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Porter Square, Cambridge, Middlesex Co., Massachusetts, USAi
Regional Level Types
Porter Square- not defined -
Cambridge- not defined -
Middlesex Co.County

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Latitude & Longitude (WGS84): 42° 23' 18'' North , 71° 7' 9'' West
Latitude & Longitude (decimal): 42.38833,-71.11917
GeoHash:G#: drt3jss9g
Köppen climate type:Dfa : Hot-summer humid continental climate
Nearest Settlements:
Somerville80,318 (2017)1.6km
Cambridge110,402 (2017)1.8km
Medford57,403 (2017)3.5km
Arlington42,844 (2017)4.3km
Belmont24,729 (2017)5.0km

A camptonite dyke was exposed during excavation of a subway station. Xenoliths in the dyke included granulite, dunite, lherzolite, werhlite, harzburgite, and pyroxene peridotite.

Regions containing this locality

North America PlateTectonic Plate

Select Mineral List Type

Standard Detailed Strunz Dana Chemical Elements

Mineral List

7 valid minerals.

Detailed Mineral List:

Formula: Na(AlSi3O8)
Albite var: Andesine
Formula: (Na,Ca)[Al(Si,Al)Si2O8]
Formula: Ca(Al2Si2O8)
Formula: (CaxMgyFez)(Mgy1Fez1)Si2O6
Formula: CaCO3
Formula: {Na}{Ca2}{Fe2+3AlTi}(Al2Si6O22)O2
Formula: {Na}{Ca2}{Mg3AlTi}(Al2Si6O22)O2
Formula: (Ca,Na)[Al(Al,Si)Si2O8]
Formula: Fe2+Fe3+2O4
Formula: Ca0.25(Mg,Fe)3((Si,Al)4O10)(OH)2 · nH2O
Saponite var: Bowlingite
Formula: Ca0.25(Mg,Fe)3((Si,Al)4O10)(OH)2 · nH2O
'Smectite Group'
Formula: A0.3D2-3[T4O10]Z2 · nH2O

List of minerals arranged by Strunz 10th Edition classification

Group 4 - Oxides and Hydroxides
Group 5 - Nitrates and Carbonates
Group 9 - Silicates
var: Andesine9.FA.35(Na,Ca)[Al(Si,Al)Si2O8]
Saponite9.EC.45Ca0.25(Mg,Fe)3((Si,Al)4O10)(OH)2 · nH2O
var: Bowlingite9.EC.45Ca0.25(Mg,Fe)3((Si,Al)4O10)(OH)2 · nH2O
Unclassified Minerals, Rocks, etc.
'Smectite Group'-A0.3D2-3[T4O10]Z2 · nH2O

List of minerals arranged by Dana 8th Edition classification

Group 65 - INOSILICATES Single-Width,Unbranched Chains,(W=1)
Single-Width Unbranched Chains, W=1 with chains P=2
Group 66 - INOSILICATES Double-Width,Unbranched Chains,(W=2)
Amphiboles - Mg-Fe-Mn-Li subgroup
Group 71 - PHYLLOSILICATES Sheets of Six-Membered Rings
Sheets of 6-membered rings with 2:1 clays
Saponite71.3.1b.2Ca0.25(Mg,Fe)3((Si,Al)4O10)(OH)2 · nH2O
Group 76 - TECTOSILICATES Al-Si Framework
Al-Si Framework with Al-Si frameworks
Unclassified Minerals, Mixtures, etc.
var: Andesine-(Na,Ca)[Al(Si,Al)Si2O8]
var: Bowlingite
-Ca0.25(Mg,Fe)3((Si,Al)4O10)(OH)2 · nH2O
'Smectite Group'-A0.3D2-3[T4O10]Z2 · nH2O

List of minerals for each chemical element

H Smectite GroupA0.3D2-3[T4O10]Z2 · nH2O
H Saponite (var: Bowlingite)Ca0.25(Mg,Fe)3((Si,Al)4O10)(OH)2 · nH2O
H SaponiteCa0.25(Mg,Fe)3((Si,Al)4O10)(OH)2 · nH2O
C CalciteCaCO3
O Kaersutite{Na}{Ca2}{Mg3AlTi}(Al2Si6O22)O2
O Albite (var: Andesine)(Na,Ca)[Al(Si,Al)Si2O8]
O Labradorite(Ca,Na)[Al(Al,Si)Si2O8]
O MagnetiteFe2+Fe23+O4
O Augite(CaxMgyFez)(Mgy1Fez1)Si2O6
O Smectite GroupA0.3D2-3[T4O10]Z2 · nH2O
O Saponite (var: Bowlingite)Ca0.25(Mg,Fe)3((Si,Al)4O10)(OH)2 · nH2O
O CalciteCaCO3
O AlbiteNa(AlSi3O8)
O Ferro-kaersutite{Na}{Ca2}{Fe32+AlTi}(Al2Si6O22)O2
O AnorthiteCa(Al2Si2O8)
O SaponiteCa0.25(Mg,Fe)3((Si,Al)4O10)(OH)2 · nH2O
Na Kaersutite{Na}{Ca2}{Mg3AlTi}(Al2Si6O22)O2
Na Albite (var: Andesine)(Na,Ca)[Al(Si,Al)Si2O8]
Na Labradorite(Ca,Na)[Al(Al,Si)Si2O8]
Na AlbiteNa(AlSi3O8)
Na Ferro-kaersutite{Na}{Ca2}{Fe32+AlTi}(Al2Si6O22)O2
Na SaponiteCa0.25(Mg,Fe)3((Si,Al)4O10)(OH)2 · nH2O
Mg Kaersutite{Na}{Ca2}{Mg3AlTi}(Al2Si6O22)O2
Mg Augite(CaxMgyFez)(Mgy1Fez1)Si2O6
Mg Saponite (var: Bowlingite)Ca0.25(Mg,Fe)3((Si,Al)4O10)(OH)2 · nH2O
Mg SaponiteCa0.25(Mg,Fe)3((Si,Al)4O10)(OH)2 · nH2O
Al Kaersutite{Na}{Ca2}{Mg3AlTi}(Al2Si6O22)O2
Al Albite (var: Andesine)(Na,Ca)[Al(Si,Al)Si2O8]
Al Labradorite(Ca,Na)[Al(Al,Si)Si2O8]
Al Saponite (var: Bowlingite)Ca0.25(Mg,Fe)3((Si,Al)4O10)(OH)2 · nH2O
Al AlbiteNa(AlSi3O8)
Al Ferro-kaersutite{Na}{Ca2}{Fe32+AlTi}(Al2Si6O22)O2
Al AnorthiteCa(Al2Si2O8)
Al SaponiteCa0.25(Mg,Fe)3((Si,Al)4O10)(OH)2 · nH2O
Si Kaersutite{Na}{Ca2}{Mg3AlTi}(Al2Si6O22)O2
Si Albite (var: Andesine)(Na,Ca)[Al(Si,Al)Si2O8]
Si Labradorite(Ca,Na)[Al(Al,Si)Si2O8]
Si Augite(CaxMgyFez)(Mgy1Fez1)Si2O6
Si Saponite (var: Bowlingite)Ca0.25(Mg,Fe)3((Si,Al)4O10)(OH)2 · nH2O
Si AlbiteNa(AlSi3O8)
Si Ferro-kaersutite{Na}{Ca2}{Fe32+AlTi}(Al2Si6O22)O2
Si AnorthiteCa(Al2Si2O8)
Si SaponiteCa0.25(Mg,Fe)3((Si,Al)4O10)(OH)2 · nH2O
Ca Kaersutite{Na}{Ca2}{Mg3AlTi}(Al2Si6O22)O2
Ca Albite (var: Andesine)(Na,Ca)[Al(Si,Al)Si2O8]
Ca Labradorite(Ca,Na)[Al(Al,Si)Si2O8]
Ca Augite(CaxMgyFez)(Mgy1Fez1)Si2O6
Ca Saponite (var: Bowlingite)Ca0.25(Mg,Fe)3((Si,Al)4O10)(OH)2 · nH2O
Ca CalciteCaCO3
Ca Ferro-kaersutite{Na}{Ca2}{Fe32+AlTi}(Al2Si6O22)O2
Ca AnorthiteCa(Al2Si2O8)
Ca SaponiteCa0.25(Mg,Fe)3((Si,Al)4O10)(OH)2 · nH2O
Ti Kaersutite{Na}{Ca2}{Mg3AlTi}(Al2Si6O22)O2
Ti Ferro-kaersutite{Na}{Ca2}{Fe32+AlTi}(Al2Si6O22)O2
Fe MagnetiteFe2+Fe23+O4
Fe Augite(CaxMgyFez)(Mgy1Fez1)Si2O6
Fe Saponite (var: Bowlingite)Ca0.25(Mg,Fe)3((Si,Al)4O10)(OH)2 · nH2O
Fe Ferro-kaersutite{Na}{Ca2}{Fe32+AlTi}(Al2Si6O22)O2
Fe SaponiteCa0.25(Mg,Fe)3((Si,Al)4O10)(OH)2 · nH2O

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

298.9 - 323.2 Ma

ID: 3184341
Paleozoic sedimentary rocks

Age: Pennsylvanian (298.9 - 323.2 Ma)

Comments: Narragansett And Boston Basins

Lithology: Sedimentary rocks

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]

Early Cambrian - Neoproterozoic
511 - 1000 Ma

ID: 2919431
Cambridge Argillite

Age: Neoproterozoic to Cambrian (511 - 1000 Ma)

Stratigraphic Name: Cambridge Argillite

Description: Gray argillite and minor quartzite; rare sandstone and conglomerate. Contains acritarchs. Cambridge Argillite of Boston Bay Group contains sandy horizons which are in some places quartzite. Most prominent are Milton quartzite unit of Billings (1976), and Tufts Quartzite Member (described by Billings, 1929, and LaForge, 1932) in northern part of basin. Red sandstone and sandy argillite in Chelsea, Revere, and Milton-Quincy areas intertongue with green argillite (Kaye, 1980). Red beds lie above cleaner quartzites such as Tufts and Milton units. Core analysis by D.A. Ashenden (Metropolitan District Commission, 1980, written commun.) indicates that Cambridge and Braintree Argillites are identical. Age of Boston Bay rocks has been controversial and was once thought to be Cambrian to Pennsylvanian (the latter based on lithologic similarity to rocks of Narragansett basin and now discredited plant fossils). Age of Cambridge and of Boston Bay Group as a whole is Proterozoic Z and possibly Early Cambrian based on presence of acritarchs in Cambridge. Acritarchs are diagnostic species that ranges in age from Proterozoic Z to Early Cambrian, but is most abundant in Proterozoic Z time (Lenk and others, 1982; [also see Goldsmith and others, 1982]). Age is also supported by the following: 1) plant fossils so numerous in strata of Narragansett basin are absent in the Boston Bay Group strata, 2) Late Ordovician and Early Silurian Quincy Granite contains argillite inclusions that are on strike with Cambridge Argillite, and 3) Boston Bay Group stratigraphy is primarily marine, not similar to terrestrial stratigraphy of Narragansett basin (Goldsmith, 1991).

Comments: Part of Milford-Dedham Zone (Tertiary and Older Rocks). Secondary unit description per MA023. Original map source: Unpublished Digital Geologic Map of Massachusetts received from Rudi Hon of Boston College in 1998; based on the 1983 USGS paper version (MA002).

Lithology: Major:{argillite}, Minor:{quartzite}, Incidental:{sandstone, conglomerate}

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]

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


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Ross, Martin E. (2010): An Early Triassic 40Ar/39Ar age for a camptonite dyke in Cambridge, Massachusetts, USA (Atlantic Geology, Volume 46).

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