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: Na_1.38Ca_6.0(Ca_0.37,K_0.13,Fe_0.08,Sr_0.07,Mg_0.05)_Σ0.70 Al_3.90Si_6.02P_0.54O_26.20F_1.06C_0.06 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.