Trigonal

-3 2/m (D3d)

Tres ejes iguales en ángulos iguales, o ejes hexagonales con simetría de 3 pliegues (a1 = a2 = a3 ≠ c)

El sistema cristalino trigonal, a veces considerado subdivisión del hexagonal, presenta simetría de rotación trifold. Los cristales típicamente forman romboedros, escalenoedros o prismas trigonales. Es uno de los sistemas más ricos en minerales, con cuarzo, calcita, turmalina, dolomita y corindón (rubí, zafiro).

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.

Minerales notables

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.

Minerales (9)

Dolomite

CaMg(CO3)2

05 Carbonates and Nitrates

Rhodochrosite

MnCO3

05 Carbonates and Nitrates

Calcite

CaCO3

05 Carbonates and Nitrates

Siderite

FeCO3

05 Carbonates and Nitrates

Cinnabar

HgS

02 Sulfides and Sulfosalts

Tourmaline

Complex borosilicate

09 Silicates

Corundum

Al2O3

04 Oxides and Hydroxides

Hematite

Fe2O3

04 Oxides and Hydroxides

Quartz

SiO2

09 Silicates

Preguntas frecuentes

What is the Trigonal crystal system?
El sistema cristalino trigonal, a veces considerado subdivisión del hexagonal, presenta simetría de rotación trifold. Los cristales típicamente forman romboedros, escalenoedros o prismas trigonales. Es uno de los sistemas más ricos en minerales, con cuarzo, calcita, turmalina, dolomita y corindón (rubí, zafiro).
What minerals crystallize in the Trigonal system?
There are 9 minerals that crystallize in the Trigonal crystal system on MineralFYI.
What symmetry does the Trigonal system have?
The Trigonal crystal system has -3 2/m (D3d) symmetry.
What are the axes of the Trigonal crystal system?
The Trigonal system is characterized by Tres ejes iguales en ángulos iguales, o ejes hexagonales con simetría de 3 pliegues (a1 = a2 = a3 ≠ c).