• journal_title：American Mineralogist
• Contributor：Paola Bonazzi ; Frank N. Keutsch ; Luca Bindi
• Publisher：Mineralogical Society of America
• Date：2012-
• Format：text/html
• Language：en
• Identifier：10.2138/am.2012.4004
• journal_abbrev：American Mineralogist
• issn：0003-004X
• volume：97
• issue：7
• firstpage：1199
• section：Articles

Manganoquadratite, ideally AgMnAsS₃, is a new mineral from the Uchucchacua polymetallic deposit, Oyon district, Catajambo, Lima Department, Peru. It occurs as dark gray, anhedral to subhedral grains up 0.5 mm across, closely associated with alabandite, Mn-rich calcite, Mn-rich sphalerite, proustite, pyrite, pyrrhotite, tennantite, argentotennantite, stannite, and other unnamed minerals of the system Pb-Ag-Sb-Mn-As-S. Manganoquadratite is opaque with a metallic luster and possesses a reddish-brown streak. It is brittle, the Vickers microhardness (VHN₁₀) is 81 kg/mm² (range 75–96) (corresponding Mohs hardness of 2–2½). The calculated density is 4.680 g/cm³ (on the basis of the empirical formula). In plane-polarized reflected light, manganoquadratite is moderately bireflectant and very weakly pleochroic from dark gray to a blue gray. Internal reflections are absent. Between crossed polars, the mineral is anisotropic, without characteristic rotation tints. Reflectance percentages (R_min and R_max) for the four standard COM wavelengths are 29.5, 31.8 (471.1 nm), 28.1, 30.5 (548.3 nm), 27.3, 29.3 (586.6 nm), and 26.0, 28.2 (652.3 nm), respectively.

Manganoquadratite is tetragonal, space group P4₃22, with unit-cell parameters: a = 5.4496(5), c = 32.949(1) Å, V = 978.5(1) ų, c:a = 6.046, Z = 8. The structure, refined to R1 = 0.0863 for 907 reflections with F_o > 4σ(F_o), consists of a stacking along [001] of alabandite-like Mn₂S₂ layers connected to each to other by a couple of AgAsS₂ sheets where As³⁺ forms typical AsS₃ groups, whereas Ag⁺ cations are fivefold coordinated. The six strongest lines in the observed X-ray powder-diffraction pattern [d in Å (I/I₀) (hkl)] are: 3.14 (60) (116), 2.739 (50) (0 0 12), 2.710 (100) (200), 1.927(70) (2 0 12 + 220), 1.645 (25) (3 0 16), and 1.573 (20) (22 12).

Electron microprobe analyses gave the chemical formula (on the basis of six atoms) (Ag_0.95Cu_0.05)_∑=1.00 (Mn_0.96Pb_0.04)_∑=1.00(As_0.87Sb_0.14)_∑=1.01S_2.99, leading to the simplified formula AgMnAsS₃.

The name was chosen to indicate the close analogy of the formula and unit-cell dimensions with quadratite, Ag(Cd,Pb)(As,Sb)S₃. The new mineral and mineral name have been approved by the Commission on New Minerals, Nomenclature and Classification, IMA 2011-008.