Arsenopyrite is an iron arsenic sulfide mineral and the major host for arsenic in the Franklin Marble. Few analyses exist; one of cobaltoan material was given by Palache (1935). Several semiquantitative analyses by [Dunn] indicate that local material is relatively close to end-member composition. The arsenopyrite associated with the nickel arsenides from Franklin has Fe 32.5, As 47.6, S 19.3, total = 99.4 wt. % (Oen et al., 1984).
Arsenopyrite is one of the two dominant, opaque arsenic minerals occurring locally; the other is loellingite. In general, arsenopyrite is found in the Franklin Marble, and loellingite is found in the orebodies. Arsenopyrite was first noted from Franklin by Nuttall (1822) and was mentioned in subsequent lists. Fine crystals of arsenopyrite from the Franklin Marble were found in a number of the Franklin quarries in 1905 and were described by Palache (1910, 1935) and Palache et al. (1930). He also described superb crystals from the realgar occurrence at Sterling Hill (1941b). The crystal structure of arsenopyrite was described using Franklin crystals by Buerger (1936a, 1936b).
Arsenopyrite occurs well-crystallized in euhedral crystals up to 22 mm in length, although most are much smaller. They are generally prismatic, highly lustrous, and of simple to complex habit.
The morphology has been described in great detail by Palache (1910, 1935, 1941b). Many crystals were illustrated by Dunn (1979c). Crystals are commonly isolated and have chevron-like striations, but intergrowths of crystals are known and are rarely arcuate. Arsenopyrite twins to form cruciform or star-shaped trillings (Palache et al., 1944); massive material is known as well. Arsenopyrite is silver-white when freshly broken, is opaque, has a metallic luster which is commonly dulled by exposure, and has two commonly indistinct cleavages. The density is 6.223 g/cm3.
The vast preponderance of local arsenopyrite occurs as crystals associated with calcite in the Franklin Marble. It has been found in the local quarries, notably the Fowler Quarry and the Franklin Iron Company Quarry, and within the ore deposits, the latter only sparsely. In the marble, it is associated with pyrrhotite, graphite, amphiboles, uvite, titanite, norbergite, and many other species.
Arsenopyrite was found in the Franklin orebody associated with the nickel arsenide assemblage from the Trotter Shaft (Oen et al., 1984) and was reported from the 900 level at Sterling Hill by Palache (1941b), associated with calcite, realgar, graphite, arsenic, pyrite, diopside, and zinkenite. Arsenopyrite also occurs in the margarite assemblage in the marble adjacent to the Sterling Hill orebody; the assemblage is described under margarite (Dunn and Frondel, 1990). An occurrence from the 1100 level at Sterling Hill was reported by Dunn (1979c) who found arsenopyrite associated with calcite, realgar, microcline, and clinopyroxene.
Additionally, massive arsenopyrite was found in the late 1970's associated with bornite and calcite on the 1680 level. It also was found on the 1300 level, and in the north orebody, and near both the hanging wall and footwall on the 1000 level, all at Sterling Hill.
Cook (1973) and Peters et al. (1983) reported arsenopyrite from the Buckwheat Dolomite, wherein it is a rare mineral. (Dunn, 1995)

 Location Found: Franklin and Ogdensburg
 Formula: FeAsS
 Essential Elements: Arsenic, Iron, Sulfur
 All Elements in Formula: Arsenic, Iron, Sulfur
 IMA Status: Valid - first described prior to 1959 (pre-IMA) - "Grandfathered"
 To find out more about this mineral at minDat's website, follow this link   Arsenopyrite

Dunn, Pete J. (1995). Franklin and Sterling Hill New Jersey: the world's most magnificent mineral deposits. Franklin, NJ.: The Franklin-Ogdensburg Mineralogical Society. p.526

Frondel, Clifford (1972). The minerals of Franklin and Sterling Hill, a checklist. NY.: John Willey & Sons. p.43

The Picking Table References
 PT Issue and PageDescription / Comment
View IssueV. 34, No. 2 - Fall 1993, pg. 14Recent Mineral Finds From The Sterling Mine Ogdensburg, New Jersey - Arsenic-Bearing Minerals
View IssueV. 32, No. 1 - Spring 1991, pg. 8An Uncommon Margarite/Corundum Assemblage From Sterling Hill, New Jersey, Pete J. Dunn, Clifford Frondel, Arsenopyrite (small description)
View IssueV. 27 No. 2 - Fall 1986, pg. 5Minerals of the Franklin Quarry, Philip P. Betancourt, Arsenopyrite
View IssueV. 24 No. 2 - Fall 1983, pg. 11Minerals of the Buckwheat Dolomite Franklin, New Jersey, Arsenopyrite (small description)
View IssueV. 20, No. 2 - September 1979, pg. 5Contributions to the Mineralogy of Franklin and Sterling Hill New Jersey, Arsenopyrite (small article)

Arsenopyrite on Franklin marble from Braen Franklin Quarry, Franklin, NJ
Arsenopyrite (metallic silver) on Franklin marble from Braen Franklin Quarry, Franklin, NJ. 4" x 2". Photo by WP.

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