PHLOGOPITE-1M

KMg₃AlSi₃O₁₀(F,OH)₂
Monoclinic, C2/m, a = 5.3078, b = 9.1901, c = 10.555 Å,
b = 100.08^o, Z = 2

	Figure 18-2. Sharp crystals of brown phlogopite in the white calcite of the Franklin Marble from Franklin. Specimen is 12 cm in maximum dimension. Smithsonian Institution, #164056. Photo by the author.

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. 

Description

	Figure 18-3. Euhedral crystal of phlogopite in the white calcite of the Franklin Marble from Franklin. Specimen is 10 cm in maximum dimension. Smithsonian Institution, #C3644-5. Photo by the author.

The best crystal specimens come from the Franklin Marble and are the 1M polytype. The crystals are well-formed, sharp, prismatic on [001], and up to 18 cm in length, occurring as single isolated crystals (Figures 18-2, 18-3, and 18-4), 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. Optically, Franklin phlogopite is biaxial, negative, 2V = 0^o, with a = 1.530, and b = g = 1.558. Some local phlogopite has a moderate yellow fluorescence in shortwave ultraviolet (Bostwick, 1982), which is sometimes brighter in colorless crystals.

Composition

Phlogopite is a potassium magnesium aluminum silicate fluorine hydroxide mineral of the mica group. There are few extant analyses of local material, and they are given in Table 13. 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.

	Table 13. Chemical analyses for micas and brittle micas.

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. 

Occurrence and paragenesis

The prismatic to tabular brown crystals (Figures 18-2, 18-3, and 18-4) 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).

	Figure 18-4. Prismatic crystal of phlogopite in the white calcite of the Franklin Marble from Franklin. Specimen is 12 cm in maximum dimension. Mineralogical Museum, Harvard University, #121883. Photo by Chip Clark.		Figure 18-5. Previously-existing mica (light gray reflective) with andradite (granular gray) from Franklin. This now-replaced mica is locally referred to as caswellite. Specimen is 12 cm in maximum dimension. Smithsonian Institution, #84974. Photo by the author.

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 (Figures 12-23, 12-31, 15-34, 18-1, and 18-5). It is discussed further under grossular.

Copyright © 1995 by Pete J. Dunn			Website by Herb Yeates
Link to homepage
This page created: January 14, 2001