Trigonal
-3 2/m (D3d)
Three equal axes at equal angles, or hexagonal axes with 3-fold symmetry (a1 = a2 = a3 ≠ c)
The trigonal crystal system, sometimes considered a subdivision of hexagonal, features three-fold rotational symmetry. Crystals typically form rhombohedra, scalenohedra, or trigonal prisms. This is one of the most mineral-rich systems, with quartz, calcite, tourmaline, dolomite, and corundum (ruby, sapphire).
The trigonal crystal system possesses three-fold rotational symmetry, with its principal axis showing a crystal that repeats every 120° of rotation. Sometimes treated as a subdivision of the hexagonal system (sharing the same axis convention), trigonal minerals are distinctive in their characteristic rhombohedral or scalenohedral crystal forms. This system includes quartz — the second most abundant mineral in Earth's continental crust — along with calcite, the most common carbonate mineral.
Symmetry
Trigonal symmetry means a crystal looks identical after a 120° rotation around its principal axis (three-fold symmetry, as opposed to hexagonal six-fold). The axes convention is the same as hexagonal (a₁ = a₂ = a₃, 120° apart, ≠ c), but the symmetry is lower. Some crystallographers use rhombohedral axes (a = b = c, α = β = γ ≠ 90°) instead. The trigonal system contains 5 crystal classes. The most familiar crystal form is the rhombohedron — a 'squashed cube' shape seen in calcite. Quartz crystals appear hexagonal but are actually trigonal, as revealed by subtle differences in face sizes.
Известные минералы
Quartz is the quintessential trigonal mineral, occurring in more varieties than any other mineral: amethyst (purple), citrine (yellow), rose quartz (pink), smoky quartz (brown-black), and chalcedony (microcrystalline). Calcite (CaCO₃) displays over 800 different crystal forms — more than any other mineral — all within the trigonal system. Its extreme birefringence (double refraction) is a defining optical property. Tourmaline forms distinctive trigonal prisms with rounded triangular cross-sections, often with color zoning. Corundum (ruby and sapphire) is trigonal, with Mohs hardness 9, second only to diamond.
Минералы (9)
Tourmaline
Complex borosilicate
09 Silicates
Corundum
Al2O3
04 Oxides and Hydroxides
Dolomite
CaMg(CO3)2
05 Carbonates and Nitrates
Siderite
FeCO3
05 Carbonates and Nitrates
Cinnabar
HgS
02 Sulfides and Sulfosalts
Rhodochrosite
MnCO3
05 Carbonates and Nitrates
Hematite
Fe2O3
04 Oxides and Hydroxides
Quartz
SiO2
09 Silicates
Calcite
CaCO3
05 Carbonates and Nitrates