New Mexico Mineral Symposium — Abstracts

Microscopically crystallized minerals were found many years ago in a basaltic sill emplaced between Devonian-age Millboro shales 5.5 mi west of Sugar Grove, adjacent to Highway 21, Pendleton County, West Virginia. The sill has a maximum thickness of approximately 9 ft and is exposed for approximately 125 ft. It is an amygdaloidal olivene-free pyroxene-basalt, with cavities to 10 cm, which contain numerous zeolites, and associated minerals. According to Mindat.org's Sugar Grove page, the basalt is Eocene, 45 Ma, and “These are the youngest volcanic rocks east of the Rocky mountains.”

Mineralogy

Pyrite, FeS₂, despite being a very common mineral, is quite interesting at this location. The pyrite crystals can form cubes and pyritohedrons, with octahedral and trapezohedral modifications. However, the main attraction is when filiform crystals, ± right-angle bends occur. Acicular pyrite is believed to form rapidly from a mechanism called a “screw dislocation.” One might think that such crystals should have rounded edges, but electron photomicrographs of some lab-grown n-hectane crystals show that a crystal can retain its sharp edges during spiral growth. A screw dislocation can also occur at right angles to the pre-existing crystal, causing a right-angle bend. The right-angle bends usually occur at the tip of an elongated crystal, when one screw dislocation ends, and another takes off at 90 degrees.

Henderson and Francis (1989) studied 35 locations with filiform pyrite and concluded that twinning was not the mechanism causing the right-angle bends. Pyrite crystals commonly show striations caused by an alternation between cube and pyritohedral faces. A twinned crystal should have striations at 90° to each other at the twin plane. Right-angled Sugar Grove pyrite crystal faces show a single striation direction. Henderson and Francis also found that when performing a single-crystal X-ray diffraction upon the bend portion of a Sugar Grove right-angle pyrite, that the diffraction pattern showed two-fold symmetry, not four-fold as would be expected from a twin junction.

The thinnest crystals are sometimes curved. Relatively thick curved crystals have not been observed. Henderson and Francis deduce that if a later period of growth occurs then the acicular crystals thicken, and upon doing so, straighten out.

One might consider rapid hopper growth as a simple answer to the occurrence of acicular pyrite. This would have to be dramatic growth in just one direction, which although it can occur in theory, challenges the accepted idea of screw dislocations.

Pyrite is the earliest mineral to form in the Sugar Grove cavities. Normal cubic or pyritohedral crystals have reached 3 mm, whereas the filiform crystals have reached 6 mm. Nontronite commonly coats the pyrite.

Nontronite, Na_0.3Fe₂³⁺(Si,Al)₄O₁₀(OH)₂^.H₂O, is generally gray in color and forms spheres and coatings. It commonly coats pyrite and harmotome. Nontronite, when freshly exposed, is green to blue in color, but it rapidly dehydrates and then turns black. Others have suggested that the nontronite oxidizes; however, the author has observed that freshly opened nontronite-lined cavities leave adjacent wet spots, indicating that a water-filled cavity has been breached. It can form wild spindly/wirey coatings, which when broken, usually reveal a tiny core of pyrite, but not always.

Chabazite-Ca, Ca₂(Al₄Si₈O₂₄)^.13H₂O, is the most common zeolite found. It usually forms “phacolite” penetration  twins, which are formed by a 60° rotation about the c-axis. It is rhombohedral, transparent, and usually colorless. It sometimes has an attractive pale peach color.

Analcime, NaAlSi₂O₆^.H₂O, forms transparent colorless trapezohedra and is the earliest formed zeolite, often forming drusy cavity coatings.

Harmotome, Ba₂(NaKCa_0.5)(Al₅Si₁₁O₃₂)^.12H₂O, forms beautiful transparent Marburg-type penetration twins. Nontronite frequently coats harmotome crystals, while leaving adjacent chabazite-Ca crystals alone.

Thomsonite-Ca, Ca₂Na[Al₅Si₅O₂₀]^.6H₂O, forms white divergent fans, and also translucent gray spheres of acicular crystals. The fans are often associated with nontronite and mesolite.

Mesolite, Na₂Ca₂(Al₆Si₉O₃₀)^.8H₂O, almost always is found as colorless acicular crystals emerging from a fan of thomsonite. Nontronite spheres are sometimes attached to the tips or sides of the crystals.

Calcite, CaCO₃, is a common late mineral. It usually forms colorless transparent rhombohedra, which can reach 12 mm. Tiny spherical aggregates of scalenohedra are gray to white and translucent.

Aragonite, CaCO₃, is a very late mineral and is quite sparse. It forms colorless transparent long prisms. Kearns (1993) noted rare sixling twins.

Baryte, BaSO₄, is relatively rare at Sugar Grove. It is found as thin colorless to white blades, sometimes forming tiny twinned pinwheel aggregates. These tiny pinwheels has been shown by Kearns to be strontian baryte. The larger crystals show no strontium.

Quartz, SiO₂, does not occur in the vesicles, but Kearns observed it as drusy crusts on joint and fracture surfaces.

Unknowns. Kearns noted two unknowns. The author has seen three others: tiny colorless erionite (?) crystals (only three specimens found), colorless, transparent prisms, which are highly lustrous (only two found), and extremely thin wafers, which are coated with erionite (?) and then nontronite.

Acknowledgments

I would like to thank Adam Roybal for his Photoshop help and Doktor Klaus Fuhrberger for his keen insight and field direction.

References:

1. Henderson, W. A., Jr., and Francis, C. A., 1989, On right angle bends in filiform pyrite: Mineralogical Record, v. 20, pp. 457???464.
2. Kearns, L. E., 1993, The minerals of Sugar Grove, West Virginia: Rocks and Minerals, v. 68, pp. 158???167.??
3. Strickland-Constable, 1986, The defect structure of crystals; in Kinetics and Mechanism of Crystallization: Elsevier Science and Technology Books, pp. 131???145.