Variscite

Variscite is a hydrated aluminium phosphate mineral (AlPO4•2H2O). It is a relatively rare phosphate mineral. It is sometimes confused with turquoise; however, variscite is usually greener in color. The green color results from the presence of small amounts of trivalent chromium (Cr3+).

It is a mineral that can range in color from bluish-green to red. It has color patterns. It is sometimes confused with similar minerals such as turquoise, jade, and chrysoprase. It is a mineral derived from dihydrated aluminum phosphate that rarely appears in the form of crystals, forming large masses in its outcrops. Its hardness on the Mohs scale is 4 to 5.

Geology

Variscite is a secondary mineral formed by direct deposition from phosphate-bearing water which has reacted with aluminium-rich rocks in a near-surface environment. It occurs as fine-grained masses in nodules, cavity fillings, and crusts. Variscite often contains white veins of the calcium aluminium phosphate mineral crandallite.

It was first described in 1837 and named for the locality of Variscia, the historical name of the Vogtland, in Germany. At one time, variscite was called Utahlite. At times, materials which may be turquoise or may be variscite have been marketed as "variquoise". Appreciation of the color ranges typically found in variscite have made it a popular gem in recent years.

Variscite from Nevada typically contains black spiderwebbing in the matrix and is often confused with green turquoise. Most of the Nevada variscite recovered in recent decades has come from mines located in Lander County and Esmeralda County, specifically in the Candelaria Hills.

Notable localities are Lucin, Snowville, and Fairfield in Utah, United States. Most recently found in Wyoming as well. It is also found in Germany, Australia, Poland, Spain, Italy (Sardinia), and Brazil.

Classification

Already in the outdated 8th edition of the mineral classification according to Strunz, variscite belonged to the mineral class of “phosphates, arsenates and vanadates” and there to the section “hydrated phosphates, arsenates and vanadates without foreign anions”, where it is listed together with mansfieldite, skorodite and strengite in the “variscite series” with the system number VII/C.05b.

In the "Lapis Mineral Catalogue", last revised in 2018, which is still based on the old form of Karl Hugo Strunz 's classification, the mineral was given the system and mineral number VII/C.09-050. In the " Lapis Systematics ", this also corresponds to the section "Hydrous phosphates, without foreign anions", where variscite, together with kolbeckite, koninckite, malhmoodite, mansfieldite, metavariscite, parascorodite, phosphosiderite, scorodite, strengite, and yanomamite, forms the "variscite group" with the system number VII/C.09. 

The 9th edition of Strunz's mineral classification, last updated by the International Mineralogical Association (IMA) in 2009, also classifies variscite in the section "Phosphates etc. without additional anions; with H 2 O". However, this is further subdivided according to the relative size of the cations involved and the molar ratio of the phosphate, arsenate or vanadate complex to the crystal water content, so that the mineral can be found in the subsection "With exclusively medium-sized cations; RO 4: H 2 O = 1: 2" according to its composition, where it forms the "Variscite group" with the system number 8.CD.10 together with mansfieldite, redondite (Q), skorodite, strengite and yanomamite.

In the Dana mineral classification system, which is primarily used in English-speaking countries, variscite has the system and mineral number 40.04.01.01. This also corresponds to the class of "Phosphates, Arsenates, and Vanadates" and the division "Hydrous Phosphates, etc.". Here, it is found within the subdivision "Hydrous Phosphates, etc., with A 3+ XO 4 × x(H 2 O)" in the "Variscite Group," which also includes strengite, scorodite, mansfieldite, and yanomamite.

Chemistry

In the theoretically ideal, i.e. pure composition of variscite (AlPO4•2H2O), the mineral consists of one aluminum atom (Al 3+) and one phosphate ion ((PO 4) 3−, consisting of one phosphorus and four oxygen atoms) as well as two parts water (H2 O). This corresponds to a mass fraction (weight %) of the atoms of 17.08 wt.% Al, 19.61 wt.% P, 60.76 wt.% O and 2.55 wt.% H or in the oxide form 32.27 wt.% Al 2 O 3, 44.92 wt.% P 2 O 5 and 22.81 wt.% H 2 O. 

In natural mineral samples from the Lucin mining district in Box Elder County, Utah, small amounts of vanadium (0.32 wt.% V2 O5), chromium (0.18 wt.% Cr2 O3) and iron (0.06 wt.% Fe2 O3) were also measured. 

Variscite forms a continuous solid solution series with the also orthorhombic, hydrous iron(III) phosphate strengite. 

Crystal structure

Variscite crystallizes isotypically with scorodite in the orthorhombic space group Pcab (space group no. 61, position 2) with lattice parameters a = 9.82 Å; b = 9.63 Å and c = 8.56 Å and 8 formula units per unit cell. 

In the crystal structure of variscite, aluminum is octahedrally surrounded by six oxygen atoms (coordinated, Al), with two of these oxygen atoms being part of each water molecule. The Al octahedra are connected to the phosphate tetrahedra via shared vertices, forming a three-dimensional network. Hydrogen bonds provide additional stability to the network.

Deposits

As a rule, deposits occur in waters with high phosphate content in contact with rocks containing aluminum. On islands and in caves, phosphate is produced by the decomposition of guano. Variscite was used in Europe to make personal ornaments, especially beads, from the Neolithic period, having been found at multiple prehistoric sites as far north as Brittany. Its use continued through the Bronze Age and into Roman times, although it was not determined until the 19th century that all the variscite used in Europe came from three sites in Spain. The Gavá mine (Barcelona) is probably the one that began to be mined in the early Neolithic period. Since 1993, the mines, with underground galleries, have been part of an archaeological park open to visitors.   

In the Alcañices synform, in Palazuelo de las Cuevas, a town that is part of the municipality of San Vicente de la Cabeza (Zamora) there are remains of ancient work, at least from the end of the Neolithic, but which probably knew their peak in Roman times since near Palazuelo there are remains of a Roman village, with abundant fragments of variscite cut on the surface of the ground,   Variscite has also been found, with remains of ancient work, in the so-called Terena Synform, in Encinasola (Huelva).  Other notable locations for the specimens of variscite found in it are Lucin and Fairfield, both in the state of Utah (United States).

Jewelry

Despite its relatively low hardness and brittleness, variscite is often used in gemstones due to its often vibrant coloration, especially since it can resemble other valuable gemstones such as chrysocolla, jade, and turquoise. If variscite is interspersed with brown to black veins (matrix), it can also be confused with the coveted matrix turquoise.

Variscite has been used in Europe to make personal ornaments, especially beads, since Neolithic times. Its use continued during the Bronze Age and in Roman times although it was not until the 19th century that it was determined that all variscite used in Europe came from three sites in Spain, Gavá (Barcelona), Palazuelo de las Cuevas (Zamora), and Encinasola (Huelva).

Variscite is processed into either cabochons or layered stones and is occasionally offered under the misleading trade names “Australian Turquoise”, “California Turquoise”, “Nevada Turquoise”, “Utah Turquoise” or “Utalith”.

Variscite is sometimes used as a semi-precious stone, and is popular for carvings and ornamental use due to its beautiful and intense green color, and is commonly used in silversmithing in place of turquoise. Variscite is more rare and less common than turquoise, but because it is not as commonly available as turquoise or as well known to the general public, raw variscite tends to be less expensive than turquoise.

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