| Abstract | Ferroericssonite, ideally BaFe2+ 2Fe3+(Si2O7)O(OH), is a new mineral species found at both the Esquire #7 and #8 claims, situated along Big Creek in eastern Fresno County, and also at Trumbull Peak in
Mariposa County, California, U.S.A. The mineral is the Fe2+ analogue of ericssonite and is named accordingly. Ferroericssonite crystallized very late in a sequence of minerals resulting from fluids interacting with a quartz - sanbornite vein along
its margin with the country rock. It is closely associated with anandite, bazirite, celsian, devitoite, quartz and titantaramellite. The crystals occur in dark reddish brown foliated masses of irregular, undulating striated blades up to several mm in
length, flattened on {100} and elongate and striated parallel to [001]. The mineral is transparent and has a brown streak, a vitreous luster, a Mohs hardness of approximately 41/2, and two cleavages: {100} perfect and {011} good. It is brittle with
an irregular fracture. The calculated densities are 4.445 and 4.413 g/cm3 on the basis of the empirical formulas for Esquire #7 and Esquire #8 material, respectively. It is optically biaxial (+), {alpha} 1.827(3), ? 1.845(3), {gamma} 1.920(6) (white
light); 2Vmeas = 63(2)°; 2Vcalc = 63.0°; parallel dispersion, r < v; orientation: X = b, Y {approx} c, Z {approx} a; pleochroic: brown, Z > X > Y. Electron-microprobe analyses of Esquire #7 and Esquire #8 material provided: BaO 30.11, 29.76, MgO
0.21, 0.10, MnO 1.19, 0.79, FeO 26.01, 26.24, Fe2O3 15.30, 15.35, Al2O3 0.02, 0.04, SiO2 22.89, 23.34, F 0.09, 0.05, Cl 1.29, 0.88, H2O 1.33, 1.42, F + Cl{equiv} O -0.33, -0.22, total 98.11, 97.75 wt%, with FeO and Fe2O3 assignments and H2O based on
the structure. The empirical formulas for Esquire #7 and Esquire #8 material, based on Mg + Fe + Mn + Si = 5 apfu, are Ba1.02(Fe2+1.89Mn2+ 0.09Mg0.03){sum}2.01Fe3+ 1.00(Si1.99O7)O[(OH)0.77O0.02Cl0.19F0.02]{sum}1 and Ba1.01(Fe2+1.90Mn2+0.06
Mg0.01){sum}1.97Fe3+1.00(Si2.02O7)O[(OH)0.82O0.04Cl0.13F0.01]{sum}1, respectively. Ferroericssonite is monoclinic, C2/m, a 20.3459(10), b 7.0119(3), c 5.3879(4) Å, ? 94.874(7)°, V 765.89(7) Å3 and Z = 4. The strongest five lines in the X-ray
powder-diffraction pattern [dobs in Å(I)(hkl)] are: 3.708(42)(Formula11), 3.506(81)(311, 510, 020), 2.880(42)(420), 2.788(100)(221) and 2.663(83)( Formula02, 710). The crystal-structure determination (R1 = 3.56% for 885 Fo > 4{sigma}F) shows the
mineral to be a heterophyllosilicate with a Fe2+ trioctahedral (O) sheet flanked on either side by heterophyllosilicate (H) layers, forming a HOH composite sheet. The heterophyllosilicate layers are composed of Si2O7 groups linked by Fe3+ square
pyramids in a configuration referred to as a TS (Ti silicate) block, although five-coordinated Fe3+ takes the place of Ti4+. The region between the composite sheets is occupied by Ba2+. Ferroericssonite is a member of the lamprophyllite group.
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