Let's Talk Gemstones

By Edna B. Anthony, Gemologist

The Uncommon Sorosilicate Gemstones: Hemimorphite, Lawsonite, Idocrase, And The Epidote Group

The sorosilicate class of minerals is composed of more than seventy minerals. Most are rare, and only a few are used as gemstones or are cut for collectors. The exception, the lovely tanzanite of the zoisite group that forms in the orthorhombic crystal system, was discussed in a previous article. Since the optical and physical properties of the lesser known varieties vary and their use as gemstones is not common, each will be discussed in paragraph form and the "properties" table will be omitted in this article.

HEMIMORPHITE: (calamine) Zn4Si2O7[OH]2.H2O ; hydrous zinc silicate

The crystallographic polar symmetry of orthorhombic hemimorphite gave this mineral its name. A crystal displays polar symmetry (hemimorphism) when the ends of the central crystallographic axis are not symmetrical. In the hemimorphite structure, the ZnO3[OH] tetrahedra link groups of Si2O7 tetrahedra with bases parallel and apices oriented in one direction and with H2O molecules occupying the spaces. The usually tabular crystals exhibit terminations of a pyramid at one end and a combination of domes and pedion at the other. Granular, botryoidal, stalactite, massive, and fan-shaped forms can develop from such crystallization. The directional orientation of the tetrahedra causes its strong piezoelectric and pryoelectric characteristics. Crystals are brittle and exhibit a vitreous silky lustre, perfect cleavage in one direction, an uneven to sub conchoidal fracture, a Mohs hardness of 4.5 to 5 and a specific gravity variation of 3.4 - 3.5. It has a greater density than prehnite, which it sometimes resembles. The streak is white and the diaphaneity ranges from transparent to opaque. Refractive indices of 1.61 - 1.617 - 1.636 result in a birefringence of 0.022 for the biaxial positive mineral. Walter Schumann tells us in Gemstones of the World that the absorption spectrum is not usable, that there is no dispersion, no pleochroism and that weak fluorescence may be present, but it is not characteristic. Exposure to strong acids results in the formation of a silica gel.

Found worldwide, hemimorphite contains as much as 54.2 percent zinc in its composition and is valued as a rich source of zinc. Crystals are often associated with smithsonite, sphalerite, cerussite, galena, and anglesite in oxidized areas of the deposits. Until the middle of the eighteenth century, it was believed that smithsonite and hemimorphite were the same mineral called calamine. At that time, it was discovered that smithsonite is, instead, a zinc carbonate.

Dr. J. Kourimsky states in his Illustrated Encyclopedia of Minerals and Rocks that hemimorphite "most frequently occurs as the product of the oxidation of the upper parts of sphalerite accompanied by other secondary minerals that form the so-called `iron-cap' or `gossan'." By the process of metasomatism, the limestone is gradually replaced by less soluble materials from the surrounding waters. Mexico has been the only source of facetable hemimorphite crystals. Colorless gems of more than three carats are rare. Very little of the delicate blue material has been available. Cabachons of the massive blue material can be confused with smithsonite and turquoise. Specimens of white, brown, yellow, pale green, pale blue, gray, and colorless crystals are often found in mineral-enthusiasts' collections. A perfect plane of cleavage and its lack of durability limit the use of hemimorphite as a gemstone except in cabochon form.

LAWSONITE : CaAl2Si2O7[OH]2 .H2O; hydrous calcium aluminum silicate

In the structure of orthorhombic lawsonite, Si2O7 groups link [AlO,OH] octahedra with spaces filled by Ca2+ and H2O molecules in the chains. It is related to ilvaite and dimorphic with monoclinic partheite, which is related to the zeolite group. The vitreous to greasy lustered, translucent, frequently twinned crystals are found in association with glaucophane, quartz, chlorite, sphene, epidote, and garnet in gneisses and glaucophane schists formed under low temperature and high pressure, like those of the Tiburon peninsula of California. Two planes of perfect cleavage make it a fragile mineral. Dr. Joel Arem lists a hardness of 6+, but Dr. Cornelis Klein and Dr. Cornelius S. Hurlbut, Jr. state in the Manuel of Mineralogy after J. D. Dana that it is "characterized by its high hardness" of 8 and "fuses to a pebbly glass."

Lawsonite occurs in granular and massive forms and in tabular or prismatic crystals with a density of 3.05 to 3.12. Moderately high refractive indices of 1.665 -1.674 to 1.675 - 1.684 to 1.686 and a birefringence of 0.019 help to give it a high rate of dispersion. The crystals are colorless or white and can occur in delicate tones of gray, blue or pink. It is biaxial positive with pleochroism colors of blue/yellow-green/colorless or pale brownish-yellow/deep blue-green/ yellowish. The spectrum is not helpful, and it is inert to ultra-violet light.

The name lawsonite was chosen to honor Professor A. C. Lawson of the University of California. Few reference works list lawsonite, and Dr. Joel Arem tells us it "is extremely rare as a faceted stone, seldom reported and generally unavailable." Faceted gems probably would not exceed two to three carats and are likely be pale blue in color. Faceted lawsonite is truly a collector's gemstone.
 

 

IDOCRASE : Ca10Mg2Al4[SiO4]5[Si2O7]2[OH]4 + Be,Cu,Cr,Mn,Na,K,Ti,B,H2O,U,Th,Zn,Sn,Sb,

rare earths [Vesuvianite] Calcium magnesium aluminosilicate hydroxide

From the chemical composition formula shown above, taken from Dr. Joel Arem's Color Encyclopedia of Gemstones, it is apparent that a multitude of elements can replace elements in the tetragonal crystal structure of idocrase [vesuvianite]. It is a complicated sorosilicate, where isolated SiO4 tetrahedra and Si2O7 groups occur in the chains with the calcium, magnesium and aluminum and with the substitute elements occupying various sites available. The structure is closely related to and sometimes intergrown with grossular garnet. This helped to give it the name, derived from the Greek words "eidos" (likeness) and "krasis" (composition). Richard T. Liddicoat, Jr. observes in his Handbook of Gem Identification that both minerals are "calcium-aluminum silicates", and that green hydro grossular material from South Africa "grades into idocrase." Andradite garnet and diopside are also associated with idocrase.

Idocrase is found in several forms, including granular or compact aggregates, striated columnar masses, pyramidal crystals, and acicular forms, as well as the usual elongated prisms. The rather fragile vitreous to resinous crystals exhibit no plane of cleavage, a conchoidal fracture, a hardness of 6 - 7 and a varying specific gravity of 3.32 - 3.47. Intumescence accompanies fusion at 3 to produce a greenish or brownish glass. The associated minerals and the environment where the deposits developed greatly influenced the optical properties of the frequently twinned crystals.

Idocrase is double refractive and can exhibit positive or negative uniaxial optic character and sometimes anomalous positive or negative biaxial characteristics. Refractive indices of crystals found in serpentinites are highest with 1.705-1.750 and 1.702-1.761 readings with a birefringence of 0.018. A contact metamorphic environment produces material with indices of 1.655-1.733 and 1.674-1.737 and a birefringence of 0.015. Deposits in igneous formations have indices of 1.655-1.727 and 1.715-1.731and a low birefringence of 0.004. Readings of 1.697-1.698 and 1.705-1.707 with a birefringence of 0.008 occur in crystals produced in regionally metamorphosed zones. Dispersion is low at 0.019 - 0.025, no fluorescence occurs with exposure to ultraviolet light, and only the strongly colored specimens exhibit pleochroism. A weak line at 5285 and a strong one at 4610 in the spectrum can help in its identification. It can be confused with garnet, tourmaline, peridot, diopside, axinite, and with red-brown and yellow-red zircon.

Idocrase is most commonly known as vesuvianite, but the location of deposits and its colors have caused it to be known by several other names. It was differentiated from garnet by the mineralogist Abraham Gottlob Werner after he analyzed crystals from the volcanic extrusions of Mt. Vesuvius. He gave it the name vesuvianite. The famed Hazlov deposits near Eger in Bohemia yield the brown acicular variety, egeran, in association with hessonite garnet, wollastonite, and albite. The blackish green vilyuyite recovered from the banks of the Vilyuy river in Siberia, and the green variety that Dr. Joel Arem refers to as wiluite that is found near the Wilui River may be the same material. The massive green californite is sometimes sold as "California" or "Vesuvian" jade. A yellow variety is known as xantite. The unusual pale blue crystals are called cyprine.

Near Achmatovsk in the Ural Mountains, a delicate green matrix hosts lovely green to blue-green columnar idocrase crystals, which are highly prized by collectors. Beautifully transparent green crystals are retrieved from deposits near Ala in Piedmont, Italy, and brown and green varieties are produced in Kenya. Kristiansand and Eiker in Norway are famous for material with a square cross-section, and dark brownish-green druses of the same configuration are found in the limestones of Sanford, Maine.

Magnet Cove, Arkansas is the source of exceptionally large bi pyramidal crystals up to 30 centimeters in length. Idocrase is one of the many minerals that occurs in the specimen-rich gem deposits at Franklin, New Jersey. The delicate blue cyprine variety is found there. Chrome green and the unusual violet crystals accompany the numerous minerals in the noted gem deposits of Asbestos, Canada. Gem quality yellow-brown and green crystals are associated with pink grossular garnet in the Lake Jaco and Xalostoc, Morales regions in Chihuahua, Mexico. The asbestos-bearing formations near Eden Mills, Vermont yield crystals of exceptional beauty. Although it is known to collectors, idocrase is a lovely gemstone for jewelry when well cut.