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Thomaston Dam railroad cut, Thomaston Dam, Thomaston, Litchfield Co., Connecticut, USA

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Latitude & Longitude (WGS84): 41° 41' 50'' North , 73° 4' 5'' West
Latitude & Longitude (decimal): 41.6975, -73.0680555556


With the construction of this dam by the US Army Corps of Engineers following the devastating flood along the Naugatuck River in 1955, the railroad line was moved into a cut along the west side of the dam above the spillway level. At least two workmen died in accidents during the making of this cut (Zodac, 1959). The cut through the Ordovician Ratlum Mountian Schist and some outlier Devonian Nonewaug Granite and pegmatite also exposed numerous hydrothermally mineralized fault zones of unknown origin, but possibly related to Mesozoic rifting. The veins are typically about 0.25 to 1.5 meters thick and contain mostly fluorite, quartz, sulfides (galena, sphalerite, pyrite), calcite and zeolites (stilbite, heulandite, laumontite, harmotome). Other minerals are found in the metamorphic rocks, the pegmatite, or are secondary in origin.

Myer (1962) noted the paragenesis of the hydrothermal minerals in the fault veins as follows:

Hydrothermal mineralization appears to be in four separate stages. Deposition of quartz [and] sphalerite...are the first stage. Quartz cemented the shattered country rock and is the most abundant mineral,...and sphalerite [is] rooted against the quartz. Next, fluorite and calcite, along with galena, [sphalerite], and chalcopyrite, crystallized in a second set of fractures as well as within the quartz-lined cavities. Calcic zeolites, stilbite, heulandite and laumontite, crystallized with calcite in the third stage. They are intergrown and perched on quartz and fluorite druze [sic]. The less siliceous zeolites, [harmotome] and chabazite, are rare and usually found alone on fluorite. The final stage is pyrite in minute crystals that are sprinkled randomly on other minerals.


Collecting is no longer allowed there per US ACOE rules.

There has long been controversy regarding the identification of the brown, generally 6-sided pagoda-like micro-crystals of ZnS found here and generally referred to as wurtzite. This ID was first made by Myer (1962). However, Pete Dunn analyzed crystals via XRD in 1972 (see Yedlin 1973a and b) and obtained "perfect sphalerite patterns". Henderson (1979) showed diagrams of sphalerite crystals "epitaxial" on "wurtzite", and the other way around, with a (0001) (pinacoidal) face of "wurtzite" matching a (111) (tetrahedral) face of sphalerite. Yet (111) is the face that sphalerite twins on. The crystals are actually combined positive and negative tetrahedra of sphalerite twinned on a 6-sided (111) face. Vogt (1991) shows a nice photo of a tetrahedron with a (111) face capping a stack of polysynthetic sphalerite twins resting on multiple sphalerite tetrahedra, but still referring to it as "wurtzite" even though he notes the presence of sphalerite associated with the "wurtzite". Note the re-entrant angles that circumscribe the "prisms" of these pagoda crystals, plus the varying width of the crystals, which are indicative of twinning. Here are some examples of sphalerite tetrahedra with both simple twinning and polysynthetic twinning, some on the same crystal: http://www.mindat.org/photo-783527.html http://www.mindat.org/photo-783535.html and http://www.mindat.org/photo-783519.html The pagoda stacks also cleave along the dodecahedral form {011} and part parallel to the twin plane (111), which is across the "prism" of the stack of twins, whereas wurtzite's best cleavage is prismatic and is not evident here.

A mindat discussion found here http://www.mindat.org/forum.php?read,9,318431,318568#msg-318568 makes an argument that wurtzite is not a separate mineral at all, but merely a sphalerite polytype, while others hold that it is.

Mineral List


61 valid minerals. 5 erroneous literature entries.

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References

Zodac, Peter. (1959), Minerals at Thomaston Dam, Connecticut; Rocks & Minerals: 34: 3.

Schooner, Richard. (1961), The Mineralogy of Connecticut. Fluorescent House, Branford, Connecticut.

Myer, George H. (1962), Hydrothermal Wurtzite at Thomaston Dam, Connecticut. American Mineralogist: 47(7-8): 977-979.

Hiller, John. (1968), Collecting at Thomaston, Conn. Rocks & Minerals: 43(10): 746-747.

Ryerson, Kathleen. (1972), Rock Hound's Guide to Connecticut. Pequot Press: 36-37.

Yedlin, Neal. (1973a), Yedlin on Micromounting. Mineralogical Record: 4(2).

Yedlin, Neal. (1973b), Yedlin on Micromounting. Mineralogical Record: 4(6).

Januzzi, Ronald E. and Seaman, David. (1976), Mineral Localities of Connecticut and Southeastern New York State and Pegmatite Minerals of the World. The Mineralogical Press, Danbury, Connecticut.

Molon, Joseph. (1976), Minerals from Thomaston Dam, Thomaston, Connecticut; Rocks & Minerals: 51: 449.

Henderson, William A. (1979), Microminerals. Thomaston Dam. Mineralogical Record: 10: 239-241.

Shelton, Bill and Webster, Bud. (1979), Mineral Collector's Field Guide: The Northeast. Mineralogy, Wallingford, Connecticut: 46-49.

Segeler, Curt and Molon, Joseph. (1985), The Thomaston Dam Site, Thomaston, Connecticut; Rocks & Minerals: 60(3): 119-124.

Vogt, Wolfgang. (1991), Rediscovering Thomaston Dam. Lapidary Journal: April.

Januzzi, Ronald. E. (1994), Mineral Data Book - Western Connecticut and Environs. Mineralogical Press, Danbury, Connecticut.

Henderson, William A. (1995), Microminerals of Connecticut; Rocks & Minerals: 70(6): 420-425.

Weber, Marcelle H. and Earle C. Sullivan. (1995), Connecticut Mineral Locality Index. Rocks & Minerals (Connecticut Issue): 70(6): 399.

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