G119                                                                                                                       Activity Four

Name_________________________________                                                         Feldspars and Quartz

Identifying Feldspars and Quartz

This week we will begin to examine the rock forming minerals.  These minerals are different than the ore minerals but just as important.  While ore minerals are useful for the products that can be made from them, rock forming minerals are important because they make up the rocks or crust of the earth.  Rock forming minerals are more common than ore minerals and are used to help determine which rock is which. 

Feldspars

We are going to start by learning the most common group of rock forming minerals which are the feldspars.  This group of minerals (in one form or another) is found in most rocks on earth.  Feldspars are aluminum silicates that contain varying amounts of potassium (K), sodium (Na) and/or Calcium (Ca).  There is a solid solution between the calcium end member Anorthite and the potassium end member Orthoclase and the variation between these two minerals makes up the continuous series on the Bowen's reaction table.  This often time results in a texture known as perthite.  Perthite is a texture where there are actually two minerals present but they are intergrown.  This can create a feldspar sample that has "strings" of a lighter colored mineral through a darker mineral (or vise versa).  This is a common texture in the mineral microcline (more in a minute).    Below are black and white images of this perthitic texture.  Note the “lines” running through each of the feldspar samples in the images.

Your lab manual puts the feldspar minerals into two groups; Plagioclase feldspar and Potassium feldspar.  Plagioclase feldspar is associated with rocks dominated by minerals rich in iron and magnesium though the sodium feldspars can be found in non iron/magnesium rich rocks.  Most Plagioclase feldspars tend to form at high temperatures and they include the following feldspar minerals:

Albite NaAlSi₃O₈

            Oligoclase                   Na(90-70%) Ca(10-30%) (AlSi)₄ O₈

            Andesine                    Na(70-50%) Ca(30-50%) (AlSi)₄ O₈

            Labradorite               Ca(50-70%) Na(50-30%) (AlSi)₄ O₈

            Bytownite                   Ca(70-90%) Na(30-10%) (AlSi)₄ O₈

            Anorthite                    CaAl₂Si₂O₈

Potassium feldspars on the other hand tend to be found in rocks that are dominated by silica rich minerals and form at lower temperatures. Potassium feldspars include:

Microcline                  KAlSi₃O₈

            Sanidine                     (K,Na)(Si,Al)₄O₈

            Orthoclase                  KAlSi₃O₈

Note that Microcline and Orthoclase have the same formula but are considered different minerals.  They are known as polymorphs.  Polymorphs are minerals with the same composition but different crystal habits.  They are created due to different conditions of formation.  A couple of very common polymorphs are graphite and diamond.  Both have the chemical composition of "C" but graphite is very soft while diamond is very hard.  This has to do with how the atoms pack together in the minerals crystal structure.

Quartz:

Quartz is another common rock forming mineral and, like feldspar, there are a group of minerals that fall under this heading.  Quartz has the basic chemical formula of SiO₂ and quartz minerals can form from a couple of different processes which includes quartz that forms from magma and also quartz that forms due to silica rich water solutions. 

The first type of quartz, quartz that forms from magma, is found intergrown with other minerals in igneous rocks.  In the first lab this term, you looked at and drew a sample of granite to illustrate how minerals grow together as magma cools.  One of those minerals was quartz.  Often this type of quartz is grey, white or clear in color and has no regular habit. 

Quartz that grows from silica rich water solutions will often form large crystals or can also form microcrystals depending on the conditions it crystallizes in.  Crystal quartz or microcrystalline quartz forms as a solution rich in silica flows through rock cavities, over surfaces or even sits for a while in a lake or ocean.  As this happens, the silica in the water combines with oxygen and molecules of quartz (SiO₂) are deposited.  

Crystal quartz will often grow in rock cavities and create beautiful hexagonal crystals of amethyst, rose quartz, smoky quartz etc.  Microcrystalline quartz on the other hand, forms often in lakes, ponds, streams etc.   Microcrystalline quartz is also known as chalcedony (pronounced cal-sed-ney).  Chalcedony minerals include jasper, onyx, chert, agate, flint, and opals.  All of these are forms of quartz with different types/amounts of trace elements in the structure to give them their unique colors. 

Exercise:

For today's lab you will focus on learning a few of the feldspars, quartz and some chalcedonys.  The list of minerals you need to learn is found below.  Again, the numbers next to the minerals correspond to the numbers on the minerals in the box.  Please fill out a chart on the physical properties for each mineral and there are questions at the end of the lab to answer.  The idea is to be able to identify these minerals without the use of a book or chart. 

A couple of good web sites for rock forming minerals are below and if you should come across others please let me know.

                            http://www.webmineral.com/data/

                            http://mineral.galleries.com/minerals/silicate/feldspar.htm

Feldspar minerals

      1.   Perthitic Microcline KAlSi₃O₈

      2.   Microcline* KAlSi₃O₈

      3.   Albite* NaAlSi₃O₈

      4.   Oligoclase

      5.   Andesine

      6.   Labradorite                                        CaNa(AlSi)₄ O₈

      7.   Bytonite                                              

      8.   Anorthite* CaAl₂Si₂O₈

Quartz Minerals

      40.  Crystal quartz*                                 SiO₂

      41.  Milky quartz*                                    SiO₂

      42.  Flint (chalcedony)*                            SiO₂

      43.  Chert (chalcedony)*                          SiO₂

      44.  Jasper (chalcedony)*                        SiO₂

       45.  Opal  (chalcedony)*                         SiO₂

For each of the underlined minerals be able to identify each in a hand sample.  This means that if I were to give you a specimen I would expect you to be able to tell me what it is without the use of a flow chart.  Additionally, for each mineral with an asterisk (*) you should know the mineral formula

Questions:

1.   How many planes of cleavage does feldspar have?

2.   Which of the feldspar minerals that you have to learn are potassium and which are plagioclase?

3.   List two distinctive properties you could use to tell quartz and feldspar apart.

4.   In the space provided, draw an example of perthitic texture  

5.   List the distinctive colors for each of the chalcedonys you need to know.

        Chert:                                                       Flint:

        Opal:                                                        Jasper:

L:G119/activity 4