Triclinic

-1 (Ci)

Three unequal axes, none at 90 degrees (a ≠ b ≠ c, alpha ≠ beta ≠ gamma ≠ 90)

The triclinic crystal system has the lowest symmetry of all crystal systems, with three unequal axes at oblique angles. Crystals usually form tabular or platy habits. Examples include plagioclase feldspars (albite, labradorite), kyanite, turquoise, and microcline (amazonite).

The triclinic crystal system is the least symmetric of all seven crystal systems, with three unequal axes at three unequal angles (a ≠ b ≠ c, α ≠ β ≠ γ ≠ 90°). Despite this minimal symmetry, about 8% of known mineral species are triclinic, including the most abundant mineral group on Earth's surface: the plagioclase feldspars. The lack of symmetry constraints means triclinic crystals can adopt complex, irregular shapes that challenge even experienced crystallographers.

Symmetry

Triclinic crystals have either a center of symmetry only (pinacoidal class, 1̄) or no symmetry at all (pedial class, 1). This minimal symmetry means every face on the crystal is unique — there are no symmetrically equivalent faces. Despite this seeming disorder, triclinic minerals can form beautifully regular-looking crystals. The three axes are conventionally chosen so that c < a < b, and the interaxial angles are all oblique. Twinning is very common, especially in the plagioclase feldspars, where polysynthetic twinning (repeated fine twin lamellae) produces the characteristic striations visible on cleavage surfaces.

Minéraux notables

The plagioclase feldspars — albite, oligoclase, andesine, labradorite, bytownite, and anorthite — are all triclinic and together constitute the most abundant mineral group in Earth's crust. Labradorite displays the spectacular 'labradorescence' — iridescent flashes of blue, green, and gold caused by light interference in polysynthetic twin lamellae. Kyanite is notable for having different hardness values depending on direction (Mohs 4.5 parallel to length, 7 perpendicular) — a property called anisotropism that is particularly pronounced in triclinic minerals. Rhodonite (pink manganese silicate) and turquoise are popular triclinic gemstones.

Minéraux (5)

Kyanite

Al2SiO5

09 Silicates

Wollastonite

CaSiO3

09 Silicates

Labradorite

(Ca,Na)(Al,Si)4O8

09 Silicates

Amazonite

KAlSi3O8

09 Silicates

Turquoise

CuAl6(PO4)4(OH)8 · 4H2O

08 Phosphates, Arsenates, and Vanadates

Questions fréquentes

What is the Triclinic crystal system?
The triclinic crystal system has the lowest symmetry of all crystal systems, with three unequal axes at oblique angles. Crystals usually form tabular or platy habits. Examples include plagioclase feldspars (albite, labradorite), kyanite, turquoise, and microcline (amazonite).
What minerals crystallize in the Triclinic system?
There are 5 minerals that crystallize in the Triclinic crystal system on MineralFYI.
What symmetry does the Triclinic system have?
The Triclinic crystal system has -1 (Ci) symmetry.
What are the axes of the Triclinic crystal system?
The Triclinic system is characterized by Three unequal axes, none at 90 degrees (a ≠ b ≠ c, alpha ≠ beta ≠ gamma ≠ 90).