Phlogopite is a potassium magnesium aluminum silicate fluorine hydroxide mineral of the mica group. Material from the Franklin Marble is highly fluorian; that from the orebodies has not been analyzed for F except for the analysis of Sandhaus (1981), who found a Sterling Hill specimen to be F-free. In general, specimens from the Franklin Marble are near end-member composition, whereas those from the orebodies are highly manganoan and zincian, grading into hendricksite. Little is known of the relative abundances of Mg and Zn in the micas of the orebodies. Frondel and Ito (1966a) and Frondel and Einaudi (1968) provided data for Mn- and Zn-bearing micas from Franklin and from Sterling Hill, respectively, but the limited number of analyses precludes general conclusions as to which species is dominant or more widespread. Much work remains to be done on the orebody micas, particularly with respect to specific assemblages, large divalent cations such as barium, and the fluorine and chlorine contents.
Phlogopite is known both from the Franklin Marble and the orebodies at Franklin and Sterling Hill. Phlogopite was first described from Franklin (as mica) by Silliman (1850); additional data were given by Hendricks and Jefferson (1939), and thermal studies were done by Yoder and Eugster (1954). Phlogopites were also investigated as part of the studies of Mn- and Zn-bearing micas by Frondel and Ito (1966a) and Frondel and Einaudi (1968). Franklin material was used for the crystal-structure studies of Hazen and Burnham (1973) and Hazen and Finger (1978), the latter at high pressures. X-ray diffraction data were given by the ICDD (PDF #34-159). The mica earlier referred to as manganophyllite (Chester, 1894) is possibly phlogopite in part.
The best crystal specimens come from the Franklin Marble and are the 1M polytype. The crystals are well-formed, sharp, prismatic and up to 18 cm in length, occurring as single isolated crystals, clusters of crystals, and isolated platelets. Crystals may be broken or distorted; breaks are commonly healed by calcite. The crystal size varies substantially, and some fibrous habits are known from the Franklin Quarry, perhaps as replacements of tremolite or other species.
Phlogopite from the marble is light green to yellow to mostly brown; that from the orebodies is generally darker brown. The luster on cleavage surfaces is pearly to vitreous, and the density is 2.86 g/cm3 for end-member material. Some local phlogopite has a moderate yellow fluorescence in shortwave ultraviolet (Bostwick, 1982), which is sometimes brighter in colorless crystals.
The prismatic to tabular brown crystals abundant in the Franklin Marble are phlogopite, and it is likely that much of the other mica there, occurring in small crystals and masses, is also phlogopite. Commonly associated minerals are calcite, spinel, scapolite, titanite, diopside, and many calcium silicates. It was described from the Franklin Quarry, in an anomalous occurrence, associated with muscovite and margarite and altering to chlorite, by Yau et al. (1984).
The altered material referred to locally as caswellite (Chester, 1894a, 1894b, 1896) is a replacement of mica from the Franklin orebody, perhaps phlogopite in part. It exists in all degrees of alteration; the end-product shows relict mica cleavages, has a pearly to satiny luster, and varies from brown to light brown to colorless and all intermediate hues. It is discussed further under grossular. (Dunn, 1995)

 Location Found: Ogdensburg
 Formula: KMg3(AlSi3O10)(OH)2
 Essential Elements: Aluminum, Hydrogen, Magnesium, Oxygen, Potassium, Silicon
 All Elements in Formula: Aluminum, Hydrogen, Magnesium, Oxygen, Potassium, Silicon
 IMA Status: Approved 1998
Fluorescent Mineral Properties

 Shortwave UV light: Moderately bright to weak yellow
 To find out more about this mineral at minDat's website, follow this link   Phlogopite

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.465

Frondel, Clifford (1972). The minerals of Franklin and Sterling Hill, a checklist. NY.: John Willey & Sons. p.71

The Picking Table References
 PT Issue and PageDescription / Comment
V. 58, No. 1 - Spring 2017, pg. 15Fluorescent Minerals of Franklin and Sterling Hill, N.J., Part 2, Richard C. Bostwick - Phlogopite
View IssueV. 33, No. 2 - Fall 1992, pg. 11The Check List of Franklin-Sterling Hill Fluorescent Minerals - Phlogopite (Fluorescent Info)
View IssueV. 27 No. 2 - Fall 1986, pg. 7Minerals of the Franklin Quarry, Philip P. Betancourt, Phlogopite
View IssueV. 18, No. 2 - September 1977, pg. 16The Fluorescent Minerals of Franklin and Sterling Hill, NJ by Richard C. Bostwick - Phlogopite
View IssueV. 13, No. 2 - August 1972, pg. 12The Fluorescent Minerals of Franklin/Ogdensburg Area by Frank Z. Edwards - Phlogopite (Fluorescent Info)
View IssueV. 7, No. 2 - August 1966, pg. 11The Minerals of Sterling Hill 1962-65 by Frank Z. Edwards - Phlogopite

Phlogopite mica and corundum in marble, from Franklin, NJPhlogopite mica and corundum in marble, from Franklin, NJ under shortwave UV Light
Phlogopite mica (tan) and corundum (lavender) in marble (white), from Franklin, NJ. Photo by WP.
Phlogopite mica and corundum in marble, from Franklin, NJ under shortwave UV light. The phlogopite fluoresces straw yellow and the corundum dark red, marble is non-fluorescent. Photo by WP.

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