Microcline — The Triclinic Potassium Feldspar Behind Amazonite
Microcline — The Triclinic Potassium Feldspar Behind Amazonite
A low-temperature K-feldspar species best known for its blue-green amazonite variety
Microcline is a potassium feldspar with the composition KAlSi3O8, crystallising in the triclinic system. Together with orthoclase and sanidine it forms the K-feldspar end of the alkali feldspar series, and within that series it is the low-temperature member — the form into which orthoclase eventually orders as the host rock cools. Microcline matters to gemmology principally because its blue-green variety, amazonite, is one of the few coloured feldspars to enter the trade in volume, and because its perthitic intergrowths with sodium-rich albite produce some of the most striking moonstone-class optical effects.
Mineralogy and structure
The triclinic symmetry of microcline distinguishes it from its monoclinic siblings sanidine and orthoclase, even though all three share the same chemical composition. The structural difference is one of order: at low temperatures, the aluminium and silicon atoms occupy specific tetrahedral sites in a fully ordered arrangement, producing the slight angular deviation from monoclinic geometry that gives microcline its name (from Greek mikros, small, and klinein, to incline). Above approximately 500 degrees Celsius the structure disorders toward orthoclase; the sequence of cooling-driven ordering is among the better-characterised topics in feldspar mineralogy.
Hardness sits at 6 to 6.5 on the Mohs scale; specific gravity ranges from approximately 2.55 to 2.63; refractive indices fall in the 1.514 to 1.539 range, with low birefringence. Cleavage is perfect in two directions at near-right angles, a behaviour shared with the other feldspars and one that any cutter must respect.
Amazonite
The blue-green to green variety known as amazonite is the variety of microcline most often encountered in the trade. Colour ranges from a pale aqua-green through saturated turquoise-blue to a deeper teal, sometimes with white streaks of intergrown albite that give the material its characteristic patterned appearance. The cause of colour was for many decades misunderstood; current research, summarised in the GIA literature, attributes amazonite's colour to a combination of structurally bound lead in the lattice combined with water-related defects, with both contributing to a charge-transfer mechanism that absorbs in the red-orange region of the visible spectrum.
Source localities for amazonite include the Pikes Peak batholith of Colorado, the type-area pegmatites of the Ilmen Mountains in Russia, Madagascar, Brazil, Ethiopia, and parts of southern Africa. Despite the name, the Amazon basin is not a notable source — the name commemorates an early but unverified attribution of similarly coloured material to that region.
Perthite and moonstone-class material
Microcline often occurs as perthite — a microscopic to macroscopic intergrowth of microcline with sodium-rich albite, formed when the alkali feldspar exsolves into K-rich and Na-rich phases on cooling. Where the lamellar spacing is in the range of visible-light wavelengths, perthitic intergrowths produce optical effects ranging from fine adularescence (the floating blue sheen of moonstone) through the bolder iridescence of certain spectrolite-class material. Microcline-derived moonstone is less commercially common than orthoclase moonstone but does appear in the trade, particularly from Indian and Madagascan sources.
Transparent yellow to colourless microcline is occasionally faceted for collectors, where the rough is clean enough to allow it. These stones rarely exceed a few carats and command interest more from the mineral-collector market than from mainstream jewellery.
Identification
In thin section, microcline shows the diagnostic cross-hatched twinning pattern produced by its triclinic ordering — a feature so characteristic that it is one of the first indicators a petrographer reaches for. In hand specimen and as cut stones, distinguishing microcline from orthoclase requires either careful refractive-index measurement or, more commonly, X-ray methods. For amazonite specifically, the colour and characteristic albite striping are usually diagnostic without further work.
Some amazonite is treated to deepen colour, typically by gamma irradiation followed by mild heat. The treatment is generally stable and is not currently disclosed by most retailers; the trade has long accepted irradiated amazonite as commercial-grade material.
Cutting and care
Cleavage in two directions at right angles is the principal hazard. Cabochons must be oriented to set the cleavage planes parallel to the base of the dome wherever possible; faceted stones require careful dop-block work to avoid cleavage flaws propagating through the finished piece. Hardness at 6 to 6.5 makes microcline including amazonite a soft material for daily-wear ring use; bezel mountings and protective settings are preferred. Avoid ultrasonic and steam cleaners; mild soap, warm water, and a soft brush are appropriate.
Amazonite colour is generally stable to normal wear and light exposure, though prolonged direct sun on a treated stone can cause measurable fading over years.
In the trade
Amazonite is a workhorse stone in the cabochon and beaded-jewellery markets. Mainstream commercial material runs from about $1 to $20 per carat depending on colour, clarity, and size; the most desirable stones are saturated, even-coloured, and free of obvious albite veining. Larger collector-grade transparent crystals from Pikes Peak can command meaningfully higher prices, particularly when associated with smoky quartz in classic specimen pieces. Microcline outside the amazonite variety is largely a mineralogical and lapidary curiosity rather than a commercial gem.