Hexagonal
6/m 2/m 2/m (D6h)
Four axes: three equal horizontal at 120 degrees, one vertical (a1 = a2 = a3 ≠ c)
The hexagonal crystal system has four crystallographic axes: three equal horizontal axes intersecting at 120 degrees and one vertical axis of different length. Crystals often show hexagonal prisms and pyramids. Examples include beryl (emerald, aquamarine), apatite, and graphite.
The hexagonal crystal system features four axes — three equal axes in a plane at 120° to each other, plus one unique axis perpendicular to them. This six-fold symmetry produces the characteristic hexagonal prisms and bipyramids seen in many well-known minerals. The hexagonal system is home to some of the most valuable gemstones on Earth, including beryl (emerald, aquamarine) and corundum (ruby, sapphire).
Symmetry
Hexagonal crystals have a principal six-fold rotation axis (a₁ = a₂ = a₃ ≠ c, with a-axes at 120°). Looking down the c-axis, the crystal appears to have six-fold symmetry. The system contains 7 crystal classes, from the fully symmetric dihexagonal dipyramidal class to the less symmetric hexagonal pyramidal class. The Miller-Bravais index system uses four indices (hkil) rather than three, where i = -(h+k), reflecting the three equivalent a-axes. This system is sometimes confused with the trigonal system, which has three-fold rather than six-fold symmetry and is treated as a subdivision by some crystallographers.
Известные минералы
Beryl is the signature hexagonal mineral, forming the prismatic hexagonal crystals that yield emerald (green, chromium-colored), aquamarine (blue), morganite (pink), and heliodor (yellow). The largest single crystal ever found — an 18-meter beryl from Madagascar — crystallized in this system. Apatite, the mineral that forms our bones and teeth, is hexagonal and occurs in a remarkable range of colors. Molybdenite (MoS₂), the chief ore of molybdenum, forms distinctive hexagonal plates with a metallic luster. Ice itself crystallizes in the hexagonal system, explaining the six-fold symmetry of snowflakes.
Минералы (2)
Apatite
Ca5(PO4)3(F,Cl,OH)
08 Phosphates, Arsenates, and Vanadates
Beryl (Emerald)
Be3Al2(SiO3)6
09 Silicates