Abstract
Santaclaraite, ideally CaMn4[Si5O14(OH)](OH) · H2O, occurs as pink and tan veins and masses in Franciscan chert in the Diablo Range, Santa Clara and Stanislaus Counties, Calif. It is associated with four unidentified Mn silicates, Mn-howieite, quartz, braunite, calcite, rhodochrosite, kutnohorite, barite, harmotome, chalcopyrite, and native copper. Santaclaraite is triclinic, space group B1, a = 15.633(1), b = 7.603(1), c = 12.003(1)Å, α = 109.71(1)°, β = 88.61(1)°, γ = 99.95(1)°, V = 1322.0(3)Å3, Z = 4. The strongest lines of the X-ray powder pattern are (d, I, hkl): 7.04Å, 100,010; 3.003, 84, 501; 3.152, 80,410; 7.69, 63, 200; 3.847, 57, (113,400); 3.524, 39, 020. Crystals are lamellar to prismatic (flattened on {100}), with good cleavage on {100} and {010}; H = 6½; D(calc.) = 3.398 g/cm3, D(meas.) = 3.31(±0.01); optically biaxial negative, with α = 1.681, β; = 1.696, γ = 1.708 (all ±0.002), 2VX = 83 (±1)°. Although chemically a hydrated rhodonite, santaclaraite dehydrates to Mn-bustamite at about 550°C (in air). Santaclaraite is a five-tetrahedral-repeat single-chain silicate and has structural affinities with rhodonite, nambulite, marsturite, babingtonite, and inesite.
