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The pineal gland (also known as the pineal body [1] or epiphysis cerebri) is a small endocrine gland in the brain of most vertebrates. It produces melatonin, a serotonin-derived hormone, which modulates sleep patterns following the diurnal cycles. [2] The shape of the gland resembles a pine cone, which gives it its name. [3]
This category lists every crystal element that exists in a rhombohedral structure at STP. Pages in category "Chemical elements with rhombohedral structure" The following 6 pages are in this category, out of 6 total.
However, the rhombohedral axes are often shown (for the rhombohedral lattice) in textbooks because this cell reveals the 3 m symmetry of the crystal lattice. The rhombohedral unit cell for the hexagonal Bravais lattice is the D-centered [1] cell, consisting of two additional lattice points which occupy one body diagonal of the unit cell with ...
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A crystal family is determined by lattices and point groups. It is formed by combining crystal systems that have space groups assigned to a common lattice system. In three dimensions, the hexagonal and trigonal crystal systems are combined into one hexagonal crystal family. Hexagonal hanksite crystal, with threefold c-axis symmetry
Crystal structure is described in terms of the geometry of arrangement of particles in the unit cells. The unit cell is defined as the smallest repeating unit having the full symmetry of the crystal structure. [2]
An abundance of cilia and centrioles has also been found in these Type 2 cells of the pineal gland. [7] Unique to the Type 2 is the presence of vacuoles containing 2 layers of membrane. [7] As Type 1 cells contain serotonin, Type 2 cells contain melatonin and are thought to have similar characteristics as endocrine and neuronal cells. [8]
Rhombohedral cleavage occurs when there are three cleavage planes intersecting at angles that are not 90 degrees. Calcite has rhombohedral cleavage. Octahedral cleavage occurs when there are four cleavage planes in a crystal. Fluorite exhibits perfect octahedral cleavage. Octahedral cleavage is common for semiconductors. Diamond also has ...