New Mexico Mineral Symposium — Abstracts

Three new mineral species have been identified at a tin prospect on the north side of Squaw Creek, Catron County, New Mexico. The first is squawcreekite (Fe,Sb,Sn,Ti)O₂, the iron-and antimony-dominant member of the rutile group. Cassiterite is also a member of the rutile group. Trivalent iron and pentavalent antimony substitute in equal amounts (atomic) to preserve charge balance. This new mineral is associated with a high-temperature assemblage of quartz, cassiterite, hematite, pseudobrookite, tridymite, chernovite-(Y), the cerium-dominant analogue of chernovite (a new species), and the iron-analogue of durangite, NaFeAsO₄F (a new species). Squawcreekite occurs in very sparse amounts and the maximum crystal size observed is about 50x120x200 micrometers. The mineral usually occurs mantled by epitaxial overgrowths of Fe- and Sb-bearing cassiterite, adjacent to hematite-cassiterite veins, disseminated within and in small miarolitic cavities in hydrothermally altered rhyolite. The squawcreekite is light to medium yellow brown with a very pale yellow-brown streak. Cell data are: a 4.6673(7)A, c 3.1006(8)A, V 67.542 (2)A3, Z = 2; space group P42/mnm. Relative to pure SnO₂ (cassiterite), there is a reduction in squawcreekite of 1.5% for the a parameter, 2.7% for the c parameter, and 5.6% for the volume. Crystals have a prismatic habit, and all are euhedral. Forms present include (100), (110), (111), and (101). Twinning, by rotation about [101], is present in some crystals.

The iron-analogue of durangite has not yet been named but is currently before the IMA (International Mineralogical Association) for voting and approval. It is the most abundant of the three new minerals at Squaw Creek. Crystal size is 0.05 to 1 mm, and aggregates may be as much as 3 mm in maximum dimension. The crystals are euhedral to subhedral. Most crystals show some evidence of solutional etching and are somewhat opaque, but others are clear and lustrous. Crystals are medium to dark red with a medium red-orange streak. The typical crystal habit is unlike that of durangite found at the "clearing" near Boiler Peak. Higher-order crystal forms dominate rather than the simpler forms for durangite. The mineral is strongly compositionally zoned and somewhat color zoned and shows solid solution towards durangite (NaAlAsO₄F) and tilasite (CaMgAsO₄F). The average analysis is: Na₂O 8.0 wt %, CaO 8.0, Fe₂O₃ 14.0, MgO 3.5, Al₂O₃ 5.0, TiO₂ 5.0, Mn₂0₃ 0.5, SnO₂ 0.6, ZnO 0.1, As₂O₅ 51.0, ZrO₂ 0.3, Nb₂O₅ 0.2, Li₂O 0.1, F 6.0, total 102.3, O for F 2.5, total 99.8. An empirical formula calculated on the basis of 5 (F,O) atoms is: (Na_0.59Ca_0.33Li₀.₀₂)_∑0.94 (Fe⁺³_0.40Al_0.22Mg_0.20Ti_0.14Mn_0.01Sn_0.01Zn_0.01)_∑.99 (As_1.01O₄)(F_0.72O_0.28)_∑1.00. Ion microprobe studies (R. W. Hinton, Univ. of Chicago) yielded results in agreement with the electron microprobe results. Optical properties for the mineral are: biaxial (+), α 1.748, β 1.772, γ 1.798, γ -α = 0.05, 2V meas 86°, 2Vcalc 89°• Dispersion r > v, strong. Cell data are: a 7.161A, b 8.780A, c 6.687A, IS 114.58°, Z = 4, V 382.4A3. Space group Aa or A2/a.

The third new mineral appears to be the Ce-group dominant analogue of chernovite-(Y), which is YAsO₄. The mineral is bright lemon yellow and occurs in very sparse amounts. It is the rarest of the three new minerals. Maximum grain size is about 75 micrometers in length. Habit is short prismatic. Crystals are euhedral to subhedral. The mineral is easily confused with yellow cassiterite.

Stolzite, PbWO₄, has been identified from hematite-cassiterite veins at the head of Nugget Gulch. Crystals resemble cassiterite in color, lemon yellow, but are slightly more adamantine. Most crystals are euhedral and have a tapered ditetragonal pyramidal habit, much like that of some wulfenite, PbMoO₄. The mineral is very rare, and the maximum grain size observed is about 250 micrometers.

A tin prospect in rhyolite adjacent to NM-59 along Willow Springs Draw contains several interesting minerals in the miarolitic cavities. Fresh rhyolite contains cavities with hematite, pseudobrookite, calcite, clinopyroxene, titanite, sanidine, tridymite, and quartz. Some of the titanite is unusual in composition, containing elevated amounts of REE's, Pb, Nb, Fe, and F. Crystals are all clear, lustrous, and sharp. Color ranges from red brown to orange brown, and the crystals may be as much as 1 mm or more in maximum dimension. All crystals are euhedral. The clinopyroxene occurs as orange or yellow-orange, thin, slender, euhedral needles as much as several millimeters long but only 100 micrometers wide. Miarolitic cavities in hydrothermally altered rhyolite contain hematite, quartz, sanidine, tridymite, calcite, pseudobrookite, and minor amounts of tilasite and the Fe-analogue of durangite. Crystals of both minerals are extensively corroded and etched. A solid solution exists between these two minerals. Most of the material found is tilasite, and the maximum grain size is about 1 mm. Color ranges from medium red to pale pink. Only small amounts of the arsenate minerals were found whereas the titanite and clinopyroxene were fairly abundant.