Microcline
Microcline is KAlSi3O8, hardness 6 to 6.5 and density 2.54 to 2.57. The Handbook gives it crystals to 50 m and 13,500 t, the largest of any mineral species.
Microcline is potassium feldspar, KAlSi3O8, triclinic, with a hardness of 6 to 6.5 and a measured density of 2.54 to 2.57. The Handbook of Mineralogy records prismatic crystals to 50 m and 13,500 t, perhaps the largest of any mineral species. Amazonite is its green variety, and the tartan twinning on {001} is what separates it from orthoclase.
In short
- The largest crystals of any mineral species may be microcline. The Handbook of Mineralogy records prismatic crystals elongated along [001] or [100] perhaps the largest of any species, to 50 m and 13,500 t. A cabinet specimen of the same mineral weighs a few hundred grams.
- Amazonite is green microcline, not a species. The IMA list holds microcline as a grandfathered species with a Norwegian type locality dated 1830; amazonite appears on it nowhere. The Handbook records green among microcline's colours and does not give a cause for it, and neither will this page.
- Tartan twinning is the identification. Polysynthetic twinning on the albite and pericline laws gives an orthogonal grid pattern on (001). Orthoclase, the monoclinic dimorph, cannot produce it. Under a lens on a cleavage face, the grid is the answer.
- Microcline and orthoclase have the same formula and the same hardness, KAlSi3O8 at 6 to 6.5, and densities of 2.54 to 2.57 against 2.55 to 2.63. The difference is aluminium-silicon ordering, which is complete in low microcline and only partial in orthoclase.
- The name is an apology for a fraction of a degree. Microcline is Greek for little and inclined, for the small deviation of its cleavage planes from 90 degrees. Orthoclase is Greek for straight and fracture, for the same angle arriving exactly.
| Species | System | Hardness | Density | What settles it |
|---|---|---|---|---|
| Microcline | Triclinic | 6 to 6.5 | 2.54 to 2.57 | Tartan grid twinning on (001); cleavages meet at about 90 degrees |
| Orthoclase | Monoclinic | 6 to 6.5 | 2.55 to 2.63 | Carlsbad, Baveno or Manebach twins but never the grid; cleavages meet at exactly 90 degrees |
| Sanidine | Monoclinic | 6 | 2.56 to 2.62 | The high-temperature form; glassy, in volcanic rocks, perfect {001} and distinct {010} |
| Albite | Triclinic | 6 to 6.5 | 2.60 to 2.65 | Sodium rather than potassium; fine parallel albite striations, not a grid |
| Quartz | Trigonal | 7 | 2.65 | Harder, no cleavage worth the name, conchoidal fracture |
The 13,500-tonne crystal, and what it means for a collector
The Handbook's crystal-data entry for microcline is one of the strangest sentences in the whole reference: prismatic crystals elongated along [001] or [100], perhaps the largest of any species, to 50 m and 13,500 t.
Fifty metres. Thirteen and a half thousand tonnes. That is a single crystal weighing roughly as much as a small ship, from a granite pegmatite, and the Handbook is clear that it is likely the record for any mineral species. Nothing else in this site's reference set carries a mass figure in its crystal-data line at all.
The practical consequence is not romantic: it tells you what pegmatite feldspar does to everything around it. Microcline is the matrix in which most of the collectable pegmatite species sit. Beryl, tourmaline, topaz, lepidolite and the phosphates grow in cavities and interstices inside a feldspar body of that scale, which is why pegmatite specimens so often arrive as a small crystal on a large, dull, cleavable grey-white block. The block is the reason the crystal exists.
Sizes at the other end of the range, and how they are graded, are on specimen sizes and grading.
Amazonite, and what this page will not claim about it
Amazonite is the blue-green to green variety of microcline, and it is one of the most recognisable ornamental minerals in the trade. It is a variety name, not a species. The September 2026 IMA-CNMNC Master List has no entry for it; microcline is there as a grandfathered species with a type locality in Norway dated 1830.
The Handbook records microcline's colour as white, pale cream-yellow; red, green, blue and offers no explanation for the green. Published work attributes amazonite colour to trace lead in combination with structural water and radiation damage, but the primary sheet this site works from does not say so, and this page will not assert a mechanism it has not verified at source. What is safe to say is that the colour is not a copper colour, despite the resemblance to turquoise and to chrysocolla.
The classic amazonite localities are the ones the Handbook names for microcline generally: the Pikes Peak area of El Paso County and Crystal Peak in Teller County, Colorado, where amazonite grows with smoky quartz; the Ilmen Mountains and the Kola Peninsula in Russia; Klein Spitzkopje in Namibia; Minas Gerais in Brazil; and Amelia in Virginia.
One commercial caution. Amazonite is routinely dyed and routinely imitated, and a uniform saturated green with no internal grid or perthitic banding is worth a second look. Our notes on how to tell if a crystal is dyed and how to tell real turquoise from fake cover the tests.
How to separate microcline from orthoclase without a laboratory
The two are dimorphs: identical formula, identical hardness, densities that overlap. Chemistry will not help. The one thing that does is twinning, and it is visible with a hand lens on a fresh cleavage surface.
Microcline is triclinic, so it can twin polysynthetically on both the albite and the pericline laws at once. Those two sets of lamellae run at roughly right angles to each other, and the result on (001) is the orthogonal grid the Handbook describes and that every field geologist calls tartan twinning. Orthoclase is monoclinic. It twins on the Carlsbad, Baveno and Manebach laws, all of which give a single composition plane, and it cannot produce a grid.
Two cautions, both worth stating. Absence of a visible grid is not evidence of orthoclase — the lamellae can be far too fine to see without crossed polars, and much perthitic feldspar shows exsolution banding that a hopeful eye reads as twinning. And a great deal of pegmatite feldspar is genuinely intermediate. The honest label for an untwinned, unanalysed grey feldspar from a granite is alkali feldspar.
The general method, and why twinning matters beyond the feldspars, is on mineral twinning explained. The rest of this reference section is indexed at the species index, and the only commercial route here is the wanted list; we hold no catalogue and nothing is offered for sale.