This specimen shows the three basic minerals found in granite: pink microcline feldspar (KAlSi3O8), white plagioclase feldspar (Na,Ca)Al(Si,Al)4O8, and glassy quartz (SiO2).  These minerals crystallized from magma rich in silica, alumina, potassium, sodium and calcium.  The thin green vein is epidote, with a chemical formula of (Ca2Al2(Fe3+;Al)(SiO4)(Si2O7)O(OH).  Epidote was introduced by hydrothermal (hot water) activity after the granite crystallized.

This specimen, from northeastern Ontario, shows pronounced lineations (stripes). The stripes reflect a phenomenon known as crystal twinning, as individual crystals in the specimen have shared boundaries.

As this granite magma moved upward through the Earth’s crust, it moved through basalt and incorporated pieces of basalt before solidifying.

Muscovite is a sheet silicate (mica) with a formula of KAl2(AlSi3O10) (F, OH)2.  Its chemical formula resembles potassium feldspar, but with added fluorine and water.  Muscovite crystals of this size are commonly found in New England pegmatites (coarse-grained granite).  Muscovite is also known as isinglass, and was popular as oven glass in the 19th century.  It is still in use today for that purpose.  I obtained this specimen at the Wellfleet swap shop.

Kimberlites are the most common host rock for diamonds and are intruded explosively as carrot-shaped pipes.  Kimberlites are common in northern Colorado and adjacent Wyoming, and although many contain diamonds, the Green Mountain pipe does not.

The Munro Mine operated from 1950 to 1964, producing asbestos for use as thermal system insulation and a host of consumer products – vinyl floor tile, brakes and clutches, construction mastics, etc.  Chrysotile asbestos is still used in concrete water pipes.  This sample has been sealed with three coats of polyurethane varnish.

Named after Mount Dun in New Zealand, dunite consists principally of olivine (Fe,Mg)2(SiO4) and no quartz. Dunite crystallizes deep in the Earth’s mantle.  The mantle can be scraped up and onto the Earth’s surface through the plate tectonic process of obduction.

In this specimen you can see basalt with a dark border next to the granite.  This means that the basalt intruded the granite, and was chilled by the pre-existing, cooler granite.  Where the basalt was chilled it solidified quickly into smaller, darker crystals.

Anorthosite consists almost entirely of calcium plagioclase.  Anorthosites commonly occur in massifs covering large areas, such as the Adirondack Mountains, the source of this specimen.  The Adirondacks were entirely glaciated during the last ice age, and this boulder came from local outwash deposited in the same fashion as in Wellfleet.

Diorite crystallizes from magma containing less silica than for granite, and consists of mainly sodium-rich plagioclase, dark iron-magnesium minerals such as hornblende, and little quartz.