Trigonal

-3 2/m (D3d)

三条等角等轴，或具三重对称的六方轴 (a1 = a2 = a3 ≠ c)

三方晶系有时被视为六方晶系的亚类，具有三重旋转对称性。晶体通常呈菱面体、偏三角面体或三方柱形态。该晶系富含矿物种类，包括石英、方解石、电气石、白云石和刚玉（红宝石、蓝宝石）等。

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.

著名矿物

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.

矿物 (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

常见问题

What is the Trigonal crystal system?
三方晶系有时被视为六方晶系的亚类，具有三重旋转对称性。晶体通常呈菱面体、偏三角面体或三方柱形态。该晶系富含矿物种类，包括石英、方解石、电气石、白云石和刚玉（红宝石、蓝宝石）等。
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 三条等角等轴，或具三重对称的六方轴 (a1 = a2 = a3 ≠ c).