New Mexico Mineral Symposium — Abstracts

Nearly 30 years ago I collected some chabazite from a basalt road-cut south of Chili, New Mexico. I was not impressed with what I had found, rhombohedrons about 1 mm in size.

About 2 years ago a friend showed me a label and a specimen, an "apophyllite" from North of Española, New Mexico. One day, when we were on our way to the Harding Mine, we made a detour to Chili. We ended up finding much more than just “apophyllite.”

The locality is about 1 mile SE of Chili on NM Highway 84 (milepost not noted), and West of the Rio Chama. On the East side of the road the outcrop is about 200 ft long, while on the higher West side it is about 250 ft long. There is only a little room for parking and vehicles on the roadway tend to speed, creating dangerous situations. When collecting on the higher West side, about 15 ft high, one has to be careful to not drop rocks onto the roadway. While working, a “spotter” is helpful for keeping loose rocks from the roadway. Caution is needed.

It is the author's experience that when calcite occurs with zeolites, as it does at Chili, the zeolites are usually Ca-dominant. Testing and identifications on the minerals was performed using Virgil Lueth's x-ray diffractometer. The x-ray diffractometer determines structure, not chemistry, so sometimes it may not give species-definitive results. With few exceptions the XRD results for zeolites were Ca-dominant. "Chabazite" and "phillipsite" had mixed results. There was 1 identification of chabazite-Na, which may be erroneous. Phillipsite had results of “phillipsite-Ca” “phillipsite-K,” and “phillipsite-Na.”

The basalt is approximately 9.8 myo (potassium/argon dating, verbal communication with Maureen Wilks) and is a member of the middle to upper Miocene Lobato Formation (Koning, 2005). The mineralogy is similar on both sides of the roadway. The following minerals were identified:

• Analcime (NaAlSi₂O₆^.H₂O) is a very common zeolite, but at Chili very few were found. The crystals are tiny, < 1 mm, colorless, transparent trapezohedrons, and it was visually identified.
• Apophyllite species undetermined/untested, was found in perhaps just 1 specimen. It is a tetragonal-appearing, colorless, transparent prism about 0.7 mm in size.
• Baryte (BaSO₄) as opaque white crystals and aggregates to 1 mm in size. Less commonly colorless transparent crystals are observed.
• Calcite (CaCO₃)is common as rhombohedrons (to 3 mm), flat wafers (to 8 MM), scalenohedrons (to 5 mm that sometimes encase green nontronite spheres), and very tiny crystals/aggregates that resemble erionite, a usually fine-grained zeolite. Coarse calcite was visually identified, while the fine-grained material and rhombohedrons effervesced in HCl. Gemmy individual rhombohedrons resemble chabazite and need to be verified with HCl. These rhombohedrons can be elongated on one axis, which is not seen in chabazite-Ca rhombohedrons.
• Chabazite-(Ca), Ca₂(Al₄Si₈O₂₄)^.13H₂O, is the most common mineral, forming colorless transparent to translucent rhombohedrons to 5 mm, but usually much smaller. Simple penetration twins are common. Not often seen were crystals attaching to each other to form tiny, gemmy, irregular bridges across vug interiors. On the East side a few specimens of variety "phacolite" were found. The phacolite crystals are stubby flat hexagonally-shaped trillings that were generally opaque, and up to 4 mm in size.
• Covellite (CuS) was visually identified on just 3 specimens. They are thin wafers to < 1 mm with crude hexagonal outlines and were oxidized with green "halos" of a secondary mineral at their bases, hence the guess of “covellite.”
• Erionite-Ca, Ca₄K₂(Al₁₀Si₂₆O₇₂)^.30H₂O, was verified in a specimen in association with levyne-Ca. From other localities it is not uncommon for erionite to coat the outside of lévyne crystals, forming thin "sandwiches". Apparently this is no different, and it explains the layering seen in some of the Chili lévyne crystals. No erionite was identified that was in free-standing crystals. Specimens with particularly tiny crystals turned out to be calcite. I expect testing of more specimens to reveal otherwise. Erionite-Ca is very tiny, has a silky luster and grows perpendicular to the surface of the lévyne-Ca crystals.·
• Goethite, alpha-Fe³⁺O(OH), was visually identified. It forms branching dendrites that can sometimes protrude from the crystal surface they are grown upon.
• Harmotome, Ba₂(NaKCa_0.5)(Al₅Si₁₁O₃₂)^.12H₂O, forms blocky prisms to 3 or 4 mm that can be colorless to white, with the larger crystals tending to be white and more opaque. The larger crystals are sometimes etched. The etching causes thin plates to remain, which reveals differences in crystallization or, perhaps, chemistry. Harmotome and phillipsite are always twinned. What appears to be single crystals are actually twins composed of 4 crystals. These are called Morvenite twins. Harmotome from Chili can also form Marburg twins, which are composed of 2 Morvenite twins. These can be easily confused with Perier twins, but the striations are different. Some of the harmotome crystals can have "side panels", which are probably polysynthetic twinning. Harmotome aggregates in parallel growth can result in curved 001/top faces, similar to what is seen with stellerite and stilbite.
• Hematite, alpha-Fe₂O₃, is found as small metallic red aggregates to 1.5 mm. It was visually identified.
• Lévyne-Ca Ca₃(Si₁₂Al₆)O₃₆^.18H₂O, forms attractive, transparent to opaque, hexagonal plates to 2.5 mm that can be layered. This layering is generally due to a coating of erionite-Ca, which is a common association, and varies in opacity and color. Lévyne-Ca is not common at Chili. Colorless, transparent crystals, without erionite-Ca, tend to be very brittle. The erionite-Ca coating makes the lévyne-Ca crystals more resistant to shock, weathering, etc.
• Nontronite Na_0.3Fe₂³⁺(Si,Al)₄O₁₀(OH)₂^.nH₂O, is found as tiny green spheres encased in calcite. This is a visual identification. Nontronite from Sugar Grove, West Virginia displays an interesting characteristic: when exposed to air the green nontronite starts loosing water or oxidizing, and its color darkens, to gray or black. This change occurs within a few hours. Once exposed, even quickly placing nontronite into water will not stop the dehydration, or oxidation, process. At Chili the clay minerals, nontronite and saponite, can form ropey structures, vug coatings and spheres. These are usually brown-colored, representing a mixture of saponite and nontronite. Less commonly these ropey structures are dark gray-black nontronite.
• Phillipsite-Ca? (Ca_0.5Na,K)₉{Al₉Si₂₇O₇₂]^._~24H₂O, is common as white to colorless prisms. They are smaller than the harmotome crystals, and are composed of Morvenite twins that can very rarely combine to form Stemple twins. Only 3 damaged Stemple twins have been observed. They are rare. The XRD results were variable, suggesting Ca, K and Na phillipsites. It is doubtful that all occur here. The author's best guess is for “phillipsite-Ca.”
• Pyroxene, species undetermined, is rarely found as euhedral crystals. Crystals are opaque, greenish-brown prisms that are frequently coated with calcite or other minerals.
• Saponite, (Ca,Na)_0.3(Mg,Fe²⁺)₃(Si,Al)₄O₁₀(OH)₂^.4H₂O, forms common masses of white clay. Relatively long ropey "worms" are common, but are usually mixed with nontronite, giving them a brown color. Saponite can be the last mineral to form. Both are nontronite and saponite are smectite group minerals.
• Thomsonite-Ca, Ca₂Na[Al₅Si₅O₂₀]^.6H₂O, is common. It forms spheres and botryoidal coatings in cavities. Spheres can reach at least 4 mm. Under high magnification one can rarely see tiny individual laths. These crystals tend to stack against each other on the 001 face. It is one of the last minerals to form, so it can coat other minerals. It is interesting when thomsonite-Ca coats elongated calcite scalenohedrons and is subsequently broken, which reveals crystals radiating about a hexagonal calcite core.By The Way—the "apophyllite" that started the investigation is chabazite.

Acknowledgments

The author wishes to thank Dr. Paulina Iñigo for her observant field kibitzing and helpful comments on this paper.

References:

1. Koning, D., et al., 2005, Geologic map of the Chili 7.5-minute quadrangle, Rio Arriba, County, New Mexico, OF-GM-103, New Mexico Bureau of Geology & Mineral Resources.
2. Tschernich, R., Zeolites of the world, 1992, Geoscience Press.