Amazonite, also known as amazonstone, is a green tectosilicate mineral, a variety of the potassium feldspar called microcline. Its chemical formula is KAlSi3O8, which is polymorphic to orthoclase.
Amazonite from Brazil
Deep robins-egg blue color amazonite crystal on smoky quartz and albite, from Teller County, Colorado. Size: 3.4 cm × 3.3 cm × 2.5 cm (1.3 in × 1.3 in × 1.0 in).
Amazonite crystal on smoky quartz, from Pikes Peak, El Paso County, Colorado
Large deep-turquoise amazonite crystal with attached stark-white microcline, from Konso, SNNPR, Ethiopia. Size: 16.4 cm × 11.9 cm × 8.0 cm (6.5 in × 4.7 in × 3.1 in).
Microcline (KAlSi3O8) is an important igneous rock-forming tectosilicate mineral. It is a potassium-rich alkali feldspar. Microcline typically contains minor amounts of sodium. It is common in granite and pegmatites. Microcline forms during slow cooling of orthoclase; it is more stable at lower temperatures than orthoclase. Sanidine is a polymorph of alkali feldspar stable at yet higher temperature. Microcline may be clear, white, pale-yellow, brick-red, or green; it is generally characterized by cross-hatch twinning that forms as a result of the transformation of monoclinic orthoclase into triclinic microcline.
Photomicrograph of thin section of microcline showing crosshatched crystal twinning (in cross polarized light)
Image: Microcline 199473
Feldspar (amazonite)
