VOLCANOS & VOLCANIC MINERALS AND MATERIALS
There are basically 3 types of volcanos - based on the chemistry of the rocks they
produce. What we are going to do is describe a bit about each type of volcano then look more
closely at the chemistry and major minerals and then at the accessory minerals, possible ores
and other things that you might want to collect.
There are 3 main types of volcanos and 3 rare types
The common ones are:
1) the basaltic shield volcanoes like Hawaii
2) the andesitic composite cones of the island arcs
3) the rhyolitic explosive ash cones for the continental arcs
together these make up most of the world's volcanoes.
The 3 rare types are:
1) the Ultramafic diapirs
2) the ultra alkaline diapirs
3) the carbonatites (volcanic limestones)
we will look at these weird fellows in the next section.
We separate the general chemistry based on the amount of silica in the rocks - basalts have
45-55%, andesites have 55-62%, and the rhyolites have >62% silica, there are some other differences as well - basalts are rich in iron, magnesium, calcium while rhyolites are rich in aluminum, potassium and sodium. All volcanic materials tend to be very fine grained due to the rapid cooling involved which doesn't provide the time needed for larger crystals to form.
While its possible (in theory at least) to get all three types of magma from a single melting
normally they form at different places and by slightly different means.
Basalts form where mantle currents lift the crust up lowering the pressure and causing the upper
mantle to partially melt in the low pressure these start as somewhat alkaline (sodium [Na)] and
potassium (K]) rich and then as melting goes on they become more typical and calcium rich. As
they end their lives the tend to become alkaline again - sometimes greatly so.
Andesites form along subduction zones where oceanic crust is being "subducted" (sucked back
into the earth). Fairly rich in water they are a mix of melted sediments and oceanic (basaltic)
material so their compositions are intermediate between the two ends.
Rhyolitic volcanos occur on continental arcs like the Andes and the Cascades where oceanic
materials are forced under continental In this case the original material is effectively mixed or replaced by crustal material that melts as the original material plates out under the lighter crust.
Due to the high levels of silica and vapors in rhyolitic magmas and the lower temperature they
have, they tend to be very pasty and explosive where the basalts are very fluid and the andesites
are (again) intermediate between them
All this molten material starts to form mineral crystals as it cools and rises so we can tell the
types apart by the minerals found in them. They also give them distinct color differences -
basalts are black to grey due to the high levels of iron rich minerals while rhyolites are pinks to
light greys from the low temperature feldspars they are rich in.
This series of differential crystallization steps is known as the Bowen series after the guy who
first described it. The iron minerals for a sequence of different minerals while the aluminum
silicates form a continuous series called the plagiolcase feldspars.
In the iron rich the sequence is (from hottest to coolest):
olivine --> pyroxenes -> calcium pyroxenes-->amphiboles -->biotite mica
In the feldspar the high temperature ones are:
calcium feldspars --> sodium feldspars --> potassium feldspars
after that whatever is left (if anything) will form Muscovite mica and quartz.
So the basalts are made up of mostly pyroxenes (augite primarily) and calcium rich plagioclase
feldspar with a little bit of olivine. They may also have some amphibole (hornblende) and/or
quartz. The andesites are primarily a mix sodic plagioclase with augite and hornblende and
some quartz. The rhyolites are mostly potassium feldspar and quartz with some biotite and
most of these minerals form very small (often microscopic) crystals in the rocks but they can be
seen sometimes. When they can they have the following general appearances:
Olivine: clear to slightly cloudy glassy green to yellowgreen grains a few mm to a couple of cm across.
Pyroxene: (augite) generally flat black/dark-brown-to-blue blocky grains with a rectangular or square (90* angle) cross sections
Amphibole (hornblende) forms dark diamond cross section (60*-120* angles) needles and plates.
Biotite: forms dark green to brown sheets
Plagioclase feldspars all form grey plates or blocks
Potassium feldspars form white to pinkish plates or blocks when visible
Quartz (if visible) forms clear to gray-white blebs around any and everything else.
Generally of more interest to the collector are the various accessory minerals in the rocks
BASALT ACCESSORIES and Other collectibles
1) olivine - as described above when the xstals are large and well formed and solid this is the
August birth stone Peridot
2) Spinels oxides of iron and other metals - Spinel is MgAl2O4, Magnetite is Fe3O4,
Chromite is FeCr2O4, ilmenite is FeTiO3
3) apatite - the repository for any phosphate in the rock
4) pyrite and other sulfides
In addition most basalts are filled with bubbles, cracks, crevices and such that provide a means
for water and mineralizing fluids to pass thru and react with the rocks so number of secondary
minerals are frequently found associated with them:
5) zeolites - lots of different types
6) carbonates especially calcite (CaCO3) , dolomite ((Ca,Mg)CO3) and siderite (FeCO3)
7) chalcedony and/or opal
In addition the eruptions of basalts can produce some special products that are not found at other
types of volcanos -
8) Pele's tears - drops of frozen basaltic glass thrown out by fountains of lava or lava bubbles.
Pele's tears are normally about 2-5 mm long
9) Pele's hair - natural fiberglass - thin fibers of volcanic glass created as bubbles or molten
pieces pull apart like taffy
: 10) tachylite - basaltic volcanic glass - forms rinds on the outermost surface of the lava flows
where they chill in the air
11) black sand - shattered basaltic glass from the sudden quenching of lava in water at the edge
of the ocean or by air quenching of fountain lava droplets
12) limu-a-Pele (Pele's seaweed) thin translucent sheets of basaltic glass from bursting lava
bubbles - think of the small sheets of rubber left when a balloon bursts when it gets too
Essentially the same minerals as in the basalts except no olivine.
1) sphene is rarely present in small visible grains scattered in the rock
In addition to the list of minerals already named the following can sometimes be found in
rhyolites as "thumbnail or micro mount specimens"
1) Zircons -zirconium oxide this is the only ore mineral for this metal - but its
mined from other types of deposits
2) Beryl - beryllium silicate - again the only ore for the metal - but not the right rock type
3) Cassiterite - tin (Sn) oxide - not mineable either
In addition the eruptions of rhyolites can produce some special products that are not found at other types of volcanos -
5) pumice - rhyolitic foamed glass - the bubbles take up so much space and in most cases are
closed off so this material normally floats
6) obsidian volcanic glass - sometimes clear enough to see thru in small sections and most of it
is translucent. Its actually divided into 3 different types based on luster and water content.
A) obsidian - vitreous (glassy) luster and under 2% water - this can form large masses that were
used to make arrowheads and are now used to make scalpels for eye surgery
B)Pitchstone - resinous (oily) luster and 5-8% water
C)Perlite - pearly luster and 2-5% water in most cases this farms fractured balls of glass in the
rock with the centers of the balls still unfractured and looking much like the glassier
obsidian - these centers are what we call apache tears
In the last 20 years with the discovery of the undersea smokers and the deposits of minerals
found at spreading centers and some other places we have sorted out (more or less <G>) some things about the formation of what are called stratabound volcanogenic massive sulphide deposits these are associated with submarine deposits of many ages. The main sulfide ore minerals are:
1) chalcopyrite (Fe, CU)S
2) spahlerite ZnS
3) galena PbS
4) enargite and Pyrargyte AgS
5) bornite (Cu,Fe)S
the common waste (gangue) materials at these locations are pyrite, quartz, barite, gypsum, and
Questions and Answers
1) Re obsidian being divided into 3 types based on luster and water content, then perlite is actually obsidian? or is it just the centers that are?
A) all three are actually obsidian. The water content has an effect on the luster and on the strength of the material
2) Where do the sheens and snowflakes and the mahoganies, etc. etc. come from? Different
minerals and cooling process I assume?
A) The sheen is that luster and so is based on the water content
3) so how does the golden sheen differ from like rainbow..has something to do with the
A) The snowflakes and other things are dependant on the types of crystals trapped inside
the glass and their sizes and orientations - also in the case of the snowflakes on
secondary mineralization of micro fractures.
Website created: January 22, 1998