Staurolite

FEATURED MINERAL – STAUROLITE

Taos Rockers featured mineral is Staurolite. The word staurolite is derived from the Greek stauros - cross, and lithos - stone: referring  to its cruciform twinning.  Staurolite can form opaque cross shapes when the crystal forms twins. These penetration twins commonly form as 60°  X-shaped (St. Andrews cross) and less commonly as 90° cross-shaped (Roman or Maltese cross) crystals. Numerous folk stories have evolved over time to explain the occurrence of the twined forms. One such story is that the tears of  angels (or fairies) crying upon the news of the crucifixion of Christ turned into these stones. An alternative name for these stones is "fairy cross" or "fairy stone." Elders of Taos, NM are known to carry larger Roman cross Staurolites as a good luck token.

  Mineral: Staurolite

  Chemistry:  (Fe2+, Mg, Zn)2Al9Si4O23(OH) - trivalent iron can substitute for aluminum

     Common Impurities: Ti,Cr,Mn,Co,Zn,Li,H2O

     Common Associates: Almandine garnet, Chloritoid, Kyanite, Muscovite, Quartz & Tourmaline

  Class: Neosilicate

  Crystal system: Monoclinic (pseudo-orthorhombic)

  Color: Dark brown, brownish-black, red-brown, yellow brown, black, dark gray; rarely blue; pale golden yellow in thin-section 

  Refractive index:  1.739 to 1.761

  Luster: Sub-Vitreous, Resinous

  Specific gravity: 3.74 to 3.83

  Mohs Hardness: 7-7.5

  Cleavage: Distinct/Good; Distinct on {010}

  Habit: Prismatic crystals, typically twinned at 60° or 90°

  Fracture: Sub-Conchoidal

  Streak: White (harder than streak plate)

  Pleochroism:  Trichroic - green/red/yellow (blue in presence of cobalt)

  Twinning:  Commonly 60-degree twins on {231}, may be cyclic. Less commonly 90-degree cruciform twins on {031}. On (320) (very rare).

  Geological Setting: Amphibolite grade of regionally metamorphosed pelitic rocks.

Staurolite is distinguished from other minerals by its six-sided (prismatic) crystals that are typically larger than the grains of other minerals in the rock, by its hardness and color. Frequently associated with muscovite mica and almandine garnet in schist and gneiss and often twinned. The twin is a classic penetration twin where it appears as if two crystals grew into & out of each other.

Used in the past as an abrasive, there are no industrial uses for Staurolite in the present day. Commercial usage is limited to jewelry making and as mineral specimens. Faceted Staurolites are prized by collectors, mainly due to their extreme scarcity. Transparent Staurolites suitable for faceting are found in a very limited area in Brazil and Switzerland. Most faceted stones are small, under 2 carats. Staurolite is used in geologic field work to assess the temperature-pressure conditions of a rock's metamorphic history. Staurolite is one of the index minerals that are used to estimate the temperature, depth, and pressure at which a rock undergoes metamorphism.  Staurolite is a common constituent of medium grade regionally metamorphosed argillaceous sediments.

While quality twinned Staurolite crystals are not rare, simulations of the cruciform twin crystals have been "created" for over a century. An article in a 1934 issue of American Mineralogist reported that. 90° “Roman Cross” shapes dominated the market, despite being the rarer variety. The author discovered that many were cut from much softer talcose material and filed into shape and then soaked in oils to darken them. Oddly the demand for transparent facetable Staurolite has not generated the same level of "industrious deception." If you see a selection of staurolite crosses offered for sale that are all the same size, same shape, and have air bubbles on close examination, they might be manufactured.

Locations for Staurolites are numerous - essentially anywhere that pelitic rocks have been subjected to amphibolite grade of metamorphism. Twinned crystals have a smaller area of distribution. Significant localities for Staurolites include:

Connecticut, Georgia (Fannin County), Idaho (Island Park), Maine, Minnesota (Blanchard Dam), New Hampshire, New Mexico (Taos), North Carolina, Vemon, Virginia (Fairy Stone State Park/Patrick County); Brazil, Canada, France, Norway Russia, Scotland, Spain, Switzerland and Zambia

It is the official state mineral of Georgia and the unofficial stone of Taos, New Mexico. It is abundant in Patrick County, Virginia and one locale is known as Fairy Stone State Park.

Some of the locales from which specimens of Staurolite are available at Taos Rockers include:

New Mexico

Russia

Madagascar

The Staurolites in the Taos area occur in a quartz-muscovite-biotite-staurolite-garnet schist and schistose phyllite (Hondo Member of the Rinconada Formation) of early Proterozoic age (approximately 1700 million years ago) and are known for their exceptionally fine twinning.

References: mindat.org; wikipedia.org, IGS, geology.com