The Triclinic System: Feldspars, Turquoise, Kyanite and Rhodonite
The triclinic system is the least symmetrical: three unequal axes and no right angles. Albite, labradorite, amazonite, turquoise, kyanite and rhodonite are examples.
Key points
| Axes | a ≠ b ≠ c |
|---|---|
| Angles | α ≠ β ≠ γ; none is required to be 90° |
| Symmetry | None (class 1) or a simple centre of symmetry (class -1) |
| Symmetry classes | 2 of 32 |
| Bravais lattice | 1 (primitive) |
| Typical minerals | Albite, labradorite, microcline (amazonite), turquoise, kyanite, rhodonite, larimar |
| Notable feature | Hardness that can differ greatly depending on direction (kyanite) |
No constraints, except periodicity
The triclinic system is the least symmetrical of the seven crystal systems. The three axes of the cell have different lengths, and none of the three angles is required to be 90°. There is neither a rotation axis nor a mirror plane: the richest symmetry possible is a simple centre of inversion (each point of the crystal has a “twin” located opposite it).
The name comes from Greek and means “three inclinations”. Of the seven systems, it is the most general: any crystal could in theory be placed in it, were one not looking for higher symmetry. It has 2 symmetry classes out of 32, and a single Bravais lattice.
Angles and axes, all different
| Mineral | Axes (Å) | Angles (°) |
|---|---|---|
| Albite (sodium feldspar) | a ≈ 8.14; b ≈ 12.79; c ≈ 7.16 | α ≈ 94.3; β ≈ 116.6; γ ≈ 87.7 |
| Kyanite | a ≈ 7.13; b ≈ 7.85; c ≈ 5.57 | α ≈ 90.0; β ≈ 101.1; γ ≈ 106.0 |
These cell values, taken from the literature (they vary by a few hundredths depending on the source), illustrate the system well: no equalities, no imposed angles. Kyanite is an amusing case: one of its angles is almost exactly 90°, without this being a rule. Since the other two differ clearly from it, it is still correctly classed as triclinic.
Plagioclase feldspars: the most widespread triclinic family
Feldspars form the most abundant group of minerals in the Earth’s crust. Among them, the plagioclases (albite, oligoclase, andesine, labradorite, anorthite) are triclinic: they form a continuous series between a sodium end-member (albite) and a calcium end-member (anorthite). Microcline, a potassium feldspar, is also triclinic at low temperature; its green variety is amazonite.
A characteristic sign of plagioclases is polysynthetic twinning: very fine twinned lamellae, visible as fine parallel striations on a cleavage face. It is a simple way to recognise them in a granite or a gabbro. The labradorescence of labradorite, those blue or golden flashes that appear at a certain angle, comes from the interference of light on tiny lamellae (a few hundred nanometres thick) formed inside the crystal.
More information on our pages labradorite, amazonite, moonstone and sunstone. Trade names sometimes cover different feldspars (moonstone is sometimes a monoclinic potassium feldspar, sometimes a triclinic plagioclase), which is why it is worth reading the entry carefully.
Other triclinic minerals
| Mineral | Simplified formula | Mohs hardness | Density | Reference point |
|---|---|---|---|---|
| Albite | NaAlSi3O8 | 6 to 6.5 | about 2.62 | Polysynthetic twinning |
| Labradorite | (Na,Ca)(Al,Si)4O8, intermediate | 6 to 6.5 | 2.70 to 2.72 | Labradorescence |
| Microcline (amazonite) | KAlSi3O8 | 6 to 6.5 | about 2.56 | Blue-green; colour linked, according to studies, to traces of lead and water |
| Turquoise | hydrated copper aluminium phosphate | 5 to 6 | 2.6 to 2.9 | Porous; isolated crystals are very rare |
| Rhodonite | manganese silicate | 5.5 to 6.5 | 3.5 to 3.7 | Pink, with black veins of manganese oxide |
| Kyanite | Al2SiO5 | 4.5 to 5 in one direction, 6.5 to 7 in the other | 3.5 to 3.7 | Blue, elongated blades |
| Larimar (pectolite) | calcium and sodium silicate | 4.5 to 5 | 2.7 to 2.9 | Pale blue, from the Dominican Republic |
| Ulexite | hydrated borate | 2.5 | about 2 | “Television stone”: fibres that transmit the image |
Related entries: turquoise, rhodonite, kyanite, larimar.
Kyanite: hard in one direction, soft in the other
Kyanite is the best example of hardness anisotropy. Along the length of its blades it resists only 4.5 to 5 on the Mohs scale; across them it reaches 6.5 to 7. Its French name, disthène, comes from the Greek for “double strength”, for precisely this reason. In practice, a steel point scratches a kyanite blade in one direction but not in the other. For the same reason, kyanite must be handled with care: it is fragile and has a sharp cleavage. See cleavage and fracture. Do not attempt this test on a stone you want to keep: the scratch is permanent.
Turquoise: a triclinic mineral, but above all a porous material
Turquoise is indeed triclinic, but its visible crystals are extremely rare: it is almost always found as a compact mass of microscopic grains, filling fissures. It is a porous material, sensitive to oils, perfumes, household products and sweat, and it can turn green or fade. Medium-quality stones are often stabilised (impregnated with resin) and sometimes reconstituted (agglomerated powder), while dyed howlite is often sold in its place. See how to recognise real turquoise and our commitments on treatments.
A harmless experiment
Look for a worktop or slab of polished granite, or a granite pebble. With a magnifying glass, pick out the white or pink crystals of feldspar; under a lamp, turn the stone slightly: on some grains a flat reflection appears or disappears all at once, the sign of a cleavage face. On plagioclases, these faces carry fine parallel striations, the mark of polysynthetic twinning. You have just observed triclinic minerals without any equipment.
Frequently asked questions
Why is the triclinic system the least symmetrical?
Because no constraint is imposed on its cell: three different axes and three different angles. At best it has a single centre of symmetry.
Is labradorite triclinic?
Yes. It is an intermediate plagioclase between albite and anorthite. Its blue flashes come from very fine internal lamellae, not from its own colour.
Is amazonite coloured by copper?
This was long believed. Spectroscopic studies have instead attributed its colour to small amounts of lead and water in the feldspar structure. The question remains debated.
Is turquoise fragile?
It is soft (5 to 6) and porous: avoid prolonged contact with water, oils, perfumes and knocks. A stabilised turquoise is a little tougher but remains sensitive.
Is kyanite really harder or softer depending on the direction?
Yes: 4.5 to 5 along its blades and about 6.5 to 7 across them. It is a unique case among common collectors’ stones.
Also read
The meanings mentioned belong to tradition and symbolism. They are neither promises of results nor medical advice. Back to Mineralogy for beginners.