The Monoclinic System: Gypsum, Mica, Malachite, Jade and Orthoclase
In the monoclinic system, only one angle differs from 90°: the cell is tilted in one direction. Gypsum, micas, talc, orthoclase, malachite, azurite, jade and epidote belong here.
Key points
| Axes | a ≠ b ≠ c |
|---|---|
| Angles | α = γ = 90°, β ≠ 90° |
| Characteristic symmetry | One two-fold axis (or one mirror plane) |
| Symmetry classes | 3 of 32 |
| Bravais lattices | 2 (primitive and base-centred) |
| Typical minerals | Gypsum, talc, micas, orthoclase, malachite, azurite, jadeite, nephrite, spodumene, epidote |
| Mohs reference points | Talc (1), gypsum (2) and orthoclase (6) are monoclinic |
A tilted brick
Imagine a shoebox tipped to one side, like a book pushed slightly askew: two angles remain right angles, but the third no longer is. This is the monoclinic system. The three axes have different lengths, and the angle β (between a and c) differs from 90°; the other two, α and γ, are 90°. The name comes from Greek and means “one inclination”.
Its characteristic symmetry is modest: a single two-fold axis (a half turn) or a single mirror plane. It is nonetheless enough for hundreds of minerals to fall into this system, including three of the ten reference points of the Mohs scale: talc (1), gypsum (2) and orthoclase (6).
Numerical examples of the angle β
| Mineral | Angle β | Remark |
|---|---|---|
| Muscovite (mica) | about 95.8° | Cell: a ≈ 5.20 Å, b ≈ 9.03 Å, c ≈ 20.11 Å |
| Orthoclase | about 116° | Usual value in tables; depends on the choice of cell |
| Malachite | about 98.7° | Value from the literature |
The values of β can vary depending on how the cell is chosen (there are several equivalent choices for the same crystal). This is why tables sometimes give slightly different figures for the same mineral, for example for gypsum.
Monoclinic minerals: reference points
| Mineral | Simplified formula | Mohs hardness | Density | Distinctive feature |
|---|---|---|---|---|
| Talc | Mg3Si4O10(OH)2 | 1 | 2.7 to 2.8 | Scratched by a fingernail; soapy feel |
| Gypsum (selenite, alabaster) | CaSO4·2H2O | 2 | 2.3 | Scratched by a fingernail; soluble in water |
| Muscovite | potassium aluminium silicate | 2 to 2.5 | 2.8 to 2.9 | Cleaves into elastic sheets |
| Malachite | Cu2CO3(OH)2 | 3.5 to 4 | 3.6 to 4.0 | Contains copper; sensitive to acids |
| Azurite | Cu3(CO3)2(OH)2 | 3.5 to 4 | 3.7 to 3.9 | Deep blue; contains copper |
| Diopside | CaMgSi2O6 | 5.5 to 6.5 | 3.2 to 3.4 | Bottle green |
| Orthoclase | KAlSi3O8 | 6 | 2.55 to 2.63 | Two cleavages at 90° |
| Nephrite (jade) | calcium-magnesium amphibole | 6 to 6.5 | 2.9 to 3.1 | Extremely tough |
| Jadeite (jade) | NaAlSi2O6 | 6.5 to 7 | 3.3 to 3.4 | Pyroxene, rare in fine quality |
| Spodumene (kunzite) | LiAlSi2O6 | 6.5 to 7 | 3.1 to 3.2 | Perfect cleavage: fragile despite its hardness |
| Epidote | calcium, aluminium and iron silicate | 6 to 7 | 3.3 to 3.5 | Pistachio green |
| Staurolite | iron and aluminium silicate | 7 to 7.5 | 3.7 to 3.8 | Cross-shaped twins |
To find these stones: malachite, azurite, jade, epidote, kunzite, selenite and muscovite.
Gypsum, a soft monoclinic mineral to know
Gypsum takes many forms. Selenite (from the Greek selênê, the moon) is the variety in transparent blades, with a pearly lustre; the fibrous variety with a silky lustre is called “satin spar”; alabaster made of gypsum is a fine, translucent mass used in sculpture (note: the calcareous alabaster of the ancients is, by contrast, calcite). Gypsum twins shaped like a spearhead or a swallowtail are typical.
The most spectacular known example is the Cave of Crystals at Naica, in Mexico, discovered in 2000 about 300 metres below ground in a mine: beams of selenite more than 11 metres long, formed in hot water saturated with calcium sulphate. The cave remained hot, up to about 58 °C, and dangerous without protective equipment.
For care, remember that gypsum, barely 2 on the hardness scale and soluble in water (about two grams per litre), must not be soaked or rinsed under running water.
Micas: stacked sheets
The word “mica” comes from the Latin micare, to shine. Muscovite and biotite cleave into flakes so thin that they are transparent and elastic: you can bend them without breaking them, and they spring back into place. This cleavage is called basal and perfect. It is the direct consequence of the mineral’s layered structure, with weak bonds between the sheets. See cleavage and fracture. The glittering flakes in aventurine or in granite are often mica.
Observe and understand
- A harmless experiment: take a sheet of mica (sold by mineral dealers, or a fragment found in granite) and peel off a layer with your fingernail; it bends without breaking. Compare it with a chip of quartz, which does not split into sheets.
- Jade: two different minerals, both monoclinic, bear the name jade (jadeite and nephrite). The name “jade” therefore does not refer to a single species. See how to recognise real jade.
- Malachite and azurite: these are copper minerals. Do not put them in your mouth, do not prepare any elixir, do not sand them dry, and wash your hands after handling them.
- Orpiment and realgar, arsenic sulphides that are also monoclinic, are toxic: do not handle them without precautions, and never leave them within reach of children.
Frequently asked questions
What does monoclinic mean?
From the Greek “one inclination”: two angles of the cell are right angles and the third, β, is not. The three axes have different lengths.
What are the most common monoclinic minerals?
Gypsum, micas, talc, orthoclase, amphiboles and pyroxenes (including jade), epidote, malachite and azurite.
Is jade always monoclinic?
Yes, whether it is jadeite or nephrite: they are two different minerals, but both are monoclinic.
Why does selenite scratch so easily?
Its Mohs hardness is 2, like a fingernail. It is also soluble in water, so clean it dry.
Is mica dangerous?
Mica sheets are harmless to the touch. As with any fine dust, avoid inhaling large quantities, for example when crushing or sanding.
Also read
The meanings mentioned belong to tradition and symbolism. They are neither promises of results nor medical advice. Back to Mineralogy for beginners.