Crystal Prospect

Small laumontite crystals turning to leonhardite. The crystals sit on top of an unknown black mineral with a velvet appearance (Figure 43). The mineral is suspected to be goethite. The diabase also contains several chalcopyrite crystals distributed throughout the matrix. Ref #18100

Figure 40  ▶

Vulcan Materials Co. Crushed Stone Quarry

Stilbite, apophyllite and calcite over prehnite on diabase matrix. Here are two halves to a cavity filled with euhedral crystals. The majority of the crystals are stilbite. Most are fractured from when the cavity was split apart because the cavity was filled almost completely with zeolite crystals. Several excellent stilbite crystals reaching 15 millimeters project outward from the coarse-grained green prehnite coating the cavity (Figure 26).

▲  Figure 35  ▲

Figure 21  ▶

They are accompanied by many double-terminated hydroxyapophyllite crystals (Figure 27) reaching up to 3 millimeters in length. A single beige calcite crystal (Figure 28) on each piece is 3.5-4 millimeters and fluoresces blue-white in short and long wave radiation. The composition of the diabase is zoned (Figure 29). There is a higher concentration of pink plagioclase closer to the crystal cavity with a narrow layer of black pyroxene between the plagioclase-rich diabase and the zeolite-bearing cavity. Specular hematite also appears to be present in the black layer.

Red silica-based rock with chalcopyrite grains. Jasper is suspected as the matrix material. Green-hued hornfels runs along one side of the matrix. The chalcopyrite is found as microscopic grains close to the hornfels (Figure 22). Ref #18080

Figure 19  ▼

Figure 36  ▼

◀  Figure 30

#18097

▼  Figure 43

Figure 32  ▶

Laumontite with calcite on prehnite in diabase matrix. Small laumontite crystals are scattered on the prehnite coating. A fractured piece of a transparent calcite crystal sits in one corner of the specimen. The calcite crystal may be scalenohedral in form, but has yet to be determined. This piece came from the same rock as the specimens in Figures 11, and 42-46. Ref #18091

Figure 25 ▼

The orange crystals in Figure 35 are cubic to pseudorhombic with a greater concentration of orange impurities closer to the surface with the center of the crystals appearing almost completely transparent. The crystals formed in a prehnite-lined vug with a matrix of diabase. Questions arose when looking at images for visual reference on the Mindat.com page for the Manassas quarry. Examples collected from the quarry and described as “chabazite” appeared visually similar to that of calcite. Images for both calcite and chabazite showed orange pseudorhombic orange crystals. However, poor resolution and focus of the images on the Mindat gallery pages made observation of subtle details difficult, which could aid in determining the difference between the two minerals.

Figure 18 ▼

This specimen comes from the same rock as the specimen in Figure 39. It consists of hornfels matrix with micro-pyrite cubes on the facing side with several individual stellerite crystals on the opposing side. Just as with Figure 39, the stellerite formed in cavities filled with grayish mud that cannot be completely removed without damaging the crystals. The pyrite is imbedded in the hornfels and is best visible when the matrix is moist. Ref #18079

The specimens in Figures 35 and 36 came from the same rock. Since the validity of previously collected samples on Mindat was questionable, the specimens in Figure 36 were tested to determine their composition. A single specimen containing the mineral in question was broken in half. The piece on the left was left untouched. The piece on the right was placed in vinegar for a 48-hour period. The orange mineral bubbled readily and was completely dissolved within the 48-hour period. This indicated that the orange mineral was likely calcite. Dissolving the calcite revealed vitreous terminated prehnite crystals. The prehnite and diabase matrix appeared unaltered by the vinegar.

▼  Figure 39

Figure 29  ▶

Figure 26  ▶

This example of stellerite is similar to Figure 4 collected during the May 21, 2016 field trip. It differs from that example by having one face coated with crystals. The coating has more damage from tumbling, and the crystals are compacted together. The pyroxene-dominant diabase matrix of both this specimen and in Figure 4 is stained brown.

The crystal-bearing cavity of this specimen was found shortly after completion of the previously described acid test to confirm the identification of orange calcite. The pseudorhombohedral calcite crystals in this specimen are much larger than the previous examples, with the largest crystal measuring 14 mm (Figure 38). The stepped appearance of some of the crystal faces is similar to calcite crystals from the Pugh quarry in Idaho. The calcite is accompanied by stilbite and micro laumontite crystals in a prehnite-lined cavity in diabase. Ref #18081

Figure 20  ▼

Prehnite cavity with micro-stilbite crystals. The botryoidal prehnite masses are very coarse grained due to the larger scale of the crystals. Individual units are distinguishable when observed under a loop. Tabular transparent crystals of stilbite protrude from the prehnite toward the center of the cavity (Figure 19). A layer of loosely dispersed plagioclase and pyroxene crystals cover one side on the exterior of the cavity. Fibrous green byssolite crystals have grown between the plagioclase and pyroxene (Figure 18). The entire cavity is sandwiched between two layers of diabase with slickenside surfaces.  Ref #18082

◀  Figure 41

◀  Figure 37

Clear calcite crystals coat pink-white dolomite on a hornfels matrix. The calcite crystals are rounded as apposed to the pseudorhombohedral crystals that are commonly found elsewhere in the quarry. The dolomite takes the form of a vein in the hornfels with only a small area of euhedral crystals visible and coated in the clear calcite. The matrix is abundant with parallel veins of calcite and dolomite. Small grains of purple bornite were observed in the hornfels near these veins. 

Ref #18088

White dolomite in hornfels with calcite. Specimen was found along the road next to the berms from which Figure 33 was collected.

  ▲ Figure 23  ▲

▼ Figure 24

Figure 34  ▶

▲ Figure 27

The surrounding mineralogy also differs for the pumpellyite. Epidote crystals of Figures 12 and 13 formed in a vein of calcite, albite, and quartz. Byssolite in Figures 18 and 19 are in a cavity with individual plagioclase and pyroxene crystals on the exterior of a prehnite-lined cavity. The pumpellyite in this example is in a vein of prehnite with micro apophyllite crystals. Ref #18085 

Figure 38  ▶

Calcite and stellerite in hornfels matrix. The calcite is pseudorhombic, colorless, and phosphorescent. The crystals glow blue white in short and long-wave radiation and emits light for 7-10 seconds after the black light is removed. The stellerite crystals formed in small cavities in clusters and individual crystals. Most crystals are not terminated because of the small size of the cavity relative to the size of the crystals. The hornfels matrix is black in color with a very, very fine grain size. The cavities are filled with a grayish mud and cannot be completely removed without damaging the crystals in the process. Ref # 18078

#18098

Figure 42  ▼

Hydroxyapophyllite, pumpellyite and prehnite in diabase. Apophyllite crystals protrude from prehnite coating the inside of a cavity. The crystals are clear, elongated, box-shaped with filleted corners. Pumpellyite is found in several small cavities on the exterior that are near the vein with the apophyllite on prehnite. The pumpellyite differs from epidote with darker green fibrous crystals, compared to the gemmy pistachio green granular coatings and veins of the epidote in Figures 12 and 13. It also differs from the byssolite in Figures 18 and 19 with a saturated green color and radial aggregates of crystals as apposed to randomly oriented masses of fibers that blanket other crystals.

Figure 28  ▼

Ref #18097 & 18098

In the process of identifying the minerals present in various specimens from the quarry, I have noticed that calcite in cavities with prehnite and various zeolites tended to be orange in color, whereas clear to white calcite crystals formed in hornfels. Perhaps the difference in the calcite is due to the formation of the crystals in hydrothermally altered zones or in the hornfels, contact metamorphosed sedimentary rocks and pyroclastics as described in Minerals of Washington D.C. and Vicinity by Lawrence Bernstein: 1976.

◀  Figure 33

◀  Figure 31

▼ Figure 22  ▼