Labradorite from Australia
Labradorite is a feldspar mineral in the category tectosilicate.
It has an anorthite percentage (%An) of between 50 to 70%; Anorthite is the calcium endmember of the plagioclase feldspar mineral with the formula of pure anorthite CaAl2Si2O8; found in mafic igneous rocks; a rare mineral on Earth but abundant on the Moon.
Twinning is common.
Labradorite displays an iridescent optical effect or schiller known as labradorescence making the mineral glow with yellow, green and blue by rotating it in different angles.
The cause of this optical phenomenon is phase exolution lamellar structure, occurring in the Boggild miscibility gap. Effect visible when the lamellar separation is between 128 and 252 nm, the lamellae are not necessarily parallel; and the lamellar structure is found to lack long range order.
The lamellar separation only occurs in plagioclases of a certain composition in particular, those of calcic labradorite and bytownite (anorthite content of 60-90%).
Another requirement for the lamellar separation is very slow cooling of the rock that contains the plagioclase. Slow cooling is required to allow the Ca, Na, Si and Al ions to diffuse through the plagioclase and produce the lamellar separation.
And therefore, not all labradorites exhibit labradorescence, by not be correct composition and/or they cooled too quickly and not all plagioclases that exhibit labradorescence are labradorites by exemple bytownite.
Labradorite belongs to the plagioclase feldspar group, an isomorphous solid solution series.
Albite is one member containing sodium and no calcium, Anorthite contains calcium and no sodium. Labradorite is an intermediary member of this series.
Labradorite is considered by some authorities as a variety of Anorthite rather then separate mineral.
Some gemstone varieties of labradorite exhibiting a high degree of labradorescence are called spectrolite.
IMA does not recognize it as individual mineral species, but rather a sodium-rich variety of Anorthite.
Labradorite contains between 30-50% Albite and 50-70% Anorthite.
Characteristics:
Category: Sodium-rich Calcium Aluminum Silicate; ratio of sodium to calcium is 7:3
Formula: Na0.4Ca0.6Al1.6Si2.4O8
Actual formula (?): (Ca,Na)Al1-2Si3-2O8
Sodium 3,38%; Calcium 8,85%; Aluminium 15,88%; Silicon 24,80%; Oxygen 47,09%
Crystal system Triclinic
Cleavage: 3 directions: perfect, good, distinct
Color colorless, gray, gray-white; white; light green
Density 2,68-2,71
Fracture uneven
Habit: crystalline, granular, striated
Hardness: 6-6,5 Mohs
Luster vitreous glassy
Streak white
Refractive index 1,559-1,573
Twinning common by albite, pericline, carlsbad, baveno or maneback twin laws
Optical properties biaxial (+)
Rock type: Igneous, Metamorphic
Special characteristics:
Labradorescence (iridescent)
May have fluorescence violet in long wave and red in short wave UV
Activator peak Eu at 403nm
May show thermoluminescence
Common mineral associations: Olivine, Pyroxenes, Amphiboles, Magnetite, Muscovite, Biotite, Augite, Hornblende
Varieties:
Specrolite - variety of labradorite from Finland that displays a schiller collor (labradorescence) with intense colors such as dark red, orange, yellow, blue, green.
Albite - belongs to the plagioclase feldspar group, an isomorphous solid solution series.
Contains sodium and no calcium, on the other end member, Anorthite contains calcium and no sodium.
Albite sometimes forms in association with the feldspar mineral microcline in alternating patterns, and forms a feldspar rock known as Perthite.
Uses: ceramics
Formula NaAlSi3O8
Composition Sodium Aluminium Silicate ; often with the sodium partially replaced by calcium or potassium
Color: white, colorless, cream, light yellow, light blue, light green, pale red, light brown, gray
Some may show iridescent with labradorescence
Streak white
Hardness 6-6,5 Mohs
Crystal system Triclinic
Habit tabular, prismatic, stubby
Specific gravity 2,6-2,63
Luster vitreous to pearly
Cleavage basal, prismatic, pinacoidal
Fracture subconchoidal to uneven
Tenacity brittle
Transparent to translucent
Rock type: igneous, sedimentary, metamorphic
Special characteristics:
May rarely show iridescence with labradorescence
Soluble in hydrofluoric acid
Common mineral associations: Quartz, Muscovite, Biotite, LEpidolite, Potassium feldspar group, Torumaline, Hornblende, Spessartine, Apatite
Distinguishing similar minerals:
Potassium feldspar group dont exhibit striations on twinned crystal surfaces
Spodumene - has a splintery fracture
Barite - lower hardness and much heavier
Calcite - much lower hardness
Anorthite - member of plagioclase feldspar group, an isomorphous solid solution series; contains calcium and no sodium.
CaAl2Si2O8
Composition Calcium Aluminum Silicate ; may contain some sodium replacing the calcium but that amount must be less than 10% to be strictly Anorthite
Color colorless, white, cream, gray, brown, pink, pale yellow, pale green
Hardness 6-6,5; Streak white
Soluble in hydrochloric acid
Rick type igneus, metamorphic
Bytownite
Andesine - member of plagioclase feldspar group, an isomorphous solid solution series, contains calcium and no sodium
Analbite - variety of albite, containing small percentage of potassium replacing the sodium, the ratio of sodium to potassium is greater than 9:1
Cleavelandite - thin, bladed crystals of plates of albite, usually white in color
Pericline - elongated, white prismatic crystals of albite
Peristerite - iridescent variety of albite, colored sheen similar to labradorite but not as pronounced and usually on white or light colored backgrounds
Other names: labradorite labradorit labradoritul spectrolite spectrolit spectrolitul feldspar feldspat feldspatul feldsparul carnatit mauilit radauit silicid spectrolith albit albitul albite andesitul andesit anortit anortitul anortite anorthit anorthitul
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