April - Labradorite
MINERAL OF THE MONTH – LABRADORITE
Taos Rockers mineral of the month is Labradorite, a plagioclase feldspar. The name labradorite comes from the area in which it was first discovered in 1770: Ford Harbour, Paul Island, near Nain, off the east coast of Labrador, Canada.
· Mineral: Labradorite
· Chemistry: (Ca,Na)[Al(Al,Si)Si2O8] with Na (30-50%) and Ca (70-50%)
· Class: Tectosilicate - feldspar group
· Crystal system: Triclinic
· Color: Usually clear, white, yellow or gray in reflected light.
Labradorescent colors can include blue, green, purple, yellow, orange, and red
· Refractive index: 1.554-1.573
· Luster: Sub-Vitreous
· Specific gravity: 2.69-2.72
· Mohs Hardness: 6-6.5
· Cleavage: perfect - two directions of perfect cleavage intersecting at 86 or 94 degrees
· Streak: white
Plagioclase is the term for a group of feldspar minerals that form a solid solution series that range from albite Na(AlSi2O8) to anorthite Ca(Al2Si2O8). Minerals in the series are a homogenous mixture of albite and anorthite and are named based upon their relative abundance of albite and anorthite: albite, oligoclase, andesine, LABRADORITE, bytownite, and anorthite. The name plagioclase is frequently used in lieu of the more specific mineral name due to the physical similarity of the feldspar minerals and the difficulty of identification without lab testing (x-ray diffraction, chemical analysis, optical tests, and specific gravity determinations). Labradorite, however, can exhibit strong labradorescence which allows it to be more easily identified from the other members of the plagioclase series. Labradorescence is a schiller effect, an irridescent optical effect, in which a strong play of lustrous blue and green, and sometimes the complete spectrum including red, orange, and yellow, colors appear. Labradorescence is not a result of surface reflection but rather a result from light entering the mineral and striking a twinning surface within the stone and reflecting off that surface. Different twinning surfaces within the feldspar reflect different colors of light and can thereby show multi-colors. The quality, hue, and brilliance of the labradorescence varies from one specimen to another and within a single specimen. Many specimens of labradorite do not exhibit labradorescence
Technically, labradorescence is caused by phase exsolution lamellar structure, occurring in the Bøggild miscibility gap. The effect is visible when the lamellar separation is between 128 to 252 nm, the lamellae are not necessarily parallel; and the lamellar structure is found to lack long range order. The lamellar separation only occurs in plagioclases of a certain composition, in particular, those of calcic labradorite and bytownite (anorthite content of ~60 to 90%). Another requirement for the lamellar separation is very slow cooling of the rock that contains the plagioclase. Slow cooling is required to allow the Ca, Na, Si, and Al ions to diffuse through the plagioclase and produce the lamellar separation. Therefore, not all labradorite exhibit labradorescence (they might not be the correct composition and/or they cooled too quickly), and not all plagioclases that exhibit labradorescence are labradorite (they may be bytownite)." Source: Wikipedia.org
Labradorite most often occurs as a primary mineral in mafic igneous rocks - basalt, gabbro and norite. It is the most abundant mineral in the igneous rock known as anorthosite and can form a syenitic rock called larvikite. In metamorphic environments, it occurs in amphibolites and gneiss. It can also be found in sediments and sedimentary rocks that are derived from the weathering of other rocks that contain labradorite. Olivine, pyroxenes, amphiboles and magnetite are commonly associated with labradorite in igneous rocks.
Labradorite is used as a gemstone; and anorthosite, a rock rich in labradorite, is used as an architectural stone. Due to its labradorescence, labradorite is a popular gemstone. It can be fashioned into cabochons or faceted stones. Many specimens of labradorite do not exhibit labradorescence but can still produce gemstones. The state gemstone of Oregon (Oregon Sunstone) is a labradorite without labradorescence but does display a schiller effect due to included micro-platelets of copper. Due to labradorite's perfect cleavage and hardness of 6, one should not subject these gemstones to impact. Anorthosite has been used for small sculptures, countertops, tiles, window sills, facing stone and other architectural products. When light strikes at the right angle, multitudes of labradorite crystals reflect brilliant color flashes, glittering in the light.
Labradorite occurs throughout the world - on every continent, with reported occurrences in over 50 countries and 34 states in the US.
Some of the mineral locales from which specimens of Labradorite are available at Taos Rockers include:
Varieties of Labradorite include Lynx Eye, Oregon Sunstone and Spectrolite. Lynx Eye is the variety with green iridescence. Oregon Sunstones contain inclusions of copper resulting in a pink to red coloration. Mindat.org says that crystallographically oriented nanoncrystals of protoenstatite and clinoenstatite in association with copper nanocrystals are responsible for the unusual green and "watermelon" coloration sometimes found in the Oregon Sunstones. Spectrolite, the most common variety seen on the mineral market, is the variety showing strong labradorescence.
Melody, in her book Love Is In The Earth, says Labradorite "protects ones aura, and helps to keep the aura clear, balanced, protected, and free from energy leaks...facilitates the transformation of intuition into intellectual thought...It can help to provide clarity to the inner sight."