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Accessory Rock Forming Minerals
Igneous and Sedimentary

Our last topic was the Major rock forming minerals, this week we are going to be looking at the accessory rock forming minerals (Igneous and Sedimentary) - and their uses.

logs of all classes are available at:


Before we start on the minerals tho I would like to review some of the diagnostic methods that I'll be talking about the main tools are color, streak, hardness, density, and crystal form/habit.

Color is the normal color(s) the mineral is found in - most are fairly variable but do have one or a few main colors.

{T} like emerald is beryl but green makes it an emerald?

yes like Beryl - its always beryl but if rich green its emerald, and if blue its aquamarine

{T} cool

Streak is the color of the powdered mineral - and is constant - but many have a white streak so it will be mentioned when its other than white *G*

Hardness is measured on Mohs scale with talc at 1 and diamond at 10; the higher the number the harder the mineral. We usually use a simple field scale:

Vsoft (under 2.5) - scratched by a fingernail

soft - 2.5 - 5.5 - scratched by a hard knife edge

hard (5.5 - 7) scratched by Quartz

very hard (7+) scratches quartz

Density (or specific gravity) is a measure of mass per unit volume, and again we can get exact numbers or use a relative field scale - I will mention just the number but the scale reads like this:

Very light- under 2.5 gm/cc

light - 2.5 - 3.3

heavy - 3.3 - 5

Very heavy 5 - 10

ultra heavy - 10+

Cleavage and fracture will be mentioned where appropriate - fracture is an uneven breaking where cleavage is an (attempt) to break along smooth planes.

Crystal form/habit is the normal shape or form of the mineral when its found in clear (collectable) crystals.

The last thing that I will provide with each mineral is the chemical formula - so we can see about what its made of. Some of these minerals have industrial uses and others have geological uses and where appropriate I'll try to describe them and provide some ideas of prime sites to trade for or collect from.

Sedimentary Accessories

Sylvite (KCl)
- better known to us as epsom salt or as sodium free salt. it is V soft (2) and very light (2); forming diamond shaped crystals and like Halite it dissolves in water with a salty taste.

Fluorite (CaF2)
H: 4, D: 3.2 - fluorite forms cubic crystals of many colors and has a perfect octahedral (8 sided) cleavage - it is the primary ore for fluorine and is normally flourescent under a UV light glowing a rich blue color. Fluorite also has uses in spectrography and optics. Good locations include: Bolzano Italy, Switzerland, Brazil, Konigsberg Norway and several places in Canada and Cornwall England.

Aragonite (CaCO3)
H: 3.5-4 D: 2.9-3 - prismatic cleavage, soluble in HCL - with bubbles forming. Aragonite can be separated from Calcite primarily by its crystal form so only where the xstals can be seen can we normally be sure of which we have collected. Aragonite is strictly a collectors mineral. Over time it normally converts to calcite wich is the industrial mineral.

Apatite (Ca, Na, K)5(PO4)3(F, Cl, OH)
H: 5, D: 3.1-3.2 - Apatite is found in both igneous and sedimentary rocks. In sedimentary form Apatite consist of phosphatic remains of organisms (fossils) and are mined as phosphate fertilizer in places like Florida. Apatite forms hexagonal prismatic xstals and is often an olive green color. Good Apatite xstals are more generally from igneous rocks rather than sedimentary while commercial deposits are primarily sedimentary the better known crystal sites include Durango, Mexico; Auburn, Maine; Wilberforce, Ontario; and Saxony. Crystals usually form prisms with off center pyramidal single terminations

Igneous Accessories

Pyrite (FeS2)
Cubes and Pyritohedrons H: 6-6.5 D: 5.1 - Streak: green-black Color: yellow - metallic. Pyrite is the most common of the sulfide minerals and is found in small amounts in almost all igneous rocks and many sedimentary rocks normally it is of no commercial value - but on occasion deposits rich in pyrite have been used as iron ore - especially in war times when no other ores were available. Such pyrite mines and the smelter remains can be found in Alabama and the southern Appalachians. the Montgomery AL steel industry was founded on super rich pyrite ores. Pyritohedrons are easily recognized as they show pentagonal faces and have 12 sides.

Magnetite (Fe3O4)
opaque black octahedron (diamond shaped) H: 6; D: 5.2 it is found in all igneous rocks and in most metamorphic and clastic sedimentary rocks. In large concentrations magnetite is the ideal iron ore - and preferred above all other non-recycled ores. Well known deposits of magnetite include Kiruna Sweden, Bushveldt SA, the Adirondacks in NY, and as lodestone (magnetic form) at Magnet Cove Arkansas. Magnetite forms the primary mineral in almost all black sands

Ilmenite (FeTiO3)
H :5-6; D: 4.5 - 5 - Ilmenite is the primary ore of titanium - it forms hexagonal platy crystals/grains and is found in most basic igneous rocks. It is collected from beach sands in Australia and Florida for use in ores. Large crystals (1 in, 2.5 cm) are found in Orange and Warwick counties NY.

Zircon (ZrSiO4)
H: 7.5 D: 3.9 - 4.8 - Zircon is found in most acidic igneous rocks and many metamorphic rocks made from them. It forms stubby diamond like bipyramids - they can range from clear and admantine to opaque black when contained radioactive elements have destroyed the crystal structure over billions of years. It is the only ore mineral for zirconium and hafnium and is sometimes used for thorium as well. The ore deposits are concentrated beach sands in Florida and Australia primarily. The oldest known sample of crustal material is an Australian Zircon dated to 4.1 Billion years old

{A} hmmmm ... that's a fair bit old

{T} That is almost the beginning of time isn't it?

Yep it was found in a sandstone with microbial fossils dated to 3.8 Billion years. The meteorites and moon show that the planets formed about 4.6 BY ago, so that zircon is from the first crustal material to ever form effectively :). Gem crystals are found in Sri Lanka, the Urals, Litchfield Maine, Renfrew Canada and in North Carolina, Norway, and Sweden.

Epidote Ca2 (Al, Fe)3Si3 O12)
H: 6-7 D: 3.3 - 3.5 color: green-gray, columnar striated crystals epidote is a common accessory of almost all rocks. Occasionally used s a gemstone it is primarily a collectors mineral. Better known locations include: Haddam Conn, Riverside CA, Isere France, Arendal Norway, and Woburn Mass.

The last of the igneous accessory minerals are members of the garnet group which also contain a number of Metamorphic Accessories. All garnets share the same crystal forms forming dodecahedral (20 sided) or trapezohedral crystals that are almost instantly recognizable. They also share similar hardness and density (H: 6.5 - 7.5, D: 3.6) the main igneous Garnets are:

Pyrope (Mg3Al2(SiO4)3
blood red to violet - Pyrope is the carbuncle of the middle ages and with spinel forms many of the large "rubies" in European royal Regalia.Good locations include: Gordono Switzerland, Bohemia, Arizona and New Mexico, and Scotland.

Uvarovite: (Ca3Cr2(SiO4)3
Emerald green due to the Chrome they are found chrome rich serpentines. The best crystals are from the Urals, Finland and Turkey.

next week: Accessory Rock Forming Minerals - Metamorphic

Website created: January 22, 1998
Website last updated: October 11, 1998