Orthoclase
Orthoclase is KAlSi3O8, hardness 6 to 6.5 and density 2.55 to 2.63. The three twin laws named after localities are all named after orthoclase localities.
Orthoclase is monoclinic potassium feldspar, KAlSi3O8, hardness 6 to 6.5 and measured density 2.55 to 2.63, with perfect cleavage on {001} and {010} meeting at exactly 90 degrees. That right angle is the name: Greek for straight fracture. Crystals reach 20 cm and the three twin laws collectors use daily, Carlsbad, Baveno and Manebach, are all named after orthoclase localities.
In short
- The three twin laws every collector names are all named after orthoclase localities. Carlsbad in Bohemia, Baveno in Piedmont and Manebach in Thuringia all appear on the Handbook's own distribution list for this species. It is an unusual concentration: three working terms, one species, one distribution list.
- The name records a right angle. Orthoclase is Greek for straight and fracture, for cleavages meeting at exactly 90 degrees. Its triclinic dimorph microcline is Greek for little inclined, for missing that angle by a fraction of a degree. Same formula, same hardness, and the two names describe the same measurement.
- Most orthoclase is not pure potassium feldspar. The Handbook's microprobe analysis of Himalaya mine material from Mesa Grande, California, gives K0.86Na0.10 on a site that holds one atom, so about a tenth of the potassium is sodium. Alkali feldspar is a series, not a compound.
- Adularia and moonstone are variety names, not species. Neither appears on the September 2026 IMA list. Adularia is low-temperature potassium feldspar from Alpine clefts; moonstone is adularescent material, and the Handbook records that orthoclase may show opalescence or schiller iridescence.
- Hardness 6 to 6.5 makes feldspar the reason quartz looks tough. Feldspar is the commonest mineral in the crust and it cleaves perfectly in two directions, so a sand grain that survives transport is usually quartz. That is a fact about weathering, not about quality.
| Twin law | Named after | On the Handbook's orthoclase locality list | What it looks like |
|---|---|---|---|
| Carlsbad | Carlsbad, Bohemia, Czech Republic | Yes | Two interpenetrant prisms rotated 180 degrees about [001]; the commonest feldspar twin of all |
| Baveno | Baveno, Piedmont, Italy | Yes | A contact twin on {021} giving a near-square cross-section |
| Manebach | Manebach, Thuringia, Germany | Yes | A contact twin on {001}, so the composition plane is the main cleavage |
| Albite and pericline | The albite species and a habit name | Not applicable | Polysynthetic, and in microcline the two together give the tartan grid |
Three localities that became three words
Open the Handbook of Mineralogy sheet for orthoclase at the twinning line and it reads: common as simple, contact, or penetration twins according to the Carlsbad, Baveno, or Manebach laws. Open the same sheet at the distribution line and all three places are there: Baveno in Piedmont, Carlsbad in Bohemia and Manebach in Thuringia, listed among the localities that produce fine examples.
That is unusual enough to be worth stating plainly. Three of the twin laws in general mineralogical use are named for localities rather than for a species or a habit, and all three of those localities appear on orthoclase's own sheet. A collector who learns to recognise a Carlsbad twin has learned a word coined for a specific Bohemian occurrence of this mineral, and now applies it to feldspars worldwide.
The Handbook's wider locality list for orthoclase runs through St Gotthard and Val Giuv in Switzerland, the Zillertal in the Tirol, Elba, Epprechtstein in Bavaria, the Mursinka-Alabashka area in the Urals, Maine, the Pala and Mesa Grande districts of California, Mount Antero in Colorado — and Cornwall, England, where the granite is the reason the county has a mining history at all. Twinning in general is covered on mineral twinning explained.
Adularia, moonstone, and what is actually being named
Neither word is on the September 2026 IMA-CNMNC Master List. Orthoclase is, listed as approved with the year 1962 s.p. and no recorded type locality.
Adularia is a varietal name for low-temperature potassium feldspar, classically from Alpine fissures in the Adula massif, and it is conventionally filed under orthoclase. It is a habit-and-origin term rather than a compositional one: adularia is typically clear, pseudo-orthorhombic and simply terminated, and much of it is structurally intermediate between orthoclase and microcline.
Moonstone is a gemmological term for material that shows adularescence — a floating blue-white sheen caused by light scattering from fine exsolution lamellae. The Handbook records only that orthoclase may exhibit opalescence or schiller iridescence, and this page will not put a more specific mechanism than that behind the sheet's own wording. Most commercial moonstone is an alkali feldspar perthite rather than a single homogeneous species.
Both are legitimate names to write on a label, as long as the species is written beside them: adularia (orthoclase), Val Giuv. The general principle is on how to label and catalogue a mineral collection.
Orthoclase against microcline, and the honest limit of a hand determination
The two are dimorphs. Both are KAlSi3O8. Both are hardness 6 to 6.5. Orthoclase's measured density is 2.55 to 2.63 and microcline's 2.54 to 2.57, so the ranges overlap over microcline's entire width and a specific gravity measurement cannot separate them.
The real difference is structural. The Handbook notes that in orthoclase the aluminium and silicon are commonly only partially ordered, while in low microcline the ordering is complete. Ordering lowers the symmetry from monoclinic to triclinic, and the visible consequence is twinning: microcline can show the orthogonal tartan grid on (001) and orthoclase cannot.
The limit is worth being blunt about. A grey or flesh-coloured feldspar crystal from a granite, with no visible grid and no analysis, is not identifiable to species in the hand. It is alkali feldspar. Writing orthoclase on it is a convention rather than a determination, and if the specimen matters, say so on the label. The comparison from the other side is on microcline.
The rest of this reference section is indexed at the species index. There is no catalogue here and nothing is offered for sale; the wanted list is the only commercial route.