Abstract: The historical background leading to the discovery of sarcolite by Thomson is reviewed. Many mineralogical reference works quote Thompson (1807) but no paper on sarcolite by Thomson has been found; also the spelling of Thomson is incorrect on numerous occasions. The history of sarcolite from its original discovery is reviewed. Chemically, sarcolite is inadequately characterized in spite of a structural study by Giuseppetti et al. (1977). All known chemical analyses are collated and discussed. Two new analyses, using a combination of gravimetric, colorimetric, atomic absorption, and electron probe methods, are presented which show sarcolite to contain 2 wt. % flourine. The presence of this element in sarcolite was reported in 1860 but no quantitative data given; the new determinations cast some doubt over certain aspects of the sarcolite structure.
Measured densities of sarcolite range from 2.93 to 2.96 (mean 2.95) gm/cm3; the tetragonal unit cell of a 12.32 and c 15.48 Å leads to a calculated density of 2.925 gm/cm3 using the empirical formula. Empirically sarcolite may be expressed as: Na1.38Ca6.0(Ca0.37,K0.13,Fe0.08,Sr0.07,Mg0.05)Σ0.70 Al3.90Si6.02P0.54O26.20F1.06C0.06 and ideally Na2Ca12(Ca,K,Fe,Sr,Mg)2Al8Si12(P,Si)O52F2 and suggests 27 (oxygen and fluorine) atoms in the quarter unit cell.
Details are given of the mineralogical assemblage, optical properties, infra-red and thermal behaviour of sarcolite which, after 176 years, is still known from only one locality—Monte Somma.