium carbonate, and can reach several centimetres in length.
		The crystals have a complex structure bounded by smooth, curved surfaces, unlike calcite crystals grown in the lab, which take on an angular shape with six flat faces, called a rhombohedron.
		Lab-grown crystals of calcium carbonate form directly from solution without an amorphous stage, so they simply adopt their rhombohedron shape.
	www.hindu.com /seta/2004/12/23/stories/2004122300011400.htm (529 words)

	Crystal Lattice Structures (Site not responding. Last check: 2007-11-05)
		The cell is a rhombohedron, with a = b = c = 2r, and alpha = beta = gamma = 60 degrees.
		[A cube is a rhombohedron with alpha = beta = gamma = 90 degrees!]
		Simple Hexagonal (SH) - Spheres of equal size are most densely packed (with the least amount of empty space) in a plane when each sphere touches six other spheres arranged in the form of a regular hexagon.
	members.iworld.net /joo/physics/curri-sub/crystal/lattice.html (1058 words)

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