With the exception of orthorhombic zoisite (tanzanite and thulite), discussed in the first article on the sorosilicate class of minerals, the members of the epidote group form in the monoclinic crystal system. Allanite, clinozoisite (the dimorph of zoisite), epidote, hancockite, mukhinite, and piedmontite are members of this group.
Epidote, Allanite, Clinozoisite
According to Dr. Joel Arem, mukhinite is a very rare vanadium-bearing mineral [Ca2(Al2V)Si3O12(OH)] that occurs in tiny grains which has not been faceted. It was named to honor the Russian geologist, A. S. Mukin, who discovered it in Gornaya Shoriya. Because the optical and physical characteristics and the chemical composition of the others vary, each will be covered separately. The table of properties will be omitted in this article.
Ca2[Al,Fe3+]3[SiO4]3[OH] Hydrous Calcium Iron-Aluminum Silicate
Epidote is the iron-rich end member of a solid solution series with aluminum-rich clinozoisite. Fe3 ions occupy the octahedral sites that occur outside the chains of AlO4[OH]2 and AlO6 linked by independent groups of Si2O7 octahedra and SiO4 tetrahedra. The brittle, typically slender, deeply striated, prismatic or tabular, transparent to translucent crystals are frequently twinned and terminated at one end only. Needle-like (acicular) crystals, fibrous and granular, and massive forms occur. The massive forms are generally lighter in color than the crystalline varieties. Its streak is gray. It has a plane of perfect cleavage, a Mohs hardness of 6 to 7, a specific gravity of 3.2 to 3.5, and a conchoidal or splintery fracture. A diagnostic black slag is formed when it fuses with intumescence at 3-4. After ignition, it gelatinizes with hydrochloric acid. Massive material is dull although crystals have a vitreous lustre. Cleavage planes exhibit a pearly lustre. A unique deep green color, shading to black that helps to identify some material, masks a dispersion range of 0.019 to 0.030 and limits its use as a gemstone substance. This dispersion and its double refraction indices of 1.733 to 1.768, with a varying birefringence of 0.015-0.049, do make the lighter green and yellow transparent crystals suitable for faceting as gems. The refractive indices and the birefringence increase with iron content.
Dr. Arem tells us that the low-iron, trichroic yellow-green crystals from Minas Gerias, Brazil have refractive indices of 1.718/1.737/1.743, a birefringence of 0.021, and a specific gravity of 3.3-3.5. Gem quality yellow-brown crystals from Sri Lanka possess very similar characteristics. Material exhibiting the higher indices may be positive, but epidote is usually biaxial negative. Its strong trichroic pleochroism colors include various shades of pale yellow, yellow-green, green, dark brown and colorless. Very careful directional orientation of a specimen usually reveals a very strong line at 4550 and a sometimes a weak one at 4750 in the spectrum. It is not diagnostic.
Epidote usually occurs in high-pressure, low-temperature calcium-bearing metamorphic rocks and as precipitates from magmatic fluids in igneous formations. It is frequently associated with zeolites, garnet, idocrase (vesuvianite), diopside, and calcite. Deposits in limestones that form epidote rocks are called epidosite. Fine crystals of epidote are found in the Ural mountains and in many alpine areas in Austria, Switzerland and France. Finland, Norway, Italy, and Czechoslovakia are other European sources. Kenya, Madagascar, Australia, Japan, Korea, China, Mexico, and the United States produce excellent material for collectors. According to Richard Pearl’s book, How to Know the Minerals and Rocks, the rare colorless epidote is found at Tierra del Fuego at the tip of South America.
The name epidote is derived from the Greek word “epidosis”, meaning increase. Dr. Joel Arem in his Color Encyclopedia of Gemstones and Michael O’Donoghue in The American Nature Guides to Rocks and Minerals state this is a reference to the longer length of one side of the base of the crystal prism than the other. However, in The Illustrated Encyclopedia of Minerals and Rocks, Dr. J. Kourimsky interprets it to denote its addition to the mineral system when its differentiation from tourmaline was made by the French mineralogist and crystallographer Rene Just Hauy in 1901. In Gemstones of the World, Walter Schumann claims its numerous crystal faces account for the name.
The pistachio green variety of epidote is sometimes called pistacite. Basil Booth in Identifying Rocks and Minerals mentions a variety called withamite that varies in color from pale yellow to bright red. A search of numerous references reveals nothing more. It is not listed in the Glossary of Mineral Species by Michael Fleisher and Joseph A. Mandarino, so information about its chemical composition, physical, and optical properties and sources are unavailable. The emerald green tawmawite found in Burma and Finland contains chrome. Dr. Arem lists it as epidote, but Michael O’Donoghue states it is a rock. The granite composed of greenish epidote and pink feldspar found in Zimbabwe and in extensive deposits in the United States is called unakite. It is used primarily for cabochon and bead material. Switzerland is the source of lovely pink and yellow mixed crystals of epidote, piedmontite, and clinozoisite.
Excellent cutting of fine epidote rough can produce lively gems, usually of less than 5 carats. Cabochons cut from material housing fibrous inclusions can produce cat’s-eye stones. An epidote gemstone can be a choice for those who seek the unusual, and it is a specimen found in almost every collector’s cabinet.
Hydrous Calcium Aluminum Silicate
Allanite is of interest primarily to collectors of mineral specimens. It is faceted and cut en cabochon only as a curiosity for gem enthusiasts. This sub-translucent, slightly radioactive substance can be found as bladed aggregates in massive and granular forms or as disseminated grains in igneous rocks, schists, and crystalline limestones. Rare earth elements, such as cerium, lanthanum, thorium, or sodium replace some of the calcium, and partial substitution of aluminum is replaced by ferric and ferrous iron. Magnesium and manganese occurs in its epidote structure. Dr. Joel Arem and The Glossary of Mineral Species indicate that yttrium can also replace calcium. Dr. Kourimsky classifies orthite as a variety of epidote and mentions cesium as one of the rare earth elements in its composition.
The complete destruction of its structure by “self-irradiation” leads to an isotropic vitreous material that can absorb fairly large amounts of water. A yellow-brown alteration product frequently coats the usually pitch black, brownish or grayish mineral. The streak is greenish-grey to brown. It fuses with intumescence at 2.5 to a black magnetic glass. Only before fusion does exposure to hydrochloric acid form a gel. Its lustre varies from sub-metallic to pitchy and resinous. The fracture is uneven to conchoidal. Simon and Schuster’s Guide to Rocks and Minerals states that the cleavage is indistinct. The hardness varies from 5.5 to 6.0, and the density varies from 3.5 to 4.2. Dichroscope examination of very thin slices of the material exhibits pleochroism colors of brownish-yellow/greenish-brown/reddish-brown or deep red-brown/brown/pale-brown or light green/colorless/green. The basis of its wide range of birefringence [0.013-0.036] is the spread of the refractive indices of the biaxial substance from 1.640 to 1.828.
Allanite was named to honor the English mineralogist Thomas Allan. The Ural mountains, Madagascar, Greenland, Norway, Sweden, Italy, Canada, and several localities in the United States are some of the sources for this mineral.
Ca2Al3Si3O12[OH] Hydrous Calcium Aluminum Silicate
This monoclinic, iron-poor dimorph with zoisite forms a series with epidote. The frequently zoned, transparent to translucent, elongated and poorly terminated, striated crystals are found in regional and contact metamorphic rocks and plutonic formations. It develops as a secondary hydrothermal alteration of the calcic plagioclase materials present in the igneous formations. Deposits in Turin, Italy and Madagascar produce beautiful pink crystals. Gem quality brownish facetable crystals and massive material are found in Baja, Mexico. Grey-green crystals occur in Kenya. Other sources are India, Austria, Czechoslovakia, Italy, Switzerland, Ireland, Iceland, Canada, and in Colorado and Nevada in the United States. Its basal plane of perfect cleavage, an uneven to conchoidal fracture, the hardness of 6.0, and a specific gravity range of 3.21-3.38 are typical physical properties of the epidote group. Unlike the other members of the group, it exhibits no pleochroism. No fluorescence and an inconclusive spectrum are characteristic. The colorless, pink, light green, yellow, or green-brown crystals exhibit a vitreous lustre and 0.019 dispersion. Biaxial clinozoisite possesses the lowest birefringence [0.005-0.015] and refractive index range [1.67-1.734] of the epidote group. These rise as the proportion of iron increases.
Clinozoisite gems of greater than five carats are rare. Careful consideration should be given to its properties if one chooses to set this unusual stone in jewelry.
Few reference works mention this very rare member of the epidote group. From its chemical formula, we know that lead and strontium can replace some of the calcium in its biaxial monoclinic structure. This may be the reason for its greater density of 4.03. Its hardness of 6-7, the plane of perfect cleavage, a conchoidal fracture, the usual absence of fluorescence, and a non-diagnostic spectrum are typical characteristics of the epidote group. The pleochroism colors are tones of yellowish-brown to definite red. The crystals can be brownish, brownish-red, or black with a refractive index range of 1.788 to 1.830 and birefringence of 0.042. Hancockite exhibits a dispersion of 0.019. In the Color Encyclopedia of Gemstones, Dr. Joel Arem tells us that the only “notable” source of the small vitreous crystals is Franklin, New Jersey. Any faceted gem would be of less than one to two carats and a prized item in any gem enthusiast’s collection. It is unlikely that hancockite would be used in jewelry.
Ca2[Mn,Fe,Al]3Si3O12[OH] Hydrous Calcium Aluminum Manganese Silicate
This sub-translucent to opaque, monoclinic, biaxial mineral, also called piedmontite, is usually found in metamorphic manganese-bearing glaucophane schists in Italy, France, Sweden, Scotland, Japan and New Zealand. Areas in California, Arizona, Missouri, and Pennsylvania in the United States are other sources. Some quartz porphyries in Egypt and Pennsylvania are colored red by its presence.
Piedmontite is not commonly found in pegmatitic formations nor lavas. It occurs in granular aggregates, dense masses, and as small rod-like crystals. Hardness is 6.5, and its specific gravity varies from 3.45 to 3.52. The basal plane of perfect cleavage, a conchoidal fracture, the lack of luminescence in ultra-violet light, an indistinct spectrum, and the vitreous lustre of this mineral are normal for the epidote group. Crystals of pink, rose red, brownish-red and black show pleochroism colors of red, yellow, and violet. It exhibits the greatest birefringence [0.025-0.073] of the group with refractive index readings of 1.732 to 1.829 and a normal dispersion of 0.19. Piemontite was named for the area in Piemonte, Italy where the manganese mines yield excellent material. Many mineral collections contain specimens; and good material is used for cabochons and inlay work by lapidaries.