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O & G Southbury Quarry (Silliman Quarry; O & G No. 2 Quarry), Southbury, New Haven Co., Connecticut, USA

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Latitude & Longitude (WGS84): 41° 30' 50'' North , 73° 12' 57'' West
Latitude & Longitude (decimal): 41.51389,-73.21611
Köppen climate type:Dfb : Warm-summer humid continental climate

Note, the entrance road to this quarry is on State Route 67 in Southbury; however, the quarry starts about 0.5 mile north of the entrance road and the vast majority of it is mostly in Woodbury. Because O & G operates another quarry farther north in Woodbury (O & G No. 1 or the "Woodbury" Quarry), this locality has almost universally been referred to as the "Southbury" Quarry to differentiate it, even though it too is technically in Woodbury. It has been referred to as the Silliman Quarry, but that name initially referred to an earlier sand pit a little to the south. However, Brunet (1977) states that the Silliman family, descended from Yale mineralogist Benjamin Silliman, owned the trap rock quarry, too, at that time.

Collecting field trips were kindly allowed until about 2000 when someone violated the rules, closing the site for all but a few with special permission.

Well known Connecticut locality for prehnite. Three varieties are found - the standard green, rare yellow prehnite, and the extremely rare white prehnite that is almost completely free of iron impurities. Particularly noteworthy are prehnite floater specimens called "hearts". According to Garabedian (1998) these formed via a 5-step process:

1. Chalcedony precipitation on vesicle walls as "fortification" agate.
2. Chalcedony (or partial white, fine-grained, chalky quartz replacement of initial chalcedony) is replaced by fine-grained, white, granular datolite.
3. Prehnite encrusts and/or partly replaces the datolite replacement.
4. Datolite dissolves, leaving a floater of prehnite with large crystals on the inside (small ones may be present on the outside where the datolite was partly replaced by initial prehnite).
5. More prehnite forms on the outside of the prehnite floater, resulting in aggregates with large prehnite crystals on both sides.

Examples of all the intermediate steps can be found in the vesicles.

Other noteworthy pseudomorphing described by Garabedian (1998) include:

- Pumpellyite replacement of chalcedony.
- Tabular anhydrite crystals epimorphed by chalcedony, quartz, a trapezohedral zeolite, or pumpellyite; the anhydrite later dissolves.
- Prehnite or pumpellyite encrust a trapezohedral zeolite (wairakite or analcime), which later dissolves, and more prehnite or pumpellyite partly or completely fill in the void.
- Sequential "water level" vesicle fillings by thin layers of ferroan calcite that are later epimorphed by quartz, chalcedony, datolite or pumpellyite.

Late forming minerals include pyrite, gemmy green sphalerite crystals to about 6mm, calcite as dogteeth or various rhombohedra, apophyllite, babingtonite, julgoldite/pumpellyite, drusy quartz (some amethystine) and zeolites.

Faulting created brecciated zones and cross-cutting veins filled with coarse-grained, columnar, parallel-growth calcite the grew from opposite walls and commonly stained with red, earthy hematite. These veins can reach around 1 meter thick.

Mineral List

23 valid minerals.

Regional Geology

This geological map and associated information on rock units at or nearby to the coordinates given for this locality is based on relatively small scale geological maps provided by various national Geological Surveys. This does not necessarily represent the complete geology at this locality but it gives a background for the region in which it is found.

Click on geological units on the map for more information. Click here to view full-screen map on

Early Jurassic
174.1 - 201.3 Ma

ID: 2767078
Holyoke Basalt

Age: Early Jurassic (174.1 - 201.3 Ma)

Stratigraphic Name: Holyoke Basalt

Description: Greenish-gray to black (weathers bright orange to brown), fine- to coarse-grained, grading from basalt near contacts to gabbro in the interior, composed of pyroxene and plagioclase with accessory opaques and locally olivine or devitrified glass.

Comments: Part of Central Lowlands; Newark Terrane - Hartford and Pomperaug Mesozoic Basins. Part of Newark Supergroup (Upper Triassic and Lower Jurassic). Part of Meriden Formation of Krynine (1950) (Lower Jurassic); CT005.

Lithology: Major:{basalt,gabbro}

Reference: Horton, J.D., C.A. San Juan, and D.B. Stoeser. The State Geologic Map Compilation (SGMC) geodatabase of the conterminous United States. doi: 10.3133/ds1052. U.S. Geological Survey Data Series 1052. [133]

Ordovician - Neoproterozoic
443.8 - 1000 Ma

ID: 3190671
Precambrian-Phanerozoic sedimentary rocks

Age: Neoproterozoic to Ordovician (443.8 - 1000 Ma)

Lithology: Mudstone-carbonate-sandstone-conglomerate

Reference: Chorlton, L.B. Generalized geology of the world: bedrock domains and major faults in GIS format: a small-scale world geology map with an extended geological attribute database. doi: 10.4095/223767. Geological Survey of Canada, Open File 5529. [154]

Data and map coding provided by, used under Creative Commons Attribution 4.0 License

This page contains all mineral locality references listed on This does not claim to be a complete list. If you know of more minerals from this site, please register so you can add to our database. This locality information is for reference purposes only. You should never attempt to visit any sites listed in without first ensuring that you have the permission of the land and/or mineral rights holders for access and that you are aware of all safety precautions necessary.


Sort by Year (asc) | by Year (desc) | by Author (A-Z) | by Author (Z-A)
Silliman, Benjamin. (1819), New localities of agate, chalcedony, chabazite, stilbite, analcime, prehnite, etc. American Journal of Science: 1: 134-135.
Brunet, William. (1977), Mineral Collecting in Woodbury-Southbury, Connecticut. Rocks and Minerals: 52(4): 182-183.
Weber, Marcelle H. and Earle C. Sullivan. (1995), Connecticut Mineral Locality Index. Rocks & Minerals (Connecticut Issue): 70(6): 407.
Garabedian, James A. (1998), Secondary Mineralization of Half-Moon Vesicles in the Mesozoic Basalt of the O&G#2 Quarry, Woodbury, Connecticut. University of Connecticut Master of Science Thesis.
Vajdak, Josef. (1998): New Mineral Finds in the Second Half of 1997, News from Josef Vajdak of Pequa Rare Minerals and Metals. Mineral News: 14(1): 4.
Moore, Thomas. (2007): What's New in Minerals. Mineralogical Record: 38(3): 214.

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