Abstract
Senaite, coexisting with inter alia titanite, richterite and barite, has been found as a constitutent of thin, hydrothermal fissure fillings at the Mangruvan Mn‐Fe deposit. It carries on average 2000 ppm U, and is in a metamict state (or nearly so). Hexagonal cell parameters for reconstituted material are a = 10.395(1) A and c = 20.795(3) A, with V = 1946.0(5) A3. The senaite is weakly bireflectant, R2‐R1 = 0.6%, and has a similar reflectance (R = 19%) at 589 nm as crystalline material from Minas Gerais, Brazil, but displays a less pronounced dispersion. The mineral is chemically heterogeneous, with an average composition (crichtonite‐type AM21O38 formula) (Pb0.75Bao0.23Ca0.06Sr0.04)S1.08 (U0.02Ti12.99Fe4.94Mn2+ 1.85Mn3+ 0.07Zn0.28Sb0.12Mg0.04Al0.01)S20.92O38.Twodif‐ferent substitution mechanisms, [M2]Mn2+ + [M3]Ti4+=2 M3+ and [A]Pb2+ =Ba2+, are invoked to explain the variations in major element chemistry. Minute grains of senaite appearing in a feldspar‐mica‐piemontite rock at the deposit show likewise h...
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