Resources written by Chas McCaw for sixth form chemistry teaching and beyond.
General interest:
Graphite Buckminsterfullerene Ice White phosphorus Benzene Cyclohexane AdamantaneCubic:
Sodium Caesium chloride Polonium Copper Halite Fluorite Antifluorite Zinc blende DiamondNon-cubic:
Hexagonal:
Magnesium WurtziteTetragonal:
RutileTrigonal:
α-quartzTriclinic:
Copper(II) sulfateOrthorhombic:
α-SulfurMonoclinic:
β-SulfurOther compounds that adopt this structure: berlinite (AlPO4).
The black wireframe just marks out the repeating unit of space in the structure - the lines are not supposed to depict any kind of chemical bond or particle. Note that the trigonal unit cell is the same shape as the hexagonal one: a prism of a rhombus, with angles of 60° and 120° in the rhombus. The difference is that within trigonal cell the structure has less symmetry, affording only three-fold symmetry within the layers, whereas there is six-fold symmetry in the layers in hexagonal unit cells.
Note that the cell occupancy of silicon atoms is three, as each silicon atom, being on a face, is only half inside the cell. All six oxygen atoms are inside the cell, so the 1:2 stoichiometry for Si:O is satisfied.
Helical chains run along the axis perpendicular to the rhombuses, which you may be able to see in the structure on the left.
Go to page 3 to look at three α-quartz unit cells stacked along the direction of the helical chains.
Other compounds with a trigonal unit cell:
Corundum (Al2O3), calcite (CaCO3), haematite (Fe2O3), dolomite (CaCO3.MgCO3), cinnabar (HgS), cubane (C8H8), hydroxyapatite (Ca5(PO4)3OH), nitratine (NaNO3), brucite (Mg(OH)2), magnesite (MgCO3), millerite (NiS), otavite (CdCO3), phenakite (Be2SiO4), rhodochrosite (MnCO3), siderite (FeCO3), smithsonite (ZnCO3), sphaerocobaltite (CoCO3), stibarsen (SbAs) and willemite (Zn2SiO4).