TERM PAPER

UNIVERSITY OF GUYANA (UG)
FACULTY OF ENGINEERING AND TECHNOLOGY (FET)
DEPARTMENT OF PETROLEUM AND GEOLOGICAL ENGINEERING
NAME OF STUDENT:
Ishwar Ramdeen
USI #: 1016409
ACADEMIC YEAR: 2019-2020
SEMESTER: One (1)-September 2019-December 2019
COURSE CODE & TITLE:
LECTURER:
GEM1102-Minerology
Sir Deon Adolph
TITLE OF TERM PAPER: Feldspars-their variety, characteristics and
applications
DATE OF SUBMISSION:
March 05, 2020 (Thursday)

TABLE OF CONTENTS
Table of contents .............................................................................................................................1
Introduction .....................................................................................................................................3
Section 1 – general overview of feldspars
Etymology .......................................................................................................................................5
General classification ......................................................................................................................6
Chemical properties ........................................................................................................................8
Physical properties ..........................................................................................................................9
Section 2 – the alkaline feldspars
Overview .......................................................................................................................................11
Atomic structure ............................................................................................................................12
Chemical properties ......................................................................................................................13
Images and observations ...............................................................................................................14
Physical properties ........................................................................................................................18
Occurrences ...................................................................................................................................19
Distribution ...................................................................................................................................20
Applications ..................................................................................................................................21
Section 3 – the plagioclase feldspar
Overview .......................................................................................................................................23
Atomic structure ............................................................................................................................24
Chemical properties ......................................................................................................................25
Images and observations ...............................................................................................................26
Physical properties ........................................................................................................................32
Critical distinguishing features ......................................................................................................33
Occurrences ...................................................................................................................................34
Distribution ...................................................................................................................................35
Applications ..................................................................................................................................36
1

References .....................................................................................................................................37
2

INTRODUCTION
Feldspar is the most important single group of rock forming silicate minerals. The mineral name
feldspar is derived from the German words field + spar. The word (field) is field in German and
(spar) is a term for light colored minerals that break with a smooth surface. feldspar minerals are
usually white or very light in color, have a hardness of 6 on the Moh` Scale of Hardness and perfect
to good cleavage (plane of breakage) in two directions.
Feldspars occur in all classes of rocks. They are widely distributed in igneous rocks, which
indicates that they have formed by crystallization from magma. Physical weathering of feldsparbearing
rocks may result in sediments and sedimentary rocks that contain feldspars; however, this
is a rare occurrence because in most environments the feldspars tend to be altered to other
substances, such as clay minerals. They also may be found in many metamorphic rocks formed
from precursor rocks that contained feldspars and/or the chemical elements required for their
formation. In addition, feldspars occur in veins and pegmatites, in which they were apparently
deposited by fluids, and within sediments and soils, in which they were probably deposited by
groundwater solutions.
Because of their abundance, feldspars are used in the classification of igneous rocks. They are also
abundant in metamorphic rocks and in some sediments and sedimentary rocks. Feldspars are
generally light colored minerals, white or buff to gray in color. One species, microcline, may also
be light brick-red or even the green to blue green variety called amazonite. Feldspars are slightly
translucent and have a glassy, or vitreous, luster rather like that of glazed porcelain. Because their
atomic framework has planes of weakness, feldspars exhibit good cleavage, breaking readily into
blocky pieces with smooth sides.
3

SECTION 1
GENERAL OVERVIEW OF FELDSPARS
4

ETYMOLOGY
The name feldspar derives from the German Feldspat, a compound of the words Feld ("field")
and Spat ("flake"). Spat had long been used as the word for "a rock easily cleaved into
flakes"; Feldspat was introduced in the 18 th century as a more specific term, referring perhaps to
its common occurrence in rocks found in fields (Urban Brückmann, 1783) or to its occurrence as
"fields" within granite and other minerals (René-Just Haüy, 1804). The change from Spat to -
spar was influenced by the English word spar, meaning a non-opaque mineral with good cleavage.
Feldspathic refers to materials that contain feldspar. The alternate spelling, felspar, has fallen out
of use. The term 'felsic', meaning light colored minerals such as quartz and feldspars, is an
acronymic word derived from feldspar and silica, unrelated to the redundant spelling 'felspar'.
5

GENERAL CLASSIFICATION
In the classification of igneous rocks of the International Union of Geological Sciences (IUGS),
the feldspars are treated as two groups: the alkali feldspars and the plagioclase feldspars. The
alkali feldspars include orthoclase, microcline, sanidine, anorthoclase, and the two-phase
intermixtures called perthite.
Alkaline / Potassium (K) feldspars
Figure 1 – showing classification of feldspars
Alkali feldspars are grouped into two types: those containing potassium in combination with
sodium, aluminum, or silicon; and those where potassium is replaced by barium.
K + Na / Al / Si
Name System Formula
Orthoclase Monoclinic KAlSi3O8
Sanidine Monoclinic (K,Na)AlSi3O8
Microcline Triclinic KAlSi3O8
Anorthoclase Triclinic (Na,K)AlSi3O8
Table 1 – showing groups of K – feldspars
K replaced by Ba / barium feldspars, Barium feldspars form as the result of the substitution of
barium for potassium in the mineral structure.
Name System Formula
Celsian Monoclinic BaAl2Si2O8
Hyalophane Monoclinic (K,Ba)(Al,Si)4O8
Table 2 – showing groups of barium feldspars
6

Plagioclase feldspar
In petrology, the plagioclase series are defined based on their percentage of albite (Ab) to anorthite
(An). Compositions of individual samples are usually written as, for example, An24Ab76 meaning
in this case that it is 24 % anorthite and 76 % albite. The word plagioclase is from the
Greek, plagios (oblique) and clasis (fracture), in reference to the two nonorthogonal cleavages.
Name Composition Formula
Anorthite An100Ab – An90Ab10 CaAl2Si2O8
Bytownite An90Ab10 – An70Ab30 (Ca,Na)Al(Al,Si)Si2O8
Labradorite An70Ab30 – An50Ab50 (Ca,Na)Al(Al,Si)Si2O8
Andesine An50Ab50 – An30Ab70 (Na,Ca)Al(Si,Al)Si2O8
Oligoclase An30Ab70 – An10Ab90 (Na,Ca)Al(Si,Al)Si2O8
Albite An10Ab90 – AnAb100 NaAlSi3O8
Table 3 – showing groups of plagioclase feldspars
7

CHEMICAL PROPERTIES
What is common to all the feldspars is the same arrangement of atoms, a framework arrangement,
and one basic chemical recipe, a silicate (silicon plus oxygen) recipe. Quartz is another framework
silicate, consisting only of oxygen and silicon, but feldspar has various other metals partly
replacing the silicon. All minerals in the feldspar group fit the generalized chemical composition
below:
X(Al, Si)4O8
In this generalized composition, X can be any one of the following seven ions: K+, Na+, Ca++,
Ba++, Rb+, Sr++, and Fe++. Feldspars that include potassium, sodium and calcium ions are very
common. Barium, rubidium, strontium and iron feldspars are very rare.
Figure 2 – showing general crystal structure of minerals
8

PHYSICAL PROPERTIES
Colour:
Crystal system:
Bright coloured; white, grey, greenish, yellow, pink (due to
inclusion of haematite), transparent.
Monoclinic (2/m), triclinic (1̅)
Cleavage: Perfect parallel to {001}, clear parallel to {010}
Fracture:
Uneven, somewhat brittle
Mohs scale (hardness): 6.0 – 6.5
Refractive index: 1.520 – 1.568
Optical properties:
Pleochroism:
Streak:
Weak to medium birefringence
None
White
Specific gravity: 2.5 – 2.8
Crystal habit:
Monoclinic / triclinic tabular crystal, elongated along [100], equal
development of {011} and {010}.
9

SECTION 2
THE ALKALINE FELDSPARS
10

OVERVIEW
Alkali feldspars are grouped into two types: those containing potassium in combination with
sodium, aluminum, or silicon; and those where potassium is replaced by barium. In this report,
focus is made on the four main K – feldspars.
K + Na / Al / Si
Name System Formula
Orthoclase Monoclinic KAlSi3O8
Sanidine Monoclinic (K,Na)AlSi3O8
Microcline Triclinic KAlSi3O8
Anorthoclase Triclinic (Na,K)AlSi3O8
Table 4 – groups of K – feldspars
11

ATOMIC STRUCTURE
Alkali feldspars are framework aluminosilicates composed of SiO4 and AlO4 tetrahedra, each of
which shares all four of its apical oxygen atoms with neighboring tetrahedra.
Figure 3 – orthoclase crystal structure
12

CHEMICAL PROPERTIES
Alkali feldspars rank with quartz and plagioclase as the most common minerals in the earth's crust.
Chemically they are represented by the formula;
(K, Na)AlSi3O8
The chemical end member KAlSi3O8 (Or) includes the minerals orthoclase, microcline, and K-
rich sanidine ; and the end member NaAlSi3O8 (Ab) is the mineral albite . In most natural
specimens, there is also a small amount of the anorthite component CaAl2Si3O8 Traces of Rb, Tl,
Pb, Ba, Sr, Fe3+, and Ge may also be present. Alkali feldspars occur extensively in a variety of
geologic environments, and, owing to their compositional and structural complexity, are especially
useful as petrologic indicators.
Name
Orthoclase
Sanidine
Microcline
Anorthoclase
Formula
KAlSi3O8
(K,Na)AlSi3O8
KAlSi3O8
(Na,K)AlSi3O8
Table 5 – chemical structure of groups
13

IMAGE AND OBSERVATIONS
Orthoclase
Figure 6 – orthoclase minerals
14

Sanidine
Figure 7 – sanidine minerals
15

Microcline
Figure 8 – microcline minerals
16

Anorthoclase
Figure 9 – anorthoclase minerals
17

PHYSICAL PROPERTIES
Properties Orthoclase Sanidine Microcline Anorthoclase
Colorless,
White, grey, greyish
White,
greenish, greyish Colorless to yellow, yellowish, tan,
Color
colourless,
yellow, white, white salmon-pink, bluish
greyish pink
pink
green, green.
Streak white White White White
Vitreous, pearly
Vitreous, pearly
Luster on cleavage
Vitreous
on cleavage
surfaces
Cleavage
Perfect = {001
good = {010}
Cleavages
intersect at 90 O .
Perfect = {001}
good = {010}
Perfect = [001]
good = [010]
Vitreous to
pearly on
cleavage planes
Perfect
Diaphaneity
Translucent to Transparent to Transparent,
transparent translucent Translucent
Transparent
Mohs 6 (defining
Hardness mineral)
6 6 – 6.5 6 – 6.5
Specific
Gravity
2.55 – 2.63 2.52 2.54 – 2.57 2.57 – 2.60
Crystal
System
Monoclinic Monoclinic Triclinic Triclinic
Tenacity Brittle Brittle Brittle Brittle
Parting
On {100} {110}
on {100}{110}
{100}
{110} {201}
{110}{201}
Prismatic
Fracture
Irregular /
Uneven,
Conchoidal
Irregular /
Uneven,
Conchoidal
Irregular / Uneven Uneven
Density
(g/cm 3 )
2.55 – 2.63 2.56 – 2.62 2.54 – 2.57 2.57 – 2.60
Polysynthetic
Carlsbad, Carlsbad,
Carlsbad, Baveno and twinning
Twinning Baveno and Baveno and
Manebach produces a grid
Manebach Manebach
pattern on [100]
Pleochroism Non-pleochroic Non-pleochroic Non-pleochroic Colourless
Table 6 – physical properties of alkaline feldspars
18

OCCURENCES
Orthoclase – The common feldspar of granites, granite pegmatites, and syenites. In cavities
in basalts; in high-grade metamorphic rocks and as a result of potassic hydrothermal alteration;
also authigenic and detrital.
Adularia - White or colorless, transparent to translucent variety of Orthoclase (or Sanidine).
Moonstone - Form of feldspar that displays a color sheen known as adularescence. Depending on
the locality, Moonstone may be Orthoclase feldspar (Adularia), or it may be the Plagioclase
feldspar Oligoclase.
Noble Orthoclase - Describes a transparent, yellow variety of Orthoclase from Madagascar.
Valencianite - White form of bladed or platy Adularia from the Valenciana Mine in Guanajuato,
Mexico.
Sanidine – Most common in felsic volcanic and hypabyssal rocks as rhyolites, phonolites,
trachytes; as spherulites in volcanic glass. Also from ultrapotassic ma¯c, high-temperature contact
metamorphic (sanidinite facies), and hydrothermally altered rocks. From eclogite nodules in
kimberlite.
Microcline – Common in plutonic felsic rocks, as granites, granite pegmatites, syenites; in
metamorphic rocks of the greenschist and amphibolite facies; in hydrothermal veins. A detrital
component in sedimentary rocks and as authigenic overgrowths.
Anorthoclase – In high-temperature sodic volcanic and hypabyssal rocks.
19

DISTRIBUTIONS
Orthoclase
Widespread. Fine examples from St. Gotthard, Ticino, and at Val Giuv, Tavetsch, GraubuÄnden,
Switzerland. In the Zillertal, Tirol, Austria. From Baveno, Piedmont, in the P¯tschtal, Trentino-
Alto Adige, and at San Piero in Campo, Elba, Italy. At Epprechtstein, Bavaria, Carlsbad, Bohemia,
and Manebach, Thuringia, Germany. From Cornwall, England. In Russia, from the Mursinka-
Alabashka area, near Yekaterinburg (Sverdlovsk).
Sanidine
In Germany, from Drachenfels, Siebengebirge, Rhine; and at Hohenfels, Mendig, Mayen, and
elsewhere around the Laacher See, Eifel district.
In France, at Mt. Dore, Auvergne, and Puy Gros du Laney, Puy-de-Dome.
From Vesuvius and Monte Somma, Campania, and Monte Cimine, Lazio, Italy.
At Daichi, Wakayama Prefecture, Japan.
Microcline
At FredriksvÄarn, Arendal, and Larvik, Norway.
In the Ilmen Mountains, Ural Mountains, and on the Kola Peninsula, Russia.
At St. Gotthard, Ticino, Switzerland.
On Mt. Greiner, Zillertal, Tirol, Austria.
At Baveno, Piedmont, Italy.
In the USA, at Amelia, Amelia Co., Virginia; Haddam, Middlesex Co., Connecticut; and
Magnet Cove, Hot Spring Co., Arkansas.
Anorthoclase
On Pantelleria and Ustica Islands, Italy.
At Larvik, Norway.
From Berkum, North Rhine-Westphalia, Germany.
On Grande Caldeira Island, Azores.
At Ropp, Nigeria.
On Mt. Kenya, Kenya.
From Kilimanjaro, Tanzania. At Chilposan, near Minchon, North Korea.
From Ogaya, Toyama Prefecture, and Madarajima, Saga Prefecture, Japan.
20

APPLICATIONS
Minerals
Orthoclase
Sanidine
Microcline
Anorthoclase
Uses
Ceramics, Glass, Abrasives, Gemstones, Mohs scale mineral
Gemstones
The most important place of use is the production of
porcelain.
Microcline is used industrially in the production of glass
and ceramic products.
It is used as ornamental lapidary material with Amazonite
in green color.
Sometimes feldspar is also used in the manufacture of
glass.
Gemstones
Table 7 – applications of alkaline feldspars
21

SECTION 3
THE PLAGIOCLASE FELDSPARS
22

OVERVIEW
In petrology, the plagioclase series are defined based on their percentage of albite (Ab) to anorthite
(An). Compositions of individual samples are usually written as, for example, An24Ab76 meaning
in this case that it is 24 % anorthite and 76 % albite. The word plagioclase is from the
Greek, plagios (oblique) and clasis (fracture), in reference to the two nonorthogonal cleavages.
Name Composition Formula
Anorthite An100Ab – An90Ab10 CaAl2Si2O8
Bytownite An90Ab10 – An70Ab30 (Ca,Na)Al(Al,Si)Si2O8
Labradorite An70Ab30 – An50Ab50 (Ca,Na)Al(Al,Si)Si2O8
Andesine An50Ab50 – An30Ab70 (Na,Ca)Al(Si,Al)Si2O8
Oligoclase An30Ab70 – An10Ab90 (Na,Ca)Al(Si,Al)Si2O8
Albite An10Ab90 – AnAb100 NaAlSi3O8
Table 8 – minerals of plagioclase
23

ATOMIC STRUCTURE
Plagioclase is a series of tectosilicate (framework silicate) minerals within the feldspar group.
Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a
continuous solid solution series, more properly known as the plagioclase feldspar series.
Figure 10 – crystal structure of mineral albite
24

CHEMICAL PROPERTIES
The composition of a plagioclase feldspar is typically denoted by its overall fraction
of anorthite (% An) or albite (% Ab), and readily determined by measuring the plagioclase
crystal's refractive index in crushed grain mounts, or its extinction angle in thin section under
a polarizing microscope.
25

IMAGES AND OBSERVATIONS
Anorthite
Figure 11 – anorthite minerals
26

Bytownite
Figure 12 – bytownite minerals
27

Labradorite
Figure 13 – labradorite minerals
28

Andesine
Figure 14 – andesine minerals
29

Oligoclase
Figure 15 – oligoclase minerals
30

Albite
Figure 16 – albite minerals
31

PHYSICAL PROPERTIES
Properties
Description
Color
Usually white or gray. Also colorless, yellow, orange, pink, red,
brown, black, blue, green.
Streak
White
Luster
Vitreous. Pearly on some cleavage faces.
Diaphaneity
Translucent to transparent
Cleavage Perfect in two directions that intersect at approximately 90 degrees.
Mohs Hardness 6 to 6.5
Specific Gravity 2.6 to 2.8
Perfect cleavage, with cleavage faces intersecting at right angles and
Diagnostic Properties striations often present on cleavage faces. Well-defined crystals are
extremely rare.
Chemical
Composition
NaAlSi3O8 - CaAl2Si2O8
Crystal System
Triclinic
Table 9 – physical properties of plagioclase
32

CRITICAL DISTINGUISHING FEATURES
Minerals
Albite
Oligoclase
Andesine
Anorthite
Labradorite
Bytownite
Common
association
Quartz, Muscovite,
Biotite, Lepidolite,
Potassium Feldspar
Group, Tourmaline,
Hornblende, Spessa
rtine, Apatite
Quartz, Muscovite,
Biotite, Potassium
Feldspar Group
Quartz, Muscovite,
Biotite, Potassium
Feldspar Group,
Hornblende
Quartz, Muscovite,
Biotite,
Hornblende, Augite
Quartz, Muscovite,
Biotite,
Hornblende, Augite
Quartz, Muscovite,
Biotite,
Hornblende
Critical features
Potassium Feldspar Group - Don't exhibit striations on
twinned crystal surfaces, whereas the Plagioclase
feldspars sometimes do. Otherwise can be difficult to
distinguish.
Other Plagioclase Feldspars - Usually cannot be
determined by practical means.
Spodumene - Has a splintery fracture.
Barite - Lower hardnessand much heavier.
Calcite - Much lower hardness.
Due to its unique color effect, Labradorite is easily
distinguished from all minerals. However, specimens that
don't exhibit labradorescence may be confused with
many minerals, especially other feldspars.
Several minerals can be confused with Bytownite, but the
localities and cleavage can usually distinguish it from all
other minerals.
Table 10 – distinguishing features
33

OCCURRENCES
Albite – granites; granitic pegmatites; low-grade metamorphic gneisses and schists; sandstones.
Oligoclase – granodiorites and monzonites; sandstones; moderate-grade metamorphic rocks.
Andesine – diorites; andesites; moderate-grade metamorphic rocks, especially amphibolites.
Labradorite – gabbros and anorthosites; diabases and basalts.
Bytownite – gabbros and anorthosites; diabases and basalts.
Anorthite – gabbros; contact-metamorphosed impure limestones; and high-grade metamorphic
rocks.
34

DISTRIBUTION
Albite – very common mineral, but localities where fine Albite crystals can be found are more
limited. Some excellent examples have come from the Swiss Alps and the Tyrol, Austria. Large,
well formed crystals come from the pegmatites of Gilgit, Pakistan; and Nuristan, Afghanistan.
Enormous Clevelandite crystals have come from several areas in Minas Gerais, Brazil, especially
in the Doce and Jequetinhonha Valleys.
Oligoclase – Good Oligoclase crystals come from the Montijos Quarry, Monte Redondo, Leiria,
Portugal. Masses of Moonstone Oligoclase come from Spruce Pine, Mitchell Co., North Carolina;
and well-shaped white and colorless crystals were found in the Hawk Mine, Mitchell Co., North
Carolina. Very good Moonstone Oligoclase was found in Middletown Township, Delaware
County, Pennsylvania. An important Connecticut locality is Timms Hill, Haddam, Middlesex Co.,
Connecticut. Much of the Feldspar in rock on the island of Manhattan (New York City) is
Oligoclase.
Andesine – prevalent in volcanic regions throughout the world. Its original locality in the Andes
Mountains is the Marmato District, Manizales, Bolivia. Large crystals are found in the San Gabriel
Mountains (Los Angeles County), California. Other localities are the Crestmore Quarry, Riverside
County, California; Goodall Farm, Sanford, York County, Maine; Mont Saint Hilaire, Quebec,
Canada; and Portland, Victoria, Australia.
Labradorite – Perhaps the most colorful Labradorite comes from Finland at the Ylamaa Quarries,
Lappeenranta. Two other outstanding localities are Golovinskoye, Zhytomyr, Ukraine; and the
Antsohamamy Quarry, Tulear Province, Madagascar. Good Labradorite also comes from Black
Hill, New South Wales, Australia. Some of the most important Labradorite deposits are in Canada
in Labrador, at Nain and Tabor Island. In the U.S., the Adirondack Mountains of upstate New
York have produced colorful Labradorite, especially at Saranac Lake. Franklin Co.; Blue Ridge
Road, North Hudson, Essex Co.; and Roaring Brook Falls, Keene Valley, Essex Co. A transparent
form of Labradorite comes from Millard Co., Utah; and from the Woodward Ranch, near Alpine,
Brewster Co., Texas.
Bytownite – Bytownite is a rare mineral, and occurs in many scattered localities usually lacking
specimens of interest. It is named after the locality of Bytown, which was the old name for Ottawa
(the capitol of Canada) where this mineral was first described. Only a handful of old specimens
are noted from this locality. Bytownite occurrences include the Dorado Mine, Casas Grandes,
Chihuahua, Mexico; Santiago Papasquiaro, Durango, Mexico; Plush, Lake Co., Oregon; and the
vicinity of Pueblo Creek, Gila National Forest, Catron Co., New Mexico.
Anorthite – is a rare member of the Feldspar group. Italian Localities include Monte Somma,
Mount Vesuvius; Val Schiesone, Sondrio; and Val Di Fassa, Trento. Japan produces excellent
crystals thinly coated with a dark layer of lava at Miyaki Jima (Miyaki Island), Tokyo Prefecture.
Other localities are Grass Valley, Nevada Co., California; and Franklin, Sussex Co., New Jersey.
35

APPLICATIONS
Minerals
Albite
Oligoclase
Andesine
Labradorite
Bytownite
Anorthite
Uses
Albite is industrially important in the manufacture of
ceramics. Albite provides the best crystallized examples of
the Plagioclase Feldspars, and these crystals are popular
among collectors. Albite is also important in the study of
mineral environments and crystal formations.
Oligoclase is industrially important in the manufacture of
ceramics. Crystals are popular among mineral collectors.
Oligoclase is also known for its gem
varieties: Sunstone and Moonstone.
There is a new gemstone called Andesine which has recently
penetrated the market. Much of the material is Chinese in
origin and has been synthetically diffused to enhance color,
and is actually produced from Labradorite.
Labradorite is a popular mineral, and it makes a
unique gemstone. It is cut and polished into cabochons and
beads, and occasionally as other facets. Some exquisite
ornaments are carved out of large Labradorite chunks.
The yellow and brown transparent forms of Bytownite
are faceted as a rare feldspar gemstone.
Primarily a rock-forming mineral, it is used in the
manufacture of glass and ceramics.
Table 11 – applications of plagioclase minerals
36

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