Feldspar                    Mineral Group: SILICATES

Feldspar is a very common mineral found in the Earth's crust, making up about 60%
  • It is often seen in road cuts along Ontario highways up North
  • Feldspar tends to be pink.
  • When feldspar is green-blue, it is called amazonite.
  • Labradorite is a well known semi-precious variety that shows dramatic blue and green schiller.
  • Feldspar is a common mineral in many rocks
  • Feldspar is commonly found in granite.
  • Feldspar is the mineral that looks like a flower in Daisy Stone and Chinese Writing Stone
  • Feldspar has perfect cleavage. It will easily break into "rhombs".

Feldspars abundance next to quartz
Bert Ellison  1999 - 2002

For reasons to be explained, this series of minerals has spawned a vast literature. But happily for most of us, the essentials can be sifted rather easily.

The way the books often present the subject is with a triangle. So lets try that. At the top we may visualize potassium (K, for kalium, the original name); at the lower left-hand corner well put Na (for sodium) and at the right hand corner put Ca, for calcium.

The sides of the triangle represent percentages so for example as one moves from the Na corner to the Ca corner we assume zero Na to 100% Ca. This side represents the most important ones for general use, though strictly, this happy simplicity is confounded by the fact that the mixture of Na (AlSi3O8) dubbed Ab (for albite) to Ca (Al2Si2O8) dubbed An (for anorthite) is continuous.

As for the pointy end of the diagram its usually labeled Or for orthoclase, formula K (ALSi3O8).

Further confusion is injected by the effects of pressure and temperature of formation which changes the crystal structure, though a lump of say the potassium feldspar orthoclase may look like a lump of the potassium feldspar microcline.

But have heart, dear readers, all is not obfuscation (?).

The series Ab to An has been arbitrarily sectioned off into six handy blocks depending on the Na (Ab) to Ca (An) content. Ready? albite, oligoclase, andesine, labradorite, bytownite and anorthite. Ab used here stands for albite, a white feldspar rich in sodium (Na). An is used to represent the calcium-rich feldspar called anorthite, dark gray to black.

How to tell them? Tough in the field but generally a clue is color; the more Na (Ab) the lighter the color; the more Ca (An), the darker. And one, labradorite is famous as building stone for its silky blue sheen in a gray mass.

Potassium feldspars orthoclase and microcline are generally flesh-to-salmon pink, have very good cleavage and, if green, the microcline kind makes a nice gemstone as amazonite.

As for those "big six" above, they are lumped together as plagioclases. So you see there is a way out of this mess!

Uses? Not many considering their abundances. Mostly microcline and orthoclase are used in ceramic glazes and mild abrasives in cleaners (H6 on Mohs scale). Kaolin, a valuable white clay may form when feldspars decay and most soil clays owe their origin to feldspars.

Now the good news.

The test announced for tomorrow on feldspars has been cancelled indefinitely. Life is not all gloom!

 

 


Bancroft, Ontario, CANADA


Feldspar rhomb
Huntsville, Ontario   CANADA
Feldspar Seams in Road cuts
  • When you drive along the roads in Muskoka, Ontario where the hills have been blasted to let the road travel straight through, you will often see seams of pink feldspar in the face of the rock.

Feldspar is often found with quartz and mica - the building blocks of Ontario granite.
If you look closely at the pink seam, you will see that the feldspar is breaking off in flat sections
 


Feldspar in Granite ~  Huntsville, Ontario, CANADA

 

 

Granite .

  • granite is one of the most commonly found rocks that contains feldspar
  • is mainly composed of feldspar, quartz and mica
  • feldspar is often salmon, pinkish in colour


 


Feldspar & Quartz ~ Huntsville, Ontario, CANADA

Feldspar in Quartz ~  Huntsville, Ontario, CANADA
Feldspar Crystals from Bancroft, Ontario CANADA
 

 

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