Minerals From the Iron Deposits of New York State

May 14, 2008

By Lupulescu, Marian

The principal goal of this article is to present the impor- tant minerals from the New York State iron deposits. Not all the minerals identified in these deposits can be seen by the naked eye; some are found only in thin sections, and others are not aesthetically impressive, making them of limited collector interest. In general, these will not be noted here, except for those that represent very rare occurrences; they will be mentioned along with their localities, but with no detailed descriptions. To write an article about the iron deposits of New York State and their minerals is a difficult task. There are numerous old and interminable debates about their origin and tectonic setting that one must consider and incorporate before pre- senting one?s own hypotheses. There are also some excellent papers about some of the deposits, and I do not want to repeat what those authors have already described. Further, there is considerable information, sometimes conflicting, that has to be assembled in a logical and understandable way. And there are a lot of other topics to be discussed!

Here I will try to present reasonably concise information about the history, geology, and origin of the iron deposits and occurrences. Where a mineral or a group of minerals has been previously described, I will refer the reader to the original paper and only add my own observations as a supplement. Thus, the reader will find detailed descriptions of some of the mineral species that were not well documented until now and only short references for those minerals that were previously described in full. This is the case for the Tilley Foster mine (Putnam County), which was well described by Nightingale in a series of articles that appeared in Matrix in summer 2001, and the geology and mineralogy of the Sterling mine, Antwerp (Jefferson County), which were well documented by Robinson and Chamberlain (1984). In such cases, I will present only some photomicrographs of a few minerals from these old mines that may help complete the previous mineral descriptions.


Iron mining in New York State has a long history. There are two main regions where iron mining was developed: the Hud- son Highlands in the south and the Adirondack Mountains in the north (fig. 1). Smock (1889) completed the first report on the iron ores in New York State and the first classification based on a ?geologico- geographical arrangement.? His clas- sification included almost all the iron occurrences known at that time and all the major ore types (magnetic iron, hematite, limonite, and carbonates).

The first discovery of iron ore deposits in the Hudson Highlands dates back to around 1730 (Lenik 1996) or 1750 (Horton 1837). Since then, but especially from the middle of the eighteenth to the end of the nineteenth centuries, many iron occurrences were found and mined in the Hudson High- lands. Horton (1837) recorded the iron mines that operated at that time and made the first substantial contribution to the geology of the iron mines of the Hudson Highlands. He listed in his report the minerals identified not only from the iron mines but also from all other known occurrences in Orange County. During his time as the assistant geologist for the First District he collected and exhibited 2,888 specimens of minerals and rocks illustrating the mineralogy and geology of this county (Horton 1837).

Horton?s information about the discovery and production of the iron ore from the highlands came from Peter Townsend, Esq., ?one of the oldest iron masters of our country; he was born in the vicinity of an iron furnace, and has engaged in this business during a long life he cast the first cannon in this country.? The Sterling mine (fig. 2) was perhaps the most important deposit found in the highlands. It was discovered in 1750 (Horton 1837) and was named after Lord Sterling. Horton also mentioned that the first anchor made in New York was in 1773 at the Sterling forge, and the great chain of 186 tons, with each link weighing from 140 to 150 pounds, that extended across the Hudson River at West Point during the Revolutionary War was also made at Sterling, using ?equal parts? of iron ore mined from the Sterling and Long mines (Horton 1837).

Mining at the Tilly Foster mine (Putnam County) started in 1810 when James Townsend obtained the mining rights in the area (Nightingale 2001), and the first report that listed the Tilly Foster mine as a mineral locality was that of Bridenbaugh (1873).

The most significant regions with iron deposits in the Adirondack Mountains are in the northwestern area that includes the Antwerp- Keene hematite belt (Sterling mine), extending from Jefferson County into St. Lawrence County, and the Jayville and Clifton mines (St. Lawrence County); the central region with the Benson mines (St. Lawrence County) and the Tahawus deposit (Essex County); and the northeastern region with the Mineville?Port Henry group of mines (Essex County) and the Ausable and Lyon Mountain mines (Clinton County). The earliest discoveries and mining operations of iron ores in the Adirondack Mountains were in the eastern part (Linney 1943).

The northeastern part of New York State was a significant supplier of large quantities of iron ore used in developing the resources and industries of the United States through the first part of the nineteeth century. The iron ore was mined on the eastern side of the Adirondack Mountains between Lake Champlain and the Adirondacks as early as 1804 (Birkinbine 1890). At the end of the nineteenth century, three main areas were mined for iron in this region: (1) Mineville?Port Henry, (2) the Chateaugay mine at Lyon Mountain where the iron ore was known as early as 1850 (Gallagher 1937), and (3) the Ausable district (fig. 1). The most prolific producers were the mines in the Mineville area, with a reported total production of 9,530,000 tons by 1889 (Birkinbine 1890), but the oldest producer was the Ausable district where iron ore was discovered in 1806 (Postel 1952). The first record of mining in Mineville?Port Henry was in 1775 at a location that later would be called the Cheever mine (Farrell 1996). The Mineville iron mining district had a sinuous pattern of boom and decline in its more than 150-year mining history, during which the various mines were repeatedly opened and shut down following the historical events, economic changes, and ownership interests in the country and in the world. The Mineville mines were mainly important and continuous producers of iron, but they were also known for their apatite byproduct and for the beauty and specific forms of the magnetite crystals that can be seen today in many museums.

The Benson iron deposit in the central region of the Adirondack Mountains was discovered around 1810 when engineers working on a military road between Albany (Albany County) and Ogdensburg (St. Lawrence County) were surprised by the magnetic effect of the iron ore on their compasses; systematic mining began in 1907 as open-pit work (Crump and Beutner 1970). The Tahawus (Sanford Lake) deposit was discovered in 1826 when an Indian directed a team of prospectors into the region (Gross 1970).

Mining operations commenced in the northwestern region in the Antwerp-Keene hematite belt as early as 1812 and continued until 1880, after which only small-scale workings were developed (Robinson and Chamberlain 1984). Beck (1842) and Emmons (1842) also mentioned some mineral specimens from this group of mines.

The Jayville iron deposit was opened for mining in 1854 and operated until 1888, when it was abandoned because of the discovery of the Little River (now Benson mines) deposit. The Clifton mine deposit was known before 1840, but mining operations began in 1865, ceased in 1870, re-opened in 1941, and were finally shut down in 1952 (Leonard and Buddington 1964).

Regional Geology

The rocks that make up the basement of New York State are metamorphic rocks formed during the Grenville Orogenic Cycle, 1.3?1.0 billion years ago, when continental plates collided with proto?North America and generated mountains. The Grenville rocks in New York are buried under younger units but crop out in the Adirondack Mountains and Hudson Highlands.

The Hudson Highlands region of New York is an uplifted area of metamorphic rocks with a northeast-southwest structural trend. Together with the New Jersey Highlands and Pennsylvania?s Reading Hill they form a geological province called the Reading Prong that connects the Blue Ridge (Virginia) and the Green Mountains (Vermont) as part of the Appalachians (Gates et al. 2004). The Hudson Highlands are divided into three major regions that are separated by old faults. The rocks from the central and eastern areas?initially sedimentary rocks deposited in a shallow sea environment (shales, limestones, and sandstones), igneous rocks (gabbro and granite), and volcanic rocks?were metamorphosed during the Grenville Orogenic Cycle and host iron deposits. Limestones, sandstones, and volcanics were the pre-metamorphic rocks of the western region. After metamorphism, the limestones became what we know today as the Franklin Marble, host for the wonderful suite of minerals from the Franklin and Sterling Hill mines (New Jersey) and from Edenville, Amity, and other famous localities in Orange County, New York.

The Adirondack Mountain region is morphologically and geologically divided into the Adirondack Highlands and Adirondack Lowlands; the two areas are separated by a wide zone of deformation called the Carthage-Colton Mylonite Zone. Rocks of igneous origin, such as anorthosites, gabbros, and granites, form the Adirondack Highlands. These rocks were metamorphosed to granulite facies between 1,090 to 1,050 Ma during the Ottawan Orogeny (McLelland et al. 1988) of the Grenville Orogenic Cycle; the peak metamorphic conditions occurred at depths of around 25 kilometers and 750?C (Bohlen, Valley, and Essene 1985). The Lyon Mountain Gneiss of 1,070 to 1,050 Ma age (McLelland et al. 1988) envelopes the Adirondack Highlands in the northeast and hosts the most important iron deposits in the eastern and northeastern part of the Adirondack Mountains. The southern and southwestern regions of the Adirondack Mountains contain sequences of metasedimentary rocks. The lowlands comprise sequences of marine sediments, volcanics, and igneous rocks that were metamorphosed to upper amphibolite facies, at temperature of 600??650?C and pressure of 6?7 kilobars (Bohlen, Valley, and Essene 1985). The marbles from the lowlands host a varied and beautiful suite of minerals that have attracted the attention of mineral collectors since their discovery.

Classification of the Iron Deposits

A detailed classification of the iron deposits of New York is difficult to prepare, and although not a principal issue of this article, it is necessary to bring to the reader?s attention the character of the deposits and thereby possibly assist in where to look for specific minerals. Based on criteria such as the main commodity of a deposit, mineralogical composition, and host rock, the iron deposits from New York State are as follows:

1. Gneiss-hosted low titanium?iron oxide deposits: these include (A) Adirondack Mountains: Mineville mining district and Skiff Mountain mine (Essex County), Ausable and Lyon Mountain group of mines (Clinton County), Benson mines (St. Lawrence County); and (B) Hudson Highlands: Hogencamp, Hogencamp, Pine Swamp, Greenwood, Boston, Bradley, Redback, Daters, Surebridge, Clove (Wilks), Sterling, Standish, and O?Neil mines (all in the Orange County) and Phillips mine (Putnam County).

2. Anorthosite- and gabbro-hosted high titanium?iron oxide deposits: Tahawus, Split Rock, and Craig Harbor mines (Essex County).

3. Skarn-hosted magnetite and vonsenite (Jayville and Clifton mines, both in St. Lawrence County) and iron oxide (Tilly Foster mine, Putnam County) deposits.

4. Sedimentary-hosted iron deposits (Dutchess and Columbia counties and Clinton-type deposits).

5. Weathering crust-hosted iron oxide and hydrated oxide deposits: Staten Island (Richmond County), Antwerp-Keene belt (Jefferson and St. Lawrence counties), and Chub Lake, Dodge mine, and other similar deposits from St. Lawrence County.

This article focusses mainly on types 1, 2, 3, and 5, because they have provided many beautiful and interesting minerals for more than 150 years.

Mineral Descriptions

Apophyllite-group, KCaO^sub 4^Si^sub 8^O^sub 22^(OH,F)?8H^sub 2^O, has been reported only from the Tilly Foster mine (Putnam County) (Trainer 1938, 1942) as millimeter-sized transparent crystals in veinlets (fig. 3).

Aragonite, CaCO^sub 3^, was reported at the Tilly Foster mine (Putnam County) (Trainer 1938) as gray to white crusts and radiating aggregates; at the Sterling mine, Antwerp (Robinson and Chamberlain 1984), as white crystals to 1 mm in length; and at the Kearney mine (St. Lawrence County) (Durant and Pierce 1878). I identified aragonite on one of the magnetite specimens in the Philadelphia Academy of Science collection, from the O?Neil mine (Orange County). Here, it occurs as ?cylinders? of fine white prisms to 1 mm on magnetite (fig. 4) and ?jenkinsite? (Fe-rich antigorite).

Barite, BaSO^sub 4^, was mentioned at the Antwerp-Keene belt (Jefferson County) (Hough 1853), and fine specimens have been preserved from the Caledonia mine in this district; it occurs also as veins at Tahawus (Essex County). The most beautiful crystals were found at the Chub Lake prospect (St. Lawrence County), where golden- yellow terminated prisms (fig. 5) to 3 cm in length were collected.

Brucite, Mg(OH)^sub 2^, reported first by Dana (1874) at the Tilly Foster mine, forms pale green to white foliated masses or transparent prismatic crystals (fig. 6).

Calcite, CaCO^sub 3^, occurred as beautiful specimens in the Sterling mine, Antwerp (Jefferson County); Lyon Mountain and Arnold Hill mines (both in Clinton County); and the Mineville mining district (Essex County). Their crystallography was described in detail by Whitlock (1910).

Calcite crystals from the Sterling mine were found in vugs in the hematitic iron ore. Robinson and Chamberlain (1984) described three generations of calcite, including pseudomorphs of stilpnomelane after scalenohedral calcite; translucent, milky rhombohedra; and crystals with a ?nailhead? habit. Whitlock (1910) recognized three types of calcite corresponding to three generations. The first generation contains rhombohedral crystals with lateral edges beveled by scalenohedral planes, and noted basal pinacoids {0001} on some crystals. The crystals of the second generation occur in association with quartz and form parallel aggregates. The calcites belonging to the third generation were described as crystals to 5 mm forming compact aggregates.

Calcite crystals (fig. 7) from the Lyon Mountain mines occurred as 3?25-mm scalenohedra in association with quartz and amphibole, as milky crystals, as crystals embedded in ?byssolite,? as yellow crystals, and as brilliant clear calcite (Whitlock 1910). Few calcite specimens associated with pyrite forming small veins in the host gneisses and the red jasper layers were collected at the Arnold Hill mine (Whitlock 1910). Scalenohedral calcite crystals to 8 mm in size were collected from the Cook shaft, Mineville mining district (Whitlock 1910). Rare scalenohedral and rhombohedral calcite crystals were reported from the Tilly Foster mine (Putnam County) (Nightingale 2001); some crystals are fluorescent (Trainer 1938, 1941, 1942).

Carbonate-fluorapatite, Ca^sub 5^(PO^sub 4^,CO^sub 3^)3^sub 3^F, was found at the Sterling mine (Jefferson County) as small, pink, short hexagonal prisms (fig. 8) terminated by basal pinacoids (Robinson and Chamberlain 1984).

Chevkinite-(Ce), (Ce,La,Ca)^sub 4^(Fe^sup 2+^,Mg)^sub 2^(Ti,Fe^sup 3+^)^sub 3^Si^sup 4^O^sub 22^, was found at the Clove (Wilks) mine (Orange County) (Lupulescu and Hawkins 2003) in a pegmatite dike cutting the iron ore. The dark brown mineral occurs as rounded equidimensional or elongated patches to 1 cm. It has a white rim of cryptocrystalline silica, unidentified titanium oxide, and clay minerals and is associated with oligoclase, amphibole, zircon, quartz, and pyrite.

Chondrodite, (Mg,Fe^sup 2+^)^sub 5^(SiO^sub 4^)^sub 2^(F,OH)^sub 2^, is one of the most spectacular minerals from the New York iron deposits. The Tilly Foster mine is the site from which some of the finest chondrodite crystals known (Trainer 1938) were collected (Cook 2007). Bridenbaugh (1873) recognized the chondrodite from this location as ?the most interesting, with respect to both occurrence and composition?; Dana (1875) described this occurrence of chondrodite and in 1876 its optical properties. Over time, chondrodite from the Tilly Foster mine has been the subject of such scientific studies as the high-pressure single-crystal X-ray and powder neutron diffraction (Friedrich et al. 2002) and high- temperature single-crystal neutron diffraction studies (Kunz et al. 2006). The mineral occurs in dark red, dark cinnamon-red (fig. 9), amber-brown, grayish-brown (Jensen 1978), or yellow crystals, mostly associated with clinochlore, magnetite, calcite, and serpentine. I strongly recommend to the reader the 2001 issue of Matrix in which Stephen Nightingale presents the detailed history and mineralogy of the Tilly Foster mine. Gillson (1926) reported another occurrence of small, dark red crystals of chondrodite associated with magnetite at the Mahopac mine (Putnam County).

Clinochlore, (Mg,Fe^sup 2+^)^sub 5^Al(Si^sub 3^Al)O^sub 10^(OH)^sub 8^, has been found in beautiful crystals only at the Tilly Foster mine (Putnam County) in association with chondrodite, magnetite, serpentine, and calcite. It occurs as scaly green aggregates, as crystals to 2.5 cm (fig. 10), and as dark emerald- green pseudohexagonal plates (fig. 11), some of which are 12 cm across. Often the clinochlore plates stand on their edges on the matrix.

Chloro-potassichastingsite, KCa^sub 2^(Fe^sup 2^^sub 4^+Fe^sup 3+^)Si^sub 6^Al^sub 2^O^sub 22^Cl^sub 2^, was identified by me at the O?Neil mine (Orange County) in association with diopside and clinoenstatite.

Datolite, CaBSiO^sub 4^(OH), has been found only at the Tilly Foster mine (Trainer 1938; Januzzi 1966), where it occurs as millimeter-sized crystals (fig. 12).

Diopside, CaMgSi^sub 2^O^sub 6^, is common in almost all of the igneous or metamorphic rock-hosted iron deposits of New York together with enstatite (Mg^sub 2^Si^sub 2^O^sub 6^), hedenbergite (CaFe^sup 2+^Si^sub 2^O^sub 6^), and ferrosilite [(Fe^sup 2+^Mg)^sub 2^Si^sub 2^O^sub 6^], but good crystals are known only from the Tilley Foster mine (Putnam County) (fig. 13). The biggest fragment of a diopside crystal (20 cm in length) was found at the Hogencamp mine (Orange County) in the Hudson Highlands (fig. 14). Here, diopside occurs regularly as grains to 1 cm embedded in marble or magnetite.

Dravite, NaMg^sub 3^Al^sub 6^Si^sub 6^O18(BO^sub 3^)3(OH)^sub 3^(OH), was found at the Tilly Foster mine (Putnam County) as small, prismatic to acicular crystals in matrix. The old literature (Whitlock 1903; Manchester 1931; Trainer 1938; Januzzi 1966) described the species as schorl. I performed electron microprobe analyses on New York State Museum specimens 12333 and 12337 from the Trainer collection and concluded the species is dravite. Edenite, NaCa^sub 2^Mg^sub 5^Si^sub 7^AlO^sub 22^(OH)^sub 2^, occurs at the Lewis iron mine (Orange County), the Mineville mining district (Essex County), and the Phillips iron mine (Putnam County) (Lupulescu 2008).

Feldspar-group minerals, NaAlSi^sub 3^O^sub 8^ – CaAl^sub 2^Si^sub 2^O^sub 8^, occur at many iron mines from New York State, but not all the locations have provided good specimens for mineral collectors and museums. The ?sunstone? from the Benson mines (Essex County) and Palmer Hill?Arnold Hill mines (Clinton County) offered very good gemlike specimens (Robinson and Chamberlain 2007). Some spectacular specimens of labradorite and labradorite-andesine were available in the large blocks of anorthosite on the mine dumps at Tahawus (Essex County) in the 1960s. Dr. George Robinson personally collected one (in the Canadian Museum of Nature?s collection now) about ?8 inches across that was as good as any material I?ve seen from Labrador or Madagascar? (pers. com., 2007).

Ferro-actinolite, Ca^sub 2^Fe^sub 5^^sup 2+^Si^sub 8^O^sub 22^(OH)^sub 2^, was found as pale blue fibrous aggregates at the Mineville mining district (Essex County) and the Tilly Foster mine (Putnam County) (Lupulescu 2008).

Ferropyrosmalite, (Fe^sup 2+^Mn^sup 2+^)^sub 8^Si^sub 6^O^sub 15^(OH,Cl)^sub 10^, was identified in thin section and confirmed by electron microprobe analysis in specimens from the Daters mine (Rockland County) in association with magnetite and actinolite (Lupulescu and Gates 2006).

Fluorapatite, Ca^sub 5^(PO^sub 4^)^sub 3^F, was found as notable occurrences at the Phillips (Putnam County) and Sterling (Orange County) mines, both in the Hudson Highlands; at the Lyon Mountain mines, Arnold Hill, Rutgers, and LaVake mines (all in Clinton County); and at the Mineville mining district (Essex County). Fluorapatite occurs at the Phillips mine as inclusions in the pyrrhotite ore; at the Sterling mine it appears in the quartz and feldspar sequences that alternate with the magnetite ore. Large, pale to dark green or brown to dark green crystals (fig. 15), to 10 cm in length and 3 cm across and associated with magnetite, microcline, and quartz, occur at the Lyon Mountain mines. A geologically significant occurrence is at Mineville, where fluorapatite locally forms quantitatively almost 50 percent of the ore in some magnetite bodies. It was found mostly in the ore from the ?Old Bed? and has unusually high thorium concentration, to 3,000 parts per million, and total REE concentrations sometimes exceeding 10 weight percent. It appears as red, brown, or yellow, small (1?3- mm), hexagonal prisms embedded in magnetite. The red and/or brown color is due to infiltrations or inclusions of hematite along the fractures or within the crystal. McKeown and Klemic (1956) statistically analyzed the fractures in apatite and found two different sets, one subparallel to {0001}, and a second one oblique to the fracture plane developed almost on {0001}. They found that some of the apatite grains display a very narrow rim of 0.05 mm of a reddish-brown aggregate of monazite, bastn?site, and hematite. I examined apatite grains in thin/polished sections under polarizing and scanning electron microscopes and by electron microprobe and found that most of the fluorapatite grains have only inclusions of hematite, not other mineral phases, and only some of them are in contact with quartz; most also show a partial leaching of the REE and a new generation of tiny grains of secondary monazite-(Ce), allanite-(Ce), and thorite. Parts of the ?Old Bed? ore are composed only of magnetite and fluorapatite in variable proportions. The fluorapatite from the Cheever mine dump is green or white and is associated with magnetite and quartz.

Fluorite, CaF^sub 2^, was found at the Tilly Foster mine (Putnam County), the Arnold Hill and Lyon Mountain mines (Clinton County), and the Benson and Jayville mines (St. Lawrence County). Bridenbaugh (1873) mentioned the first occurrence of fluorite at the Tilly Foster mine. Here, fluorite occurs as small white, yellow (fig. 16), and purple crystals or as small spherical aggregates of purple to pale purple crystals (fig. 17). Veinlets and disseminated crystals of purple fluorite with microcline and quartz occurred in a pegmatite- like segregation at the Lyon Mountain mines. Pale green to blue fluorite crystals associated with calcite, quartz, and pyrite or vugs with dark to pale purple fluorite with calcite and quartz were found at the Arnold Hill mine in the magnetite- hematite ore. Purple or pale green fluorite with calcite or botryoidal goethite is known from the Jayville deposit. By far, the most notable occurrence of fluorite in New York?s iron deposits, however, is at Benson mines. Here, pale to dark green or yellow to yellow- brown fluorite specimens to 4 cm across, occasionally in parallel growths exceeding 20 cm, were found associated with pyrite cubes.

Two generations of fluorite can be recognized in the iron mines from New York, based on the textural relationship between fluorite and the magnetite ore. The first generation, exemplified at the Jayville mine, is synchronous with the magnetite ore; the occurrence of fluorite in veinlets and vugs at the other mines is evidence for a late hydrothermal event.

Fluoro-edenite, NaCa^sub 2^Mg^sub 5^Si^sub 7^AlO^sub 22^F^sub 2^, occurs as the product of a late metamorphic-hydrothermal event in the magnetite ore at the Rutgers mine (Essex County) and Lyon Mountain mines (Clinton County) (Lupulescu 2008).

Fluorotremolite, Ca^sub 2^Mg^sub 5^Si^sub 8^O^sub 22^F^sub 2^, was found at the Redback mine (Orange County), where it forms clusters of pale green, short prismatic crystals associated with olivine and pyroxene (Lupulescu 2008).

Goethite, FeO(OH), is found as a weathering product in all the iron deposits from New York, replacing magnetite, olivine, and other iron-rich minerals. A spectacular occurrence is at the Sterling mine, Antwerp (Jefferson County), where sprays of acicular crystals (fig. 18) are on quartz and siderite or form ?botryoidal coatings on hematite? (Robinson and Chamberlain 1984).

Descriptions of the other minerals from the Sterling mine such as dolomite [CaMg(CO^sub 3^)^sub 2^]; siderite (fig. 19) (FeCO^sub 3^); talc (fig. 21) [Mg^sub 3^Si^sub 4^O^sub 10^(OH)^sub 2^]; and stilpnomelane (fig. 20) [K(Fe^sup 2+^ MgFe^sup 3+^)^sub 8^(SiAl)^sub 12^(O,OH)^sub 27^], and their specific chemical and morphological features can be found in the same reference. Goethite also forms stalactites (fig. 22) and compact masses with fibrous texture as weathering products in the Taconic iron occurrences of Dutchess County.

Hematite, Fe^sub 2^O^sub 3^, is common in almost all of the investigated iron deposits where it replaces magnetite, but the most important occurrences are at the Sterling mine, Antwerp (Jefferson County), where it occurs in both ?specular and botryoidal forms? (Robinson and Chamberlain 1984), and Chub Lake (St. Lawrence County), where beautiful aggregates of specular hematite were found (fig. 23). There are also some other locations in St. Lawrence County where attractive specimens of specular hematite were collected (e.g., Toothaker Creek prospect and others).

Ilmenite, FeTiO^sub 3^, occurred dominantly in the magnetite- ilmenite deposits from Tahawus, Split Rock, and Craig Harbor mines (Essex County) and as an accessory mineral in some iron mines from the Hudson Highlands. Ilmenite from Tahawus is partially replaced by rutile (TiO^sub 2^). It forms tabular crystals at the Tilly Foster mine (Putnam County) (Trainer 1938) and short rusty prisms to 2 cm and bladed crystals at the Benson mines (Essex County).

Isokite, CaMg(PO^sub 4^)F, occurs as short prismatic plates and radial sprays within a 2-cm-wide vein cutting wagnerite at Benson mines (Jaffe, Hall, and Evans 1992).

Ilvaite, CaFe^sub 2^^sup 2+^Fe^sup 3+^OSi^sub 2^O^sub 7^(OH), was identified by me, using the electron microprobe on a specimen from the Tilly Foster mine that is labeled as amphibole in the New York State Museum mineral collection. It is a 5.5-cm vertical black and rusty orthorhombic prism on top of a horizontal group of three smaller prisms (fig. 24).

Magnesiohastingsite, NaCa^sub 2^(Mg^sub 4^Fe^sup 3+^)Si^sub 6^Al^sub 2^O^sub 22^(OH)^sub 2^, occurs at the Wilks, Pine Swamp, Redback, and Boston mines (all in Orange County) and at the Hasenclever mine (Rockland County) (Lupulescu 2008).

Magnesiohornblende, Ca^sub 2^[Mg^sub 4^(AlFe^sup 3+^)]Si^sub 7^AlO^sub 22^(OH), was identified at the Phillips mine (Putnam County) (Lupulescu 2008).

Magnetite, Fe^sub 3^O^sub 4^, has different forms and habits in the iron deposits of New York State. The magnetite from Mineville (Essex County) seems to be by far the most spectacular. Beck (1842) reported: ?The finest crystal of magnetic oxide of iron which I have seen from this State, is in the cabinet of the Albany Institute, and was presented to it many years since by Teunis Van Vechten, Esq. of Albany. It is said to have been found in Essex County.? He described the magnetite crystal as ?cuneiform octahedron but with some edges truncated? and 1.5 inches in length. Remarkably unique specimens were found at the Lover?s Hole pit, Barton Hill mines in 1887 and 1888 (Jensen 1978). The principal crystallographic form of the crystals collected from this location was the octahedron (fig. 25). Birkinbine (1890) reported that Professor Koenig from the University of Pennsylvania described combinations of the octahedron with the rhombic dodecahedron, pentagonal dodecahedron, cube, and icositetrahedron. Not common and very spectacular are the distorted octahedra displaying ?rhombohedral? appearance (fig. 26). The thin stilpnomelane ?layers? covering parting planes (pseudocleavages) make spectacular and unique specimens. White (1979) considered that the exsolution lamellae of ilmenite in magnetite caused the octahedral parting. One of the most spectacular of the magnetite crystals collected here was called the ?Big Diamond? (Farrell 1996). It consists of a ?perfect octahedron with faces of over one inch, resting loosely in the socket? (Birkinbine 1890). It is now in the collection of the A. E. Seaman Mineral Museum, at Michigan Technological University, Houghton, Michigan. Kemp (1890) reported magnetite crystals displaying combinations of the octahedron and rhombic dodecahedra with striations parallel to the octahedral faces (fig. 27); he interpreted them as pressure-generated pseudocleavages. Gallagher (1937) reported crystals of magnetite, to 10 mm, in the miarolitic cavities from the Lyon Mountain mines (Clinton County). He mentioned that larger crystals were found here and ?one of them was as large as a baseball,? but regrettably they were lost.

Landis (1900) described the occurrence of magnetite at the Tilly Foster mine (Putnam County). Magnetite from this location occurs in two different forms: as rhombic dodecahedra (fig. 28) and as rounded crystals (fig. 29). It is mostly associated with clinochlore, chondrodite, serpentine, or, in a few specimens, with calcite and quartz. A careful examination of the larger (to 1.5-cm) rhombic dodecahedra shows the intimate association of smaller (submillimeter to millimeter-sized) rhombic dodecahedra in parallel growth as ?building blocks? for the large ones. Many crystals have pseudostriations parallel to the long axes of the rhombic faces due to multiple parallel growths of octahedral faces. Dana (1874) reported magnetite pseudomorphs after dolomite and chondrodite at this locality.

Beck (1842) wrote that the O?Neil mine (Orange County) ?has long been known as one of the most interesting localities of the crystallized variety of the magnetic iron ore.? He reported that the most common form is the octahedron, but rhombic dodecahedron, cube with octahedron, and rare cube (fig. 30) were also found. Dr. Horton (in Beck 1842) further mentioned the ?cube with the edges truncated.? Clusters of tiny octahedra in parallel growth (fig. 31) are common at this site.

Magnetite occurs at the Sterling mine, Antwerp (Jefferson County), as pseudomorphs after hematite crystals in aggregates or isolated bladed crystals (Robinson and Chamberlain 1984).

Marialite, 3NaAlSi^sub 3^?NaCl, was identified from the Barton Hill mine, Mineville mining district (Essex County), as large greenish crystals associated with magnetite, titanite, and zircon, and from the Hogencamp mine in the Hudson Highlands (Lupulescu and Gates 2006).

Millerite, NiS, has been noted only from the Sterling mine (Jefferson County). Hough found the mineral in 1848 and described it later in Hough and Johnson (1850). It occurs in sprays of very fine acicular crystals (fig. 32) displaying striated prisms, curved crystals, and a spiral growth pattern (Robinson and Chamberlain 1984). Bancroft (1973) suggested that millerite from this location is recognized among the world?s finest mineral specimens.

Molybdenite, MoS^sub 2^, is uncommon in the iron deposits from New York; it occurs at the Jayville and Benson mines (St. Lawrence County), the O?Neil and Greenwood mines (Orange County), and the Tilly Foster mine (Putnam County). Spectacular crystals displaying basal pinacoids 5 cm across (fig. 33) were collected at Benson mines (St. Lawrence County), some of which are preserved in the collection of the Canadian Museum of Nature in Ottawa.

Olivine-group, (Mg,Fe)^sub 2^SiO^sub 4^. Minerals from this group are uncommon in the iron deposits from New York, but they were mentioned as pale green crystals at the Mahopac (Gillson 1926) and Tilly Foster mines (Colony 1923; Trainer 1936) (both in Putnam County) and at the Redback mine (Orange County) (Colony 1923). Small, prismatic, and terminated crystals displaying rounded edges and corners (fig. 34) with an intermediate composition bet-ween the two end-members, forsterite (Mg^sub 2^SiO^sub 4^) and fayalite (Fe^sub 2^SiO^sub 4^), occur at the O?Neil mine (Orange County). Here, Brush and Blake (1869) described it as ?hortonolite, a new member of the chrysolite group.? The mineral is covered and partially replaced by goethite and is associated with calcite and magnetite or magnetite and molybdenite.

Pecoraite, Ni^sub 3^Si^sub 2^O^sub 5^(OH)^sub 4^, occurs in vugs (fig. 35) in hematite ore at the Sterling mine (Jefferson County) as a pale yellow to bright green alteration product that partially or entirely replaces millerite sprays. Robinson and Chamberlain (1984) discussed in detail the identity of the mineral based on X-ray diffraction, SEM, and electron probe data.

Potassichastingsite, KCa^sub 2^(Fe^sup 2+^^sub 4^Fe^sup 3+^)Si^sub 6^Al^sub 2^O^sub 22^(OH)^sub 2^, was identified at the O?Neil mine (Orange County) (Lupulescu 2008).

Potassicpargasite, KCa^sub 2^(Mg^sub 4^Al)^sub 5^(Si^sub 6^Al^sub 2^)^sub 8^O^sub 22^(OH)^sub 2^, was first described from Pargas, Finland, by Robinson et al. (1997). I identified this mineral from Monroe and the Hogencamp and Surebridge mines (Orange County). It is associated with magnetite, spinel, pyrrhotite, chalcopyrite, marcasite, and diopside at the Hogencamp mine, with magnetite and diopside at the Surebridge mine, and with large spinel and orthopyroxene crystals at Monroe.

Prehnite, Ca^sub 2^Al^sub 2^Si^sub 3^O^sub 10^(OH)^sub 2^, was mentioned at the Tilly Foster mine (Putnam County) (Trainer 1938). It was also found at the Benson mines (St. Lawrence County) as lamellar crystals and as complex globules peppered with tiny cubes of pyrite at Tahawus (Essex County).

Pyrite, FeS^sub 2^, does not commonly form noteworthy crystals or associations of crystals in the iron deposits of New York. It occurs as small grains disseminated in the ore, as rosettes of octahedral crystals (Robinson and Chamberlain 1984) in vugs at the Sterling mine, Antwerp (Jefferson County), or as globular to spheroidal aggregates (fig. 36) at the Caledonia mine, Spragueville (St. Lawrence County). Small (to 1-mm) crystals are common for many other iron deposits from the Hudson Highlands and Adirondack Mountains.

Pyrrhotite, Fe^sub 1-x^S, was found at the Tilly Foster (Bridenbaugh 1873) and Phillips mines (both in Putnam County) and at the Nickel mine (Rockland County). At the Phillips mine pyrrhotite is the dominant metallic mineral, forming masses associated with apatite; the pyrrhotite from the Tilly Foster mine forms spectacular, small (to a few millimeters), tabular to hexagonal crystals (fig. 37).

Quartz, SiO^sub 2^, is common in many of the studied iron deposits, but perhaps the more interesting crystals have been found at the Sterling mine, Antwerp (Jefferson County), Lyon Mountain mines (Clinton County), Tilly Foster mine (Putnam County), and Chub Lake (fig. 38) and Benson mines (St. Lawrence County). Quartz at the Sterling mine displays a dipyramidal habit and occurs in vugs in the earthy hematite ore. In many situations it shows a ?window? effect (fig. 39), with inclusions or the minerals behind the crystal showing through the transparent quartz. At the Lyon Mountain mines, quartz forms groups of prismatic crystals up to a few centimeters. At Benson mines, quartz occurs as prismatic, smoky, or rusty crystals (to 10 cm), terminated with pyramids. Spectacular hexagonal dipyramidal crystals associated with hematite were collected at the Chub Lake prospect, Hermon ore bed, and Lowden mine (fig. 40) (St. Lawrence County). Red jasper can be found at the Arnold Hill mine (Clinton County).

Scheelite, CaWO^sub 4^, is rare in the iron deposits from New York. The original occurrence in the state is at the Tilley Foster mine as small (millimeter-sized) crystals. Scheelite was also collected in 2004 by Robert Ballard of the Capital District Mineral Club, Albany, from the Benson mines dump (St. Lawrence County). The mineral occurs as small white grains in quartz and muscovite and displays blue fluorescence in ultraviolet radiation.

Serpentine, Mg^sub 3^Si^sub 2^O^sub 5^(OH)^sub 4^, was described from the Tilly Foster mine by Dana (1874) and from the O?Neil mine in 1892 (Dana 1892). Spectacular serpentine polymorphs with different colors and habits were collected over the years from the Tilly Foster mine. Besides the known polymorphs (crysotile, lizardite, and antigorite), Aumento (1967) reported an ?unstable polymorph of the serpentine-group minerals? and described its structure. Dana (1874) wrote about the beautiful and diverse serpentine pseudomorphs after chondrodite (fig. 41), dolomite (fig. 43), clinochlore (fig. 42), and so forth. Nightingale (2001) published a comprehensive list of minerals and pseudomorphs accompanied by representative mineral photos from this well-known mineral locality.

Shepard (1852) mentioned a serpentine-group mineral under the name ?jenkinsite? at the O?Neil mine and considered it a new species. Later, the mineral was identified as Fe-rich antigorite.

Sillimanite, Al^sub 2^SiO^sub 5^, has a notable location at Benson mines (St. Lawrence County). Here, it occurs in the magnetite- bearing quartz-feldspar gneisses and in small pegmatite bodies that cut the iron ore; it forms sprays of acicular crystals and striated prisms to 15 cm (fig. 44) that are partially replaced by muscovite.

Spinel-group (other than magnetite). Hercynite (FeAl^sub 2^O^sub 4^), ulvospinel (TiFe^sub 2^O^sub 4^), spinel (MgAl^sub 2^O^sub 4^), and gahnite (ZnAl^sub 2^O^sub 4^) have been identified optically as exsolutions (expelled phase) from magnetite (Lupulescu and Gates 2006). Small octahedral crystals of spinel-group minerals were mentioned at the Tilly Foster mine (Trainer 1938).

Stillwellite-(Ce), (Ce,La,Ca)BSiO^sub 5^ (fig. 45), has a unique occurrence in New York at Mineville (Essex County). Mei et al. (1979) identified the mineral in a sample collected from the ?Old Bed? in the vicinity of a fault on the 2100-foot level, where it occurs as 1?2-mm-wide tabular crystals with waxy luster and pink to reddish color. It is associated with fluorapatite and magnetite. Titanite, CaTiSiO^sub 5^, was described as gem-quality yellow (fig. 46) and green crystals (Whitlock 1903; Manchester 1931; Trainer 1938; Januzzi 1966) at the Tilly Foster mine (Putnam County). Titanite occurs also at the Mineville mines as small, brown or brown- yellow crystals associated with magnetite and zircon or as compact masses in association with zircon, and at the Lyon Mountain mines as brown crystals, to 1 cm, with magnetite and microcline.

Uraninite, UO^sub 2^, occurs in the magnetite ore, host gneiss, and pegmatites at the Phillips mine (Putnam County) and at the Standish mine (Orange County) as subhedral, embayed, and rounded to spherical disseminated crystals 2?10 mm in size. Klemic et al. (1959) reported crystals to 2.5 cm at the Phillips mine. Working with Dr. Joe Pyle from Rensselaer Polytechnic Institute, I dated one uraninite grain from the Phillips mine at 880 5 Ma, using the U-Th- Pb method. I interpret this age as the timing of the crustal relaxation followed by invasion of deep-seated fluids in the aftermath of the Grenville Orogenic Cycle.

Vonsenite, Fe^sup 2+^Fe^sup 3+^BO^sub 5^. Leonard and Vlisides (1961) described vonsenite at the Jayville and mentioned it at the Clifton iron mines (both in St. Lawrence County), but, as the authors write, the history of its discovery is more complex. In 1947, Leonard found a metallic mineral with strikingly unusual optical properties in the ore from a drill core at the Jayville iron deposit and considered that it might be ilvaite. In 1950, Henderson, from Princeton University, made chemical tests and prepared the mineral for spectrographic analysis and X-ray diffraction. He realized that the mineral was not ilvaite but was unable to identify it. In 1951, Axelrod and Fletcher, from the U.S. Geological Survey, identified the mineral as vonsenite.

Vonsenite occurs as gray to black aggregates of stubby or long prismatic crystals, to 5.5 cm, with metallic luster; it is associated with magnetite, pargasite, phlogopite, fluorite, titanite, diopside, and allanite. Leonard and Vlisidis (1961) reported crystal forms {001}, {010}, {100}, {3.10.0}, {250}, {120}, and {320} for the vonsenite from Jayville.

Wagnerite, Mg^sub 2^(PO^sub 4^)F, was identified at the Benson mines (Jaffe, Hall, and Evans 1992) in compact, vitreous, red-brown lenticular masses in a pegmatite body cutting the iron ore. It is associated with isokite, fluorapatite, and hematite.

Zircon, ZrSiO^sub 4^, is common in many specimens collected from the Mineville and Lyon Mountain groups of mines. The zircon crystals from Mineville (fig. 47) occur as dark, usually metamict tetragonal prisms terminated with tetragonal pyramids. The edge between the prism and pyramid faces is slightly rounded on some crystals. Small (1?2-mm), pink, transparent zircon crystals occur associated with or within magnetite, scapolite, and titanite at Barton Hill mines. The zircon at the Lyon Mountain mines (Clinton County) forms long prismatic, terminated crystals to 3 cm associated with edenite and microcline. The zircon from the Palmer Hill mine (Essex County) is twinned.

Origin of the Gneiss-, Anorthosite-, and Gabbro-hosted Iron Deposits (Types 1 and 2)

The origin of most of the low titanium?iron oxide deposits in the Grenville rocks of New York (gneiss-hosted iron deposits) is still debated in the geological literature. Many hypotheses, with more or less strong arguments, have been discussed for a long time. Wendt (1885) and Ruttman (1887) proposed a sedimentary origin for the iron deposits in the Hudson Highlands. Later, Koeberlin (1909) and Ames (1918) put forward a magmatic hypothesis, and Colony (1923) considered that the iron ore formed by a magma that replaced the host rock. Holtz (1952) and Hagner, Collins, and Clemency (1963) developed the theory of a metasomatic replacement of the host rocks by igneous source or regional metamorphism-derived solutions. Later, Foose and McLelland (1995) elaborated on the hydrothermal hypothesis, and Gundersen (2004) proposed the formation of these deposits by the exhalations from a volcanic source.

The origin of iron deposits in the Adirondack Mountains have also been the subject of many hypotheses for a long time: replacement of the host rock by igneous-derived heated solutions (Kemp 1897; Alling 1925); emplacement of an iron-rich magma (Newland and Kemp 1908); segregation of iron ore from a silicate and metal-rich magma (Kemp and Ruedeman 1910); metamorphosed iron-rich sedimentary sequence (Nason 1922); and hydrothermal or metasomatic-hydrothermal origin (Buddington 1966; Baker and Buddington 1970; Foose and McLelland 1995). Some of these deposits closely resemble Kiruna-type deposits (Rakovan 2007), for which the genetic model is also debated.

By contrast, there is general agreement on the origin of the gabbro-hosted high titanium?iron oxide deposits; they formed through the process called silicate-oxide immiscibility. That means that two magmas, a silicate-rich and an oxide-rich one, that were miscible at high temperature became immiscible, and they separated into two bodies, the silicate host rock (gabbro) and the oxide body (the ore) when the temperature dropped.

As a general feature, the iron deposits were crosscut by small pegmatite bodies dated at 900 to 1,100 Ma (Isachsen 1963) in the Adirondacks and 990 Ma (Volkert, Zartman, and Moore 2005) in the Hudson Highlands.

All the Precambrian iron deposits from the Adirondack Mountains and Hudson Highlands display, at different scale, metamorphic and hydrothermal features that changed the texture and mineral composition and obscured their original character. At this time, all we can say is that there are multiple modes of emplacement for the iron-ore bodies from the Hudson Highlands and Adirondack Mountains.

Most of the minerals from the iron deposits that highlight private or institutional collections formed in the late stages of the geological evolution of the iron deposits at the end of the Grenville Orogenic Cycle, characterized by crustal relaxation, pegmatite intrusion, fracturing, and invasion by deep-derived fluids. Gates, Krol, and Valentino (2000) dated the amphiboles generated in this late event in the Hudson Highlands at 915 to 925 Ma; Lupulescu and Pyle (2004) determined the age of a uraninite grain from the Phillips mine (Putnam County) at around 880 Ma. Thus, the most productive geological range for mineral formation in the Precambrian iron deposits from New York seems to have been between 990 and 880 Ma; the minerals were the result of the favorable mineralogical, chemical, and fluid compositions, and tectonics.


I thank Drs. John Rakovan and George Robinson for their helpful comments that greatly improved the manuscript. I especially appreciate Dr. Steven Chamberlain?s useful comments and help with the ?dirty and boring? work of ?cleaning? the photos.


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Dr. Marian Lupulescu is curator of geology at the New York State Museum.


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