Mineral Resources of the Central Lowland and Inland Basin

Illmenite is an ore of titanium and was produced for use as a white pigment in paint. Titanium is also an important metal because of its lightweight nature, strength, and resistance to corrosion. As the Grenville rocks were compressed and metamorphosed, magma from the asthenosphere welled up through overlying rocks. The magma crystallized to form the igneous rock, anorthosite. Crystallization, however, did not happen all at one time. In a process known as crystal settling, the dense, heavy minerals crystallized first and sank to the bottom. Illmenite, being a heavier mineral, became concentrated at the bottom of the crystallizing magma to form the large deposits of the ore that we see today.

Aluminum is also mined in the northernmost part of the Inland Basin. Aluminum is a common component of high-grade metamorphic minerals such as kyanite, sillimanite and andalusite.

Other rocks

Iron is also found in Pennsylvania and other parts of New York besides the Adirondacks. In particular, layers of limestone in the Clinton Group of rocks, located in the Silurian deposits at the edge of the Appalachian Plateau, contain deposits of hematite and siderite. These iron-rich layers stretch as far south as Alabama and are important indicators of sea level rise and fall. Hematite forms in shallow ocean water and siderite forms in relatively deeper water.

Wollastonite formed in the Adirondacks when the Grenville limestone was metamorphosed and intruded by magma. Ninety-nine percent of the wollastonite produced in the U.S. comes from New York.

The Adirondacks have also been a leading producer of garnets. Spectacular crystals, as large as 1 m across (though typically 2-2.5 cm across) have been found at the famous Gore Mountain garnet mine.

Though the Gore Mountain mine is now closed, the mine at nearby Ruby Mountain continues to be a leading producer of industrial garnet for use as an abrasive. When the Grenville sediments were compressed and metamorphosed over a billion years ago, the heat and pressure melted the deeply buried rocks to magma. As the magma pushed up through the overlying Grenville marble, gneiss and quartzite, it gradually crystallized to form anorthosite and other igneous rocks. When these igneous rocks were also metamorphosed, the heat and pressure recrystalized some of the rock to form the famous garnets.

Evaporite rocks

The Inland Basin was part of an inland ocean for hundreds of millions of years as the continents pulled apart and pushed together. A shallow restricted sea is the ideal environment for the evaporation of water and deposition of evaporite minerals. Halite (salt) and gypsum are examples of evaporite minerals. The Silurian, in particular, was a time of especially shallow seas with poor circulation in the region. It makes sense, therefore, that salt and gypsum are both found in Silurian sedimentary rocks exposed across central New York. The salt is at the surface, as natural salt springs, around Syracuse, New York. The gentle tilt of the Silurian rocks to the south means that salt is also found and mined underground south of the exposed salt beds, buried beneath Devonian rocks.

Salt has played a key role in the economy of upstate New York, and was the reason for the founding of cities like Syracuse, NY. The Retsof Mine near Geneseo, NY was the largest underground salt mine in the world before its collapse in 1993.

Gemstones

In addition to the abundance of metals and non-metallic minerals produced for industrial use in the Adirondacks, the Grenville rocks also contain the minerals tourmaline and beryl, prized as gemstones. Further south in Herkimer County, New York, in a patch of Cambrian rocks southwest of the Adirondacks, gem collectors seek out "Herkimer diamonds."

Herkimer diamonds are not in fact diamonds at all. Rather, they are very well formed, clear crystals of quartz found in the Little Falls dolostone. Dolostone is made in part of calcite, a mineral that is highly susceptible to reacting chemically with acids. The weakly acidic nature of rainwater and groundwater commonly dissolve away parts of dolostone, leaving open cavities in the rock. As groundwater, rich in silica, moved through the Little Falls dolostone, quartz crystallized in the cavities to form Herkimer diamonds.