dlogic.gif (44524 bytes)

blueston.gif (6585 bytes)


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 hornblende.

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 big

Andesite Accessories

Essentially the same minerals as in the basalts except no olivine.

1) sphene is rarely present in small visible grains scattered in the rock

Rhyolite Accessaries

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 arsenopyrite


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
Website last updated: October 11, 1998