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Abstract
In 1995 a collapse occurred in Helsinki, Finland, in a 10‐year old tunnel discharging treated wastewater into the Gulf of Finland. The collapse was caused by a lamprophyre dyke containing smectite. The site of the collapse is c. 70 m below the present surface of the bedrock. The rock is typical of the Helsinki area, being composed of Palaeoproterozoic migmatitic granite with portions of plutonic rocks and mica gneiss remnants varying in size and degree of preservation, and dykes of amphibolite, metadiabase, diabase, and lamprophyre. The bedrock is also cut by aplite and pegmatite veins. The age of the collapsed lamprophyre is unknown. Smectite is not a common mineral in the Helsinki area although it has been encountered in some places in recent years.
Suominen, V., 1997: Smectite alteration in a lamprophyre dyke; A case study. GFF, Vol. 119 (Pt. 2, June), pp. 149–150. Stockholm. ISSN 1103–5897.