Gaildunningite, ideally Hg2+3[NHg2+2]18(Cl,I)24, is a new mineral from the Clear Creek mine, New Idria mining district, San Benito County, California, USA. It is orthorhombic, space group Amam, Z = 4; a 26.381(6), b 45.590(10), c 6.6840(15) Å, V 8039(5) Å3. Chemical analysis by electron microprobe gave Hg 76.87, I 12.55, Cl 3.79, Br 0.56, S 0.18 wt.%; N 2.45, O 0.28, and H 0.02 wt.% were derived from the crystal structure to give a total of 96.70 wt.%. The empirical formulae (calculated on the basis of 39.44 Hg with 18 N and OH added to give electroneutrality) is Hg2+39.44N18[Cl11.00I10.18(OH)1.81Br0.73S0.58]Σ24.30 and the simplified formula is Hg2+3[NHg2+2]18(Cl,I)24. The seven strongest lines in the X-ray powder diffraction pattern [listed as d (Å), I, (hkl)] are as follows: 2.853, 100, (880, 0.16.0); 2.776, 100, (462, 5.14.0); 2.745, 100, (542); 5.717, 50, (440, 080); 5.965, 40, (131); 5.018, 40, (331); 1.673, 40, (004). Gaildunningite generally forms complex parallel intergrowths of fibrous to acicular crystals elongated along [001]. Acicular yellow to orange to darker orange-red crystals up to 0.1 mm long form matted nests lining vugs within white-grey quartz. Gaildunningite has a vitreous to adamantine luster, is transparent, and does not fluoresce under short- or long-wave ultraviolet light. Mohs hardness ≈ 3 and the calculated density is 8.22 g/cm3. It is brittle with an uneven fracture and has very good cleavage on {100} and {010} and good cleavage on {001}; neither parting nor twinning was observed. It is grey in reflected light with possibly minor bireflectance but no reflectance pleochroism, and strong yellow-orange internal reflections are evident even in plane-polarized light. The Raman spectrum indicates the presence of Hg–N bonds in the structure. The crystal structure of gaildunningite is based on a well-ordered three-dimensional [NHg2]+ net of near-linear N–Hg–N groups, where each N3– is tetrahedrally coordinated by Hg2+. The net is comprised of two five-membered, three six-membered, one seven-membered, and one eight-membered rings of N–Hg–N groups. The interstitial part of the structure is significantly disordered and consists of (Hg2+) and (I,Cl,Br,S2–,OH) ions. A new bond-valence parameter has been derived for (Hg2+–N3–) bonds: Ro = 1.964 Å, b = 0.37; this gives bond-valence sums at the N3– ions in accord with the valence-sum rule.

Gaildunningite and unnamed CCUK-18 are the only (Hg-N-I)-bearing minerals discovered at Clear Creek to date and are closely associated; gaildunningite is the later of these two minerals and tends to grow on CCUK-18. They post-date the iodine-free nitride mosesite and the (N,I)-free chlorides such as eglestonite, implying that iodine was a late addition to the overall chemical system. Both the mineral and the mineral name have been approved by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association (IMA 2018–029). The mineral is named after Gail E. Dunning (b. 1937), a prominent field-collector of minerals from the New Idria mining district who has been responsible for the discovery of many new mercury-bearing minerals.

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