Mineral of the month: serandite

Posted: January 17, 2012 - 08:47 , by ROM
Collections, Natural History, Mineralogy | Comments () | Comment

specimen image a twinned serandite crystal

World's largest twinned serandite crystal.

This is the first entry in a new series the Earth Sciences section will be running, Mineral of the Month. These blogs will feature remarkable (and perhaps some not quite so remarkable but interesting none the less) specimens from the museum’s world class mineral collection. We will also be featuring monthly meteorites and gems so there will be something for everyone to enjoy.

This is the finest and largest example of a twinned serandite crystal in the world which is on display in the Teck Gallery: Earth’s Treasures. It measures 12.5 by 11.4 by 4 centimetres. In fact, as the name implies, this specimen is composed of 2 large crystals of serandite. Minerals can form twins in many, sometimes quite complex, ways. But in some of the simpler and more obvious instances they form groups such as this where two crystals are growing together in an oriented fashion because they are joined by one common face. The net result is a mirror-image-like effect, as is the case with this specimen. You will notice a distinct line or groove running essentially down the middle from top to bottom. This is the common face of the two serandite crystals. The other minerals associated with it are white natrolite and reddish-brown magan-neptunite.

Serandite is a rare mineral occurring in only a few places on the Earth and then usually only as small, non-descript grains enclosed in rock. However in the late 1960’s, mineral collectors began finding strange crystals of an unusual orange-pink mineral from a quarry outside of Montreal. Analysis quickly showed that these were, in fact, the best crystals of serandite then known. This quarry, operating in the side of a prominent hill known as Mont St. Hilaire, was originally opened for road gravel. Fortuitously the quarry operators had chosen to open their quarry in one of the Monteregian Hills; a series of igneous intrusions that dot the southern Quebec landscape.

Space shuttle image of the Monteregian Hills

Mont St. Hilaire is the first hill on the left of the image, the middle one is Mont Rougemont, the last is Mont Yamaska. Image courtesy of NASA.

The Monteregian Hills are generally composed of rocks with very unusual chemistry. This is particularly the case at Mont St. Hilaire or “the magic mountain’ as it has been called. There, the minerals that comprise the rocks, have high concentrations of so-called alkaline elements such as sodium as well as a host of rare elements such as zirconium, beryllium, yttrium and many others. This unusual chemistry coupled with some geological peculiarities of the intrusion itself provided conditions that were perfect for the formation of a suite of rare, well crystallized minerals unlike any other locality on Earth. With the discovery of those first specimens the quarry became an instant sensation in the mineral collecting world and remained so for over 40 years. It is recognized as the locality for the finest examples not only of serandite but other rare minerals such as catapleiite, magan-neptunite, elpidite, narsarsukite, epididymite and many others. Over 388 different minerals are known from Mont St. Hilaire, 57 of which were first described from there. Some, such as carletonite (which will be featured in future Mineral of the Month blogs along with many other treasures from the magic mountain) are found nowhere else.