Grand Manan minerals




Many good examples of common minerals and rocks are found on Grand Manan. In the metamorphic formations of the eastern third of the island, people find nice crystals or masses of calcite, quartz, pyrite, barite, galena, and magnetite. In the Jurassic basalts of the western 2/3 of the island, collectible minerals are especially common in the middle member called Seven Days Work (named after its type locality). These thin lava flows were gas-rich, forming many bubbles that later filled with a variety of zeolites, amethyst, chalcedony, jasper, blue agate, calcite, prehnite, pumpellyite, native copper, and no doubt many other types.

Our fine Grand Manan Museum has a good collection of local minerals on display. Take your family and friends from away, when they visit. Or just treat yourself, the next time you are going to the library or dollar store anyway. 

In this section are photos of minerals and rocks that might be of interest to collectors or mineralogists, or to people who just appreciate their interesting colors and forms. The photos are thumbnails, so just click on them to get the full sized image.

This piece of basalt shows fluting and pipe amygdules, from Seven Days Work near Eel Brook Beach. The horizontal ridges are grooves formed by the lava as it flowed 201 million years ago. The "pipes" run across the end, and are made by gases that boiled out of the lava and streamed upward, forming finger-like tubes. The lava froze around these elongated bubble tubes. Later the tubes filled with minerals such as white calcite (calcium carbonate), and sometimes pink zeolites (silicate minerals with water molecules in their structures).  Basalt is the type of volcanic rock we have on Grand Manan. Because it makes up much of the ocean crust, it is the most abundant rock type on earth. Our formation of it is called North Mountain Basalt.

I cut this sample from Seven Days Work so that it would stand with its pipe amygdules about the way they looked in the lava flow, or nearly vertical.  The basalt is very altered to a greenish color by water and carbon dioxide, which were abundant gases in these lavas. Under pressure deep in the Earth, gases stay dissolved in the magma, but on the surface they boil out. Some of these gases can be toxic, such as sulfur, chlorine, and fluorine.

This is what pipe amygdules look like in outcrop, near the northern end of Seven Days Work. The bottom of the lava flow in this large boulder is to the upper left (the boulder is mostly overturned). Pipes don't start at the very bottom but rather a little way up into the lava, forming just as it starts to solidify. If it is still moving, the pipes bend, which can indicate the direction the lava was flowing.

More pipes. This sample is held upside down -- pipe amygdules often merge together as they rise upward. Note the little ends of red jasper (chalcedony made red by a little iron) on some of the pipes, which are filled by calcite. Seven Days Work.

Amygdules with jasper are a bit curious. The division line between the jasper and calcite or other minerals is usually very sharp and straight, as you can see here. It is almost as if one was a liquid floating on the other -- perhaps that is the actual way it formed.

My wife Nancy is also a geologist and goes with us to look at rocks. She picked this up near the northern end of Seven Days Work, if I recall correctly. We were all jealous. I think the little squared-off pink crystals are chabazite and the clear plate-shaped ones are heulandite (both are zeolites), perhaps with some calcite as well. But I need to check my references. Good crystals need space to grow, and these probably had only fluids or gases to compete with as they formed.  

This is an unusual zeolite mineral called scolecite, which is hydrated calcium aluminum silicate (chemical formula CaAl2Si3O10-3H2O).  Its crystal form is in white or pale fleshy masses of long thin needles that radiate from a point. From Seven Days Work south of Eel Brook Beach. Like most zeolites, it formed in a gas bubble, or vesicle. A vesicle filled with minerals is called an amygdule. Volcanic rocks rich in vesicles or amygdules are said to be vesicular or amygduloidal. See, you just learned several new geological terms to impress your friends when you meet them at Home Hardware or the Kwik-Way.

Something about zeolites makes them particularly interesting. Perhaps it is their crystalline shape. In this small sample, the needle-shaped (acicular) fans of scolecite are a bit more pink in color. I will try to find and photograph a few different zeolites on the island...I am sure there are others.

Another nice example of scolecite filling spaces in a vesicular basalt from Seven Days Work. But enough of gas bubbles already, let's move on to some other minerals.

In a very few places, such as near the intrusion east of Indian Beach, red mud was injected into cracks in the basalt, and lithified into mudstone. It appears to be full of little crystals of plagioclase feldspar, so the mud was probably formed by weathering and erosion of basaltic lava, probably a flow now in the Seven Days Work member. Here the mud was injected upward into the cracks under pressure, but in Nova Scotia, similar mud-filled cracks appear to have been filled from above. Iron in the basalt oxidized to make the red color. The basalt tends to break along these weak planes, showing its sides.

This is a "plate" of specular hematite, or iron oxide (Fe2O3). I found it on the beach at Bradford Cove. A similar sample in the Grand Manan Museum is also from Bradford Cove, so there might be a vein of this mineral within the lava flows at that cove (another part of the Seven Days Work member). It glitters with shiny metallic flakes, is quite heavy, and gives a rusty red streak when rubbed on a hard surface. There are pieces of weathered basalt stuck in it as well as some small areas of quartz minerals.

Beneath the basalts of the Fundy Basin is the Blomidon Formation, which is latest Triassic in age, and sedimentary in origin. For the most part it is fine grained sandstone, or siltstone, with some layers a bit coarser. Similar sediment collects today in playas, or lakes in semi-arid regions that dry up each summer. The water can become very saline (brine) due to constant evaporation and concentration of salts and minerals dissolved in the water. Minerals such as halite (sodium chloride, or "salt") and gypsum (calcium sulfate) can crystallize, forming patches in the mud as it dries up. That is what we see in this sample from Dark Harbour, from just north of the opening in the sea wall. The Blomidon is exposed at the base of the cliffs for a few km north and south of Dark Harbour.

The eastern third of the island is made of much older metamorphic rocks. Many were once sedimentary but have been recrystallized by heat and pressure from deep burial (as much as 10 kilometers or even more). Organic materials become carbonized and eventually turned into graphite. Fine-grained micas grow in planes or layers in this rock, so we can call it graphite schist. Sulfur combines with iron to make pyrite, or "fools gold" in such rocks, which later can cause the small rusty patches. This sample is from a fault zone at the northern end of Pettes Cove.