That Rocks: Muscovite

Known also as Common Mica or Isinglass is a common mineral found in igneous, metamorphic and sedimentary rocks. Muscovite occurs in a variety of brown, gray, yellow and smoky green colors but can also be found in transparent and translucent forms.

Muscovite is used for a range of commercial applications as the mineral forms in natural sheets and is an excellent insulator of electricity and heat. Cleaved into super thin sheets, muscovite has been used for lining the viewing windows of industrial furnaces and also for lining the viewing doors of old microwave ovens.

The name muscovite derives from the term Muscovy-glass as sheets of this minerals have been used as windows in Russia. The base of this names come from the Latin word "micare" which means "to shine".

Muscovite has a Mohs hardness of 2-2.25.

A sheet of muscovite is on display in the Science Alcove in the Science Library at MOSI.

This Rocks! Herkimer Diamonds

Tucked away in a lesser known gallery of MOSI you can find these gems glittering away.

Crystal Diamonds:

Known as Herkimer diamonds, these beauties are double terminated quartz crystals that each have 18 facets. The crystals are known for their clarity and became referred to as "diamonds". The most perfect crystals are usually less that 1/2 inch in length and the crystals are often found in clusters sometimes attached to each other. A Herkimer diamond scores a 7 on the hardness scale whereas a true diamond is a perfect 10. Large pockets of these crystals were discovered in the exposed outcroppings of dolostone in the late 1700's in several areas of up-state New York particularly Little Falls in Herkimer County and also along the Mohawk River Valley.

Making Herkimer Diamonds:

Approximately half a billion years ago the shallow Cambrian Sea lapped against the base of the ancestral Adirondack Mountains in the area now known as Herkimer County, New York. Limey sediments slowly accumulated and became compacted by more and more sediment under the salty waters where they formed into a rock strata of dolostone. Dolostone is a sedimentary carbonate rock rich in mineral domomite and is often known as dolomite rock. Still beneath the ocean, water seeped through this rock and dissolved away pockets known as "vugs". Inside these vugs the tiny crystals began to form, often thousands in one place.

Unusual Crystals:

Although many Herkimer diamonds are perfectly clear and glitter just like their namesake, some of the crystals form with impurities inside. Some contain anthraxolite which is decayed plant matter that appears black and coal-like in color. Some crystals contains tiny pockets of water inside and are known as enhydro crystals.

Other crystals may contain various impurities that give them a smoky color. Some, known as phantom crystals, have one crystal that has grown around another so that both crystals can be seen, one inside the other. Other usual crystals have grown together so that two or more crystals are fused together.

Several of our crystals on display contain anthraxolite and one has a distinctly smoky color.

See them at MOSI:

To see our collection of Herkimer Diamonds head into the Science Library just off of the MOSI Grand Lobby. The Science Alcove collections are located at the back of the library on the right hand side. Minerals, fossils, shells, vintage calculators, bird eggs, shark teeth and mounted butterflies are just some of the collections on display. The Herkimer diamonds are displayed on a black cloth on the bottom shelf of the display cabinets. Look for the sparkle!