MARGAROSANITE

Pb(Ca,Mn)₂Si₃O₉
Triclinic, P1, a = 6.768, b = 9.575, c = 6.718 Å, 
a = 110.36, b = 102.98, g = 83.02^o, Z = 2

Margarosanite was partially described from Franklin by Penfield and Warren (1899) and was subsequently described in full and named by Ford and Bradley (1916). New optical and X-ray data were provided by Armstrong (1963) using Franklin material. Glasser and Glasser (1964) provided additional crystallographic data and showed that margarosanite is related to artificial Ca₂BaSi₃O₉. Margarosanite was synthesized by Ito (1968). 

Crystal structure

The crystal structure of margarosanite was solved by Freed and Peacor (1969) who noted relations to walstromite, benitoite, and wadeite. They reported the structure to have layers of tetrahedra alternating with layers of calcium polyhedra between layers of close-packed oxygen atoms. They further noted that the tetrahedral sites form three-membered rings which are linked to a Ca polyhedral chain, along which Pb and additional Ca atoms, which are ordered, alternate.

Description

Margarosanite occurs as platy masses, composed of lamellae up to 4 cm, but commonly smaller, commonly foliate and warped, and sometimes in divergent arrays or sprays. Large clean samples are prized. Margarosanite is colorless to white, but may have an apparent bluish tint. It has three cleavages: {010} perfect, {100} good, and {001} fair, and it has a distinctly pearly to vitreous luster and a density of 4.33 g/cm³.

Optically, it is biaxial, negative, 2V = 78^o, with a = 1.727, b = 1.771, and g = 1.789; dispersion is strong, v > r. A striking property of Franklin margarosanite is its fluorescence: a strong violet-blue in shortwave ultraviolet. Some specimens may have red to pinkish fluorescence (Miller, 1974). Margarosanite is strongly cathodoluminescent with bright blue color, similar in intensity to that of benitoite. Margarosanite visually resembles the best minehillite and is distinguished from it easily by its indices of refraction and its strong blue fluorescence in ultraviolet.

Composition

Margarosanite is a calcium lead silicate mineral; Mn and Zn proxy for Ca in small amounts (Dunn, 1985b). Most samples conform to the ideal composition, and solid solution is limited. A representative analysis is given in Table 8.

Occurrence and paragenesis

Margarosanite is one of the more widely distributed minerals of the restricted lead silicate assemblage; nasonite is the only lead silicate which occurs associated with more species. The most commonly associated minerals are manganaxinite, andradite, and microcline. Others are minehillite, bustamite, grossular, prehnite, nasonite, wollastonite, vesuvianite, clinohedrite, willemite, and rarely vesuvianite, to mention only some. The association with minehillite is particularly noteworthy (Dunn et al., 1984a). Perhaps in part due to its strong and lovely fluorescence in ultraviolet, the species has possibly been selectively overcollected relative to the other lead silicates. Some confusion regarding the fluorescence of barylite resulted in many samples of margarosanite in microcline being mislabeled as barylite in the period 1930-1990.

Name

Margarosanite was named using the Greek words for pearl and tablet, in allusion to the luster and cleavage of this species.

Copyright © 1995 by Pete J. Dunn			Website by Herb Yeates
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