ACTINOLITE


☐{Ca2}{Mg4.5-2.5Fe0.5-2.5}(Si8O22)(OH)2,
calcium magnesium iron silicate hydroxide




Actinolite is the intermediate member of a solid-solution series that extends from tremolite, a Ca-Mg amphibole, to ferro-actinolite, a Ca-Fe amphibole.  The idealized chemical formulas for these species, with separators to more clearly indicate the chemical relationships, are as follows:

            Generic formula:            X               Z                 Si8O22         (OH,F,Cl)2
            Tremolite:                    Ca2            Mg5               Si8O22            (OH)2
            Actinolite:                    Ca2        (Mg,Fe2+)5         Si8O22            (OH)2
            Ferro-actinolite:            Ca2            Fe5                Si8O22            (OH)2

The X crystallographic site in this amphibole series is dominated by Ca, as shown, and the Z site by Mg and Fe.  If the Z site contains 90% or more Mg atoms - that is, if the Mg/(Mg+Fe2+) ratio exceeds 0.9 - the amphibole is termed tremolite.  If instead the site contains less than 50% Mg atoms (and thus more than 50% Fe) it is termed ferro-actinolite.  Actinolite as a species, then, by definition has an Mg/(Mg+Fe2+) ratio of 0.5-0.9.

It should be noted that the actual chemistry of the amphiboles is considerably more complex than the idealized formulas imply.  The local amphiboles, especially those in and near the zinc orebodies, are often markedly impure, making it difficult to identify specimens to the species level.  Comparison to analyzed specimens is helpful to a degree, but rigorously analyzed examples are few, leaving sight identification of the local amphiboles an almost hopeless proposition.  Dunn (1995, p. 451) provided some general guidelines for distinguishing the various amphiboles, but it nevertheless remains true that correct identification to the species level is generally possible only by means of combined chemical-analytical and X-ray diffraction techniques.  Many of the local amphiboles are best labeled, simply, amphibole.

Specimens known or inferred to be actinolite are common at Franklin, less so at Sterling Hill, and are most abundant in the gneisses of the calcium-silicate units.  Actinolite occurs in various familiar habits:  as coarse-grained masses; as large, prismatic crystals; as felted masses of fibrous crystals (the variety termed byssolite) enclosed in calcite; as dense masses of plumose radiating grains; and as a silky, fibrous mineral in various vein assemblages.  Its color ranges from pale to dark green and pale brown.  Outside the orebodies, much of the Franklin Marble is too iron-poor for actinolite to be present, with tremolite appearing instead, but silicate-rich gneisses within the marble unit may contain it.

References

Betancourt, P.B. (1989), The epidote-pyroxene-fluorapophyllite assemblage in the Franklin mine at Franklin, New Jersey.  The Picking Table, vol. 30, no. 1, p. 8-10.  [Mentions actinolite as a vein mineral in cavities with stilbite and epidote from the Franklin mine]

Craig, F.M. and Moritz, Harold (2024), Analysis of amphiboles from Sterling Hill, Ogdensburg, New Jersey, USA. The Picking Table, vol. 65, no. 1, p. 13-29. [Presents chemical analyses of nine amphibole specimens from Sterling Hill, three of them actinolite]

Dunn, P.J. (1995), Franklin and Sterling Hill, New Jersey:  the world’s most magnificent mineral deposits.  Privately published in five volumes, 755 pp.  [Description of actinolite on p. 451-453; includes chemical analyses of seven specimens]

Dunn, P.J. and Peacor, D.R. (1984), Nelenite, a manganese arsenosilicate of the friedelite group, polymorphous with schallerite, from Franklin, New Jersey.  Mineralogical Magazine, vol. 48, p. 271-275.  [Description of a high-Fe actinolite from the nelenite assemblage at Franklin]

Fowler, Samuel (1825), An account of some new and extraordinary minerals discovered in Warwick, Orange County, N.Y.  American Journal of Science, 1st Series, vol. 9, p. 242-245.  [Describes amianthus from Franklin, regarded by Dunn (1995) as probably corresponding to actinolite]

Frondel, Clifford (1972), The minerals of Franklin and Sterling Hill - a check list.  Wiley-Interscience, 94 pp.  [Describes local actinolite on p. 39 as “not abundant” and remarks on its commonly elevated content of zinc and manganese]

Germine, M. and Puffer, J.H., 1989, Origin and development of flexibility in asbestiform fibres. Mineralogical Magazine, vol. 53, p. 327-335.  [Reports a fibrous actinolite from Franklin with 3.8% Cr2O3]

King, V.T., King, N.E., Betancourt, P.P., Bostwick, R.C., Chin, P., Hecht, T.J., Kuitems, S.M., Moritz, H., Nemetz, J.D., Nikischer, A.J., Sanford, S., Van Fleet, J.A., and Verbeek, E.R. (2022), The Mineralogy of Franklin and Ogdensburg, New Jersey—A Photographic Celebration. Privately published, 1400 pp. [Photos of actinolite specimens shown on p. 757-758]

Klein, C. Jr. and Ito, J. (1968), Zincian and manganoan amphiboles from Franklin, New Jersey. American Mineralogist, vol. 53, p. 1264-1275.  [Good discussion of the chemistry of the local amphiboles]

Koenig, G.A. (1887), On zinc-manganese asbestos. Proceedings of the Academy of Natural Sciences of Philadelphia, p. 47-48.  [The asbestos described in this paper is regarded by Dunn (1995) as probably being fibrous actinolite]

Leake, B.E. (1978), Nomenclature of amphiboles. American Mineralogist, vol. 63, nos. 11 and 12, p. 1023-1052. [Extensive revisions to amphibole species names]

Leavens, P.B., Zullo, J., and Verbeek, E.R. (2009), A complex, genthelvite-bearing skarn from the Passaic Pit, Sterling Hill Mine, Ogdensburg, New Jersey. Axis, vol. 5, no. 1, p. 1-26. [Detailed discussion of a retrograde metamorphic mineral assemblage from a calc-silicate zone adjacent to the footwall of the east limb of ore at Sterling Hill. Numerous specimen photos]

Nuttall, Thomas (1822), Observations and geological remarks on the minerals of Paterson and the valley of Sparta in New Jersey.  American Journal of Science, 1st Series, vol. 5, p. 239-248.  [First listing of actinolite as a mineral of the Franklin-Sterling Hill area]

Palache, Charles (1935), The minerals of Franklin and Sterling Hill, Sussex County, New Jersey.  U.S. Geological Survey Professional Paper 180, 135 pp.  [Brief mention of actinolite, then regarded as a rare mineral locally, on pp. 71 and 115]



Images




Contorted mass of green, bladed actinolite crystals in a matrix of brown, iron-stained calcite
Earl R. Verbeek
A contorted mass of green, bladed actinolite crystals in a matrix of brown, iron-stained calcite. The specimen bears the label F534 of a previous collector and is accompanied by a paper label of John Albanese (1898-1969), who for several decades was a prominent dealer in Franklin-Sterling Hill minerals. It is now (2022) in the Elna Hauck collection.

Identifier: ACT7a
Locality: Trotter mine, Franklin
Specimen size: 11 x 4.5 x 4 cm





Splintery mass of dark green actinolite in tightly intergrown, contorted fibers, associated with dark brown andradite
Earl R. Verbeek
A splintery mass of dark green actinolite in tightly intergrown, contorted fibers, associated with dark brown andradite (lower right). The specimen bears several old labels, one of which reads “Actinolite- Purchase from Wards 1943?” Another gives the price as 75 cents, and the third, the number 262 in black ink on white paint, is that of Richard Hauck, in whose collection it once resided. It then passed to Steven Phillips upon his purchase of the Hauck collection, thence to Earl R. Verbeek (his specimen ERV-1744), and finally to the Franklin Mineral Museum (catalog no. FMM-2225). A label in the Phillips collection identified this specimen as tirodite, but on what basis is unknown. However, in a June 11, 1982 letter from Pete J. Dunn to his “Franklin Friends,” this specimen was shown to be actinolite on the basis of an electron microprobe analysis and calculations based on 23 oxygens in accordance with the IMA-approved nomenclature of Leake (1978).

Identifier: ACT9a
Locality: Franklin mine, Franklin
Specimen size: 13 x 8 x 4 cm





Radiating sprays of dark green, acicular actinolite crystals 0.5-2 cm long on brown andradite crystals
Earl R. Verbeek
Radiating sprays of dark green, acicular actinolite crystals 0.5-2 cm long on brown andradite crystals, both lining a vug 8 cm across in massive andradite. The actinolite crystals have been freed by dissolving the calcite that once enclosed them in acid. This specimen, formerly number 299x in the collection of Fred Parker, is now in the collection of Steven Phillips.

Identifier: ACT4a
Locality: Buckwheat dump, Franklin
Specimen size: 13 x 9 x 8 cm





Dark green, rock-locked crystals of actinolite intergrown with massive, dark brown garnet
Earl R. Verbeek
Dark green, rock-locked crystals of actinolite intergrown with massive, dark brown garnet. Vugs in the rock are lined with tiny acicular crystals of additional actinolite and locally with abundant, tiny, prismatic crystals of yellowish-green epidote. Compare to specimen ACT4 above. Formerly in the Richard Hauck collection, this specimen now resides in the collection of Steven Phillips. The dark green mineral was verified as actinolite by Pete J. Dunn, as specified in his June 11, 1982 letter to his "Franklin Friends." Verification was by microprobe analysis with calculations done on the basis of 23 oxygens and in conformance with the nomenclatural scheme of Leake (1978).

Identifier: ACT5a
Locality: Franklin mine, Franklin
Specimen size: Not recorded; estimated 8 cm tall





Actinolite in intergrown plumose masses of fibrous to acicular, pale grayish green crystals
Earl R. Verbeek
A fine example of Sterling Hill actinolite in intergrown plumose masses of fibrous to acicular, pale grayish green crystals. In 1979 this specimen was obtained by John Kolic from miner Randy Ottens and entered the Kolic collection as no. 245; thirty-five years later, in early 2014, it became part of the collection of the Sterling Hill Mining Museum.

Identifier: ACT8a
Locality: 1015 stope, 180 level, Sterling mine, Ogdensburg
Specimen size: 12 x 7.5 x 5.5 cm





Intergrown radiating masses of pale green, fibrous actinolite with pale gray calcite
Earl R. Verbeek
Intergrown radiating masses of pale green, fibrous actinolite with pale gray calcite. Originally part of the famed SPEX/Gerstmann collection, it is now on display in the Franklin Mineral Museum under catalog number FMM-959.

Identifier: ACT1a
Locality: Sterling mine, Ogdensburg
Specimen size: 10 x 8 x 6 cm





Delicate acicular crystals of green actinolite on a matrix of felted actinolite with minor purple fluorite
Earl R. Verbeek
Delicate acicular crystals of green actinolite, freed from the enclosing calcite with acid, on a matrix of felted actinolite with minor violet fluorite (right). This specimen can be traced back to April 1991, when Gary Grenier purchased it from Andy Massey, a Franklin resident and dealer in the local minerals. Most of the Grenier collection was subsequently purchased by Steven Phillips, in whose collection this fine example now resides.

Identifier: ACT3a
Locality: 700 level, Sterling mine, Ogdensburg
Specimen size: 11 x 7 x 2 cm





Tiny, dark green, acicular actinolite crystals, locally with equally tiny, brownish-green epidote crystals
Earl R. Verbeek
Tiny, dark green, acicular actinolite crystals, locally with equally tiny, brownish-green epidote crystals, lining vugs in fractured and rehealed massive actinolite. Several anhedral grains of sphalerite are included within the massive actinolite on the back of the specimen. From the Gary Grenier collection (his no. GG3033), this specimen is now in the collection of Steven Phillips.

Identifier: ACT6a
Locality: Sterling mine, Ogdensburg
Specimen size: 10 x 5.5 x 3.5 cm





Long-bladed crystals of dark green actinolite in a matrix of coarse-grained, pale gray calcite, tiny grains of franklinite
Earl R. Verbeek
Long-bladed crystals of dark green actinolite in a matrix of coarse-grained, pale gray calcite (fl. red) containing scattered, tiny grains of franklinite and, in one layer, pale olive green grains of diopside(?) along with the franklinite. The specimen is compositionally layered, as shown by changes in grain size of the calcite and by several thin layers containing franklinite. The face of the specimen containing the actinolite is parallel to these compositional layers. Gary Grenier obtained this specimen (labeled GG209) in April 1995 from collector/dealer Bob Boymistruk.

Identifier: ACT12a
Locality: Franklin mine, Franklin
Specimen size: 16 x 9 x 8 cm





Tiny needles of dark green actinolite coating highly elongate crystals of quartz
Earl R. Verbeek
Tiny needles of dark green actinolite coating highly elongate crystals of quartz, a few of which are reverse scepters. The actinolite completely coats many of the crystals but only partially coats others, including the long (1.4 cm) crystal in the center of the photo. These crystals lie on a matrix of massive actinolite that contains grains (breccia fragments?) of white calcite plus small masses of oil-green to greenish-brown sphalerite. This specimen is also host to small crystals of tennantite, plus tiny, millimeter-sized crystals of a pale violet-gray mineral, probably axinite. Formerly specimen GG3041 in the collection of Gary Grenier, who obtained it in August 1990 from Richard Hauck, it later became part of the Steven Phillips collection.

Identifier: ACT13a
Locality: Sterling mine, Ogdensburg
Specimen size: 5 x 4 x 4 cm





Fairly fine-grained pyroxene-feldspar gneiss in contact with recrystallized masses of medium-grained, dark green actinolite
Earl R. Verbeek
Fairly fine-grained pyroxene-feldspar gneiss in contact with recrystallized masses of medium-grained, dark green (nearly black in aggregate) actinolite. Druses of tiny, long-prismatic crystals of actinolite line shallow vugs in this material. The feldspar in the gneiss is pale pinkish-orange for the most part and fluoresces dim red SW, but some has been altered to epidote, as seen on the rear of this piece. Much of the feldspar is also vuggy, obviously so when viewed under a hand lens. Tiny crystals of brown titanite are present both in the fine-grained gneiss and the coarser-grained actinolite. Formerly specimen GG2561 of Gary Grenier, it later became part of the Mark Leger collection.

Identifier: ACT16a
Locality: Sterling mine, Ogdensburg
Specimen size: 9.5 x 7 x 6.5 cm





Pale green, fibrous actinolite and purple fluorite with white calcite
Earl R. Verbeek
Pale green, fibrous actinolite and violet fluorite with white calcite from a fault or fissure zone in fine-grained, well-foliated, feldspar-pyroxene gneiss. The actinolite has been freed from the enclosing white calcite with acid, but some of the calcite remains and is visible in the photo. Its bright red fluorescence suggests derivation of the specimen from within or near the Sterling Hill orebody. Now specimen FMM-9115 of the Franklin Mineral Museum, it was collected in the early 1990s by Bob Jenkins during his extensive mineralogical studies underground at Sterling Hill.

Identifier: ACT17a
Locality: Sterling mine, Ogdensburg
Specimen size: 11 x 7 x 5 cm





Massive, dark green, coarse-grained actinolite with pink rhodonite and white calcite
Jeffrey R. Glover - Indoors Outdoors Photography
Massive, dark green, coarse-grained actinolite with pink rhodonite and white calcite from the “genthelvite pit” adjacent to the footwall of the east limb of ore at Sterling Hill. At least four chemical analyses have been made of actinolite specimens from this pit, all of which support the identity of the dark green amphibole as zincian, manganoan actinolite. The most rigorous analysis, that of Craig and Moritz (2024), shows a content of 7.74 wt.% ZnO and 4.39 wt.% MnO. As discussed in Leavens et al. (2009), the actinolite and rhodonite in this area are retrograde metamorphic products formed by recrystallization of an original high-temperature assemblage with augite, calcite, bustamite, and garnet as the major phases originally present.

Identifier: ACT21a
Locality: “Genthelvite pit,” Sterling Hill, Ogdensburg
Specimen size: 11 cm tall as shown





Tiny brown crystals of axinite-(Fe) nestled among needles of green actinolite, with olive-green epidote and a little white calcite
Liz Schneider
A jewel from the Kolic collection at Sterling Hill Mining Museum, this small specimen (catalog no. SHMM-2354) features numerous, tiny brown crystals of axinite-(Fe) nestled among needles of green actinolite, with olive-green epidote and a little white calcite.

Identifier: ACT19a
Locality: Sterling mine, Ogdensburg
Specimen size: 2.6 x 2.5 x 1.7 cm





Radiating sprays of long, almost needlelike crystals of dark green actinolite intergrown with amber-brown garnet
Earl R. Verbeek
Radiating sprays of long, almost needlelike crystals of dark green actinolite intergrown with amber-brown garnet in a thin vein cutting very lean franklinite-calcite ore containing sparse, scattered, tiny grains of willemite. Also in the vein are a few tabular, centimeter-sized crystals of hematite. This specimen is in the Steven Phillips collection.

Identifier: ACT11a
Locality: Sterling mine, Ogdensburg
Specimen size: 15 x 11 x 9 cm