Minerals of Rollstone Hill, Fitchburg, Mass.

Peter Cristofono February, 2011

Rollstone Hill rises more than 400 feet above downtown Fitchburg, Worcester County, in north central Massachusetts. The city of Fitchburg currently owns 75 acres at the top of the hill, including woodland and former quarries. The name "Rollstone Hill" was in use as early as 1731, when the area was part of Lunenburg. (Fitchburg was incorporated as a town in 1764.)

Granite was first quarried on Rollstone Hill beginning sometime prior to 1830. The stone is two-mica granite of early Devonian age which has been named Fitchburg granite (Emerson, 1917). It was used both as dimension stone and as aggregate for construction purposes. At least six quarries were formerly worked there including the (John) McCauliff, (O. E.) Litchfield, and (Henry) Godbeer quarries, with the McCauliff being the largest of these. In 1928, the McCauliff Quarry Company acquired the other quarries, which had been operated by independent companies. Quarry operations ceased in 1941. The quarries are composed entirely of granite. No contacts of the surrounding schist are present on the hill, but Hitchen (1935) observed isolated clots of biotite in the granite which he regarded as xenoliths of schist assimilated to varying extent.

C. Stanfield Hitchen (1935) studied pegmatites on Rollstone Hill and named the following types: (I) Biotite type; (II) Tourmaline type; (III) Beryl type; (IV) Titanite type; and (V) Allanite type. He found that the pegmatites all occur within the granite, as veins, veinlets, pipelike bodies or irregular masses, and are without cavities. In addition, rare calcite veins also occur. Only one pegmatite, a beryl type, showed significant zonation, and gem beryl was collected from the quartz core.

A 100+ ton glacial erratic called Rollstone Boulder was once located on the summit of the hill, and was a local attraction depicted on early twentieth century postcards. Threatened by ongoing quarrying operations in the 1920s, the city of the Fitchburg, by popular demand, paid to have the boulder blasted apart and reassembled on the upper common on Main Street, where it can still be seen today. Large white feldspar phenocrysts, clearly visible on the boulder prove that the stone is not Fitchburg granite, but porphyritic Kinsman quartz monzonite, a type of stone which occurs far to the north-northwest in New Hampshire.

Quarry view — early 20th century postcard Modern view of McCauliff Quarry

Two old postcard views of Rollstone Boulder, ca. 1910

Minerals Twenty-nine minerals have been reliably reported from Rollstone Hill. Two others are considered unconfirmed, and three more are reported here as unidentified.

Albite NaAlSi3O8 Albite microcrystals occur in calcite veins (Hitchen, 1935), and also as a late mineral associated with microcline in the pegmatites. The variety, oligoclase, (Na,Ca)[Al(Si,Al)Si2O8] occurs as a primary mineral (as anti-perthite) in phenocrysts in “Type II” pegmatites (Hitchen, 1935) and was also noted by Dale (1910, 1923), Emerson (1917) and Hurlbut (1964).

Allanite-(Ce) (Ca,Ce,Y)2(Al, Fe2+, Fe3+)3(SiO4)(Si2O7)(O)(OH). Epidote Group. Allanite occurs as dark green or brown to black crystals, superficially weathering to orange or dark red-brown. Crystals can be several centimeters in length and mostly occur in the “Type V” pegmatites. Two SEM-EDS analyses (this study) show that cerium is the dominant rare earth element, followed by appreciable Nd. The mineral is radioactive due to thorium content. Crystals are frequently curved. “Gummite” was reported by Emerson (1917) as an alteration product of allanite; Hitchen (1935) reported seeing an orange-colored metamict substance fringing the crystals (in thin sections). Allanite was also reported by Lewis (1927), Lane (1933), Kitson (1938), Billings (1941), Bjareby (1962), Hurlbut (1964), Seaman (1976), Grauch & Zarinski (1976), and listed in Gleba (1978) as the variety “orthite.”

Allanite-(Ce) FOV=8mm Allanite-(Ce) 3mm crystal

Allanite “Type V” Pegmatite showing weathered allanite crystals in quartz. FOV is about 30cm. Almandine (Fe,Mn)3Al2Si3O12 Garnet Group. Manganese-rich almandine occurs in crystals in Type III and IV pegmatites. Garnet also occurs as an accessory in granite. Color can be shades of red, orange or pink. Pink crystals are associated with micro brown schorl in muscovite variety “sericite.” Orange-red to red crystals, showing a combination of the dodecahedron and the trapezohedron (according to Hitchen, 1935), can be up to 2cm in diameter. The smaller ones are frequently transparent and gemmy. Hitchen (1935) reported an analysis showing a large spessartine component to the pegmatite garnet, and SEM-EDS (this study) of an orange-red crystal also shows high Mn content though still subordinate to Fe. (See also the entry for spessartine.) “Garnet” was also reported by Dale (1910, 1923), Lewis (1927) and Hurlbut (1964); “red garnet” by Emerson (1917); “almandine” or “almandite” by Bjareby (1962) and Gleba (1978).

Almandine 1.5mm crystal Almandine (pink) and Schorl (brown) FOV=1mm Arsenopyrite FeAsS Sulfides are uncommon on Rollstone Hill; Hitchen (1935) reported that arsenopyrite is the most common.

Arsenopyrite 3mm crystal Arsenopyrite 1.5mm crystal Autunite Ca(UO2)2(PO4)2·11H2O Autunite was reported by Grauch & Zarinski (1976). Not observed in the present study. Beryl Be3Al2(Si6O18) Gem beryl has been reported from Rollstone Hill, mostly the golden variety. The earliest report was by Hamlin (1870) who stated: “At the quarries at Fitchburg, beryl of a rich golden hue, approaching the chrysoberyl and topaz in color and hardness, and closely resembling the yellow diamond in lustre, have been blasted out.” Yellow or golden beryl was again reported by Hamlin (1873) and also by Burbank (1879), Schaller (1918), and McCaskey (1919). Aquamarine was reported by Emerson (1917). Hitchen (1935) noted that beryl colors range from pale green to deep golden yellow. Most crystals are non-gem quality and occur in the “Type III” beryl pegmatite. Some crystals are offset and “healed” by quartz. The “Type III” pegmatite was observed in place (this study), and in nearby quarried boulders where very pale green beryl was observed frozen in matrix. Beryl is infrequently found now that the quarry has been inactive for so long. Nevertheless, mineral collector Mike Shih of the Boston Mineral Club discovered a new occurrence of colorless to pale green crystals in a small boulder in 2010 (photo, below right).

Beryl 26.5mm crystal (Harvard Mineralogical Museum) Beryl 4mm “goshenite” crystal

Beryl 72mm crystal, collected in 1915 (Harvard Mineralogical Museum) “Biotite” is a general name for a dark mineral of the Mica Group. Biotite occurs in “Type I” pegmatites in plates up to an inch in diameter (Billings, 1941), where it is associated with microcline, albite, muscovite and minor magnetite. It also occurs with muscovite in granite. Hitchen (1935) reported refractive indices which suggested that this black mica may be the mineral siderophyllite, KFe2+2Al(Al2Si2)O10(F,OH)2. Biotite was also reported by Dale (1910, 1923), Emerson (1917), Kitson (1938), Billings (1941) and Hurlbut (1964).

Biotite FOV=10mm Calcite CaCO3

Massive calcite occurs rarely in veins, enclosing microcrystals of quartz, muscovite, albite, chamosite, titanite, ilmenite and fluorite. Calcite veins were described by Hitchen (1935). Chalcopyrite CuFeS2 Hitchen (1935) noted chalcopyrite crystals associated with pyrrhotite.

Chalcopyrite FOV ~ 0.75mm Chamosite (Fe2+, Mg)5Al(AlSi3O10) (OH) 8 Chlorite Group. Chamosite is dark green and is found with ilmenite and titanite in calcite veins and as an alteration of biotite in “Type II” and “Type IV” pegmatites. “Chlorite” was reported by Dale (1910, 1923) and Hitchen (1935). “Prochlorite” was listed by Seaman (1976). SEM-EDS analysis (this study) of a dark green chlorite mineral from a calcite vein proved to be chamosite.

Chamosite (green) with Ilmenite FOV~ 2mm Chamosite on Muscovite FOV~ 2mm

Chamosite (green); Titanite (pink-tan); Quartz (colorless) FOV~ 2.5mm Columbite(?) Columbite was listed by Kitson (1938), Billings (1941), Grauch & Zarinski (1976) and Gleba (1978). No specimen or analysis known; may be misidentified ilmenite. Epidote Ca2Al2(Fe3+;Al)(SiO4)(Si2O7)O(OH) Epidote was reported by Dale (1910) as a secondary mineral in granite. Fluorapatite Ca5(PO4)3F Apatite was reported by Hitchen (1935) as a “very rare” mineral occurring in crystals up to 2.5 cm long in the “Type III” beryl pegmatite, and as an accessory in the “Type IV” veins. In the present study, it has also been found in the Type II pegmatites, where it is frequently light gray in color and can be difficult to distinguish in the field from quartz. The fluorapatite occurs in simple hexagonal crystals, light gray, colorless or very pale green or greenish yellow, and exhibits strong yellow fluorescence under SW ultraviolet

light. Apatite was also reported as an accessory in granite by Dale (1910, 1923) and Hitchen (1935).

Fluorapatite 9mm crystal Fluorapatite 2mm crystal Fluorite CaF2 occurs as colorless or purple cubic microcrystals in calcite veins (Hitchen, 1935).

Fluorite FOV=0.5mm Ilmenite Fe2+TiO3

Black crystals of ilmenite occur in “Type II” and “Type IV” pegmatites and in calcite veins, sometimes superficially weathering to a bluish color. Ilmenite was noted by Hitchen (1935), Kitson (1938), Bjareby (1962) and listed by Gleba (1978).

Ilmenite 1mm crystal on Titanite Ilmenite 1mm crystal in Calcite with Ilmenite-included Quartz crystal Magnetite Fe3O4

Magnetite was reported by Hitchen (1935) as microscopic grains associated with biotite in the “Type I” pegmatites. Marcasite FeS2 Marcasite was seen by Hitchen (1935) in polished section, occurring as concretionary masses resulting from supergene alteration of pyrrhotite. Microcline KAlSi3O8

Microcline occurs as a component of microperthite in granite and pegmatite. Crude crystals occur on the walls of “Type II” pegmatites, while more perfect crystals, mostly Carlsbad twins, up to 8 cm long occur embedded in massive quartz (Hitchen, 1935). Microcline was also reported by Dale (1910, 1923), Emerson (1917), Lewis (1927), Bjareby (1962) and Hurlbut (1964).

Microcline 20mm crystal Molybdenite MoS2 Molybdenite was reported by Hitchen (1935) as a rare mineral in fissures in the “Type IV” (beryl) pegmatite, associated with sericite and garnet.

Muscovite KAl2AlSi3O10(OH)2 Mica Group. Muscovite occurs in all pegmatites except “Type V” and in the “Type III” plates may be up to 4 cm across (Hitchen, 1935). It also occurs in granite and as perfect pseudo-hexagonal crystals in calcite veins; also found as the fine-grained variety “sericite” in seams, associated with micro garnet and schorl. Muscovite was noted by Emerson (1917), Dale (1923), Kitson (1938) and Hurlbut (1964). Very rarely a pink variety, so-called “rose muscovite,” occurs in feldspar in pegmatite. This color variety was first noted by Carl Francis (pers. comm., 2010) on a Harvard Mineralogical Museum specimen; subsequently it was found as a rounded 2mm fine-grained aggregate (this study). Heinrich and Levinson (1953) attributed the color of specimens of rose muscovite they studied from various localities to the absence of Fe2+ and the dominance of Mn over Fe3+. Though not a factor in the color, the examples they studied contained a small amount of lithium and occurred in replacement units in zoned pegmatites. The cause of the color in examples from Rollstone Hill remains to be resolved.

Rose Muscovite 5mm crystal (Harvard) Muscovite crystals and Chamosite (green), in Calcite FOV~ 3mm Opal (v. hyalite) SiO2·nH2O This mineral was noted (this study) as strongly fluorescent (green, SW) colorless coatings on feldspar. Orthoclase KAlSi3O8

Orthoclase was reported (as distinct from microcline) by Emerson (1917), Dale (1923) and Hurlbut (1964) as a component of granite. Pyrite FeS2 Surprisingly rare on Rollstone Hill, this mineral was only seen once in this study — as an approximately 0.25mm modified cubic crystal, associated with much smaller chalcopyrite crystals and radial “sixling” twins of arsenopyrite.

Pyrite ~ 0.25mm crystal Pyrite, Arsenopyrite, Chalcopyrite FOV~ 0.75mm Pyrrhotite Fe1-x S (x=0 to 0.17) Pyrrhotite was reported by Hitchen (1935) as the second most common sulfide, after arsenopyrite.

Pyrrhotite ~ 0.25mm crystal Quartz SiO2

Quartz is almost always massive except for the nearly colorless crystals which occur in calcite veins. The massive quartz of the pegmatites is generally a light smoky color. (Dale, 1910, 1923; McCaskey, 1919; Hitchen, 1935; Bjareby, 1962)

Quartz ~ 1mm crystal Quartz in Calcite ~ 2mm crystal Rutile TiO2

Rutile was reported in granite as micro-needles in quartz, albite and microcline (Dale, 1910, 1923; Emerson, 1917).

Schorl Na(Fe2+3)Al6(Si6O18)(BO3) 3(OH) 3(OH) Tourmaline Group. Schorl occurs abundantly in “Type II” pegmatites, sometimes in fine black crystals up to several inches in length. It also occurs in the “Type III” beryl pegmatite and as an accessory in granite. Schorl, or black tourmaline, is the most collectible mineral at Rollstone Hill and is included in all prior lists of minerals from the locality. Hitchen (1935) described crystals that had been horizontally fractured and rehealed by quartz and feldspar, and crystals with quartz cores occurring along a vertical axis. He also noted schorl occurring as sooty aggregates of minute crystals; in graphic intergrowth with quartz (similar to some pegmatites in New Hampshire and Maine); and as groups of radiating crystals. A large and outstanding specimen of these radiating or "sunburst" schorl crystals is in the collection of the Harvard Mineralogical Museum. Schorl is also found as clusters of secondary translucent green-brown microcrystals on feldspar.

Schorl 11mm crystal

Schorl with Quartz core 12mm crystal Schorl, green-brown; FOV~ 1mm

Tourmaline “Type II” Pegmatite (US quarter coin for scale)

Siderophyllite (?) Mica Group. See entry for biotite. Spessartine (?) (Mn,Fe)3Al2Si3O12 Garnet Group. Spessartine has not been verified, but is strongly suspected to occur as rare yellow to orange crystals and zones within crystals (this study). Hitchen (1935) reported an analysis of carefully selected garnet samples which were found to contain about 42% spessartine component and 53% almandine component. A garnet with more than a 50% spessartine component would by definition be considered spessartine.

Garnet crystals with yellow to orange areas, probably Spessartine; FOV=3mm

Titanite CaTiSiO5 Titanite occurs in brown crystals up to 2 cm long in “Type IV” pegmatite veins (Hitchen, 1935); also noted as pinkish-tan crystalline aggregates associated with chamosite in calcite veins. Titanite (“sphene”) was mentioned as occurring in “good crystals” by Seaman (1976). Hitchen suggests that titanite occurs as a replacement of ilmenite, with which it is frequently associated.

Titanite 17mm crystal Titanite FOV~ 0.75mm Uraninite UO2

This mineral was reported by Lane (1933) who observed a 4mm sample, but was not seen by Hitchen (1935) or in the present study. Lane reported a uranium content of 50.2% in the analyzed portion of the sample. Zircon ZrSiO4

Hitchen (1935) reports zircon from “Type II” and “Type IV” pegmatites, and as an accessory in granite. Sub-millimeter brown crystals were observed in the present study, associated with fluorapatite, almandine and ilmenite.

Zircon ~0.75 mm crystal

Other Minerals Not Yet Identified UK-1 A bright yellow mineral was found associated with pyrrhotite and chalcopyrite, as an apparent alteration product. Possibly native sulfur or a yellow iron sulfate mineral.

UK-1 FOV~ 0.5mm UK-2 Off-white, radial crystal aggregates with a crude hexagonal outline, much smaller than 1mm, were found in association with allanite-(Ce). This mineral could be an alteration product of allanite-(Ce) such as the REE fluoro-carbonate, bastnäsite-(Ce).

UK-2 FOV~ 2mm UK-3 White spheres were found in a fluorapatite-rich pegmatite vein associated with fluorapatite and schorl; possibly a clay mineral or a secondary form of apatite.

UK-3 FOV~ 4mm Conclusion Though the quarries have been inactive for 70 years, Rollstone Hill continues to be a source of fine schorl crystals for the diligent collector. Continued investigation into its microminerals has the potential to extend the species list of the locality, and to be an important contribution to the mineralogy of Massachusetts and the New England region. REFERENCES - Astell, Emilie (1991): Putting the pieces together for a bit of Fitchburg history (Worcester Telegram & Gazette, NE edition, p. A-2. Dec. 16, 1991). - Billings, Marland P. (1941): Pegmatites of Massachusetts. (Prepared under a cooperative project for geologic investigations in the Commonwealth of Massachusetts). - Burbank, L. S. (1879): in Proceedings of the Boston Society of Natural History (American Naturalist, Vol. 13, 1879). - Carter, James G. and William H. Brooks (1830): A Geography of Massachusetts (Boston: Hillard, Gray, Little and Wilkens), p. 146. - Dale, T. Nelson (1910): Supplemental Notes on the Commercial Granites of Massachusetts (USGS Bulletin 470). - Dale, T. Nelson (1923): The Commercial Granites of New England (USGS Bulletin 738). - Dietrich, Richard V. (1985): The Tourmaline Group (NY: Van Nostrand Reinhold). - Emerson, B. K. (1917): Geology of Massachusetts and Rhode Island (USGS Bulletin 597). - Fitchburg Historical Society (2005): Fitchburg (Charleston SC: Arcadia Publishing). - Mass. Dept. of Conservation and Recreation (2006): Fitchburg Reconnaissance Report (http://www.mass.gov/dcr/stewardship/histland/reconReports/fitchburg.pdf). - Gaines, R. V., et. al. (1997): Dana’s New Mineralogy, 8th Edition, p. 1264. - Gleba, Peter (1978): Massachusetts Mineral and Fossil Localities. - Gosse, Ralph (1969): A Catalogue of Massachusetts Gemstones, Part V, Worcester County (Rocks & Minerals 44:436-437). - Grauch, R. I., and Zarinski, Katrin (1976): Generalized descriptions of uranium-bearing veins, pegmatites, and disseminations in non-sedimentary rocks, eastern United States. USGS Open-File Report OF 76-582. - Hamlin, A. C. (1870): The Gems of the United States in Proceedings of the American Association for the Advancement of Science, Eighteenth Meeting, p. 212.

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