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E.S. Larsen

Publications and source records attributed to E.S. Larsen.

6 recordsLinked to original sources
USGS1952

Method for determining the age of igneous rocks using the accessory minerals

The age of igneous rocks is determinable by a method based on the included accessory minerals . In the common igneous rocks , most of the lead is concentrated in the potassium minerals , and most of the radioactivity is in the zircon and other accessory minerals . The lead in the potassium minerals is believed to be mostly primary lead; that in zircon is probably chiefly radiogenic lead. By separating the zircon of fresh igneous rocks , determining the amount of lead with the spectrograph and the radioactivity by alpha counters, the age of Paleozoic and Precambrian rocks can be determined with an accuracy of approximately 90 per cent. Basalts and gabbros may contain too few accessory minerals for satisfactory age determination by this method . Zircon is the most satisfactory mineral for this kind of determination. Apatite and sphene give high results and therefore must contain primary lead. Sphene gives erratic results.

Geological Society of America Bulletin↗
USGS1941

Igneous rocks of the Highwood Mountains, Montana: Part VI. Mineralogy

The minerals of the igneous rocks of the Highwood Mountains are described. The primary hornblende of the quartz latites is basaltic and it has been partly replaced by a common green hornblende. Hornblende is rare in the alkalic rocks. Augite is an abundant mineral of the alkalic rocks; in the syenites it contains some acmite, especially on the borders of the crystals. Aegirite is a common metamorphic mineral. The olivine of the shonkinites contains from 12 to 22 mol per cent of fayalite, that of the syenite from Square Butte 50 per cent. Monticellite twinned into trillings on (031) is an abundant mineral in one small intrusive body. In the alkalic rocks barium sanidine is abundant and plagioclase rare; leucite, primary analcime, and pseudoleucite are common and sodalite rare. Natrolite, thomsonite, and other zeolites are widespread and very abundant.

Montana↗