Grand Manan
minerals
and
rocks
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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