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American Mineralogist, Volume 59, pages 231-243, Ig74
Lunar Mineralogy:A HeavenlyDetectiveStory
Presidential
Address,part 11
JosnpnV. Surrs
Department ol the GeophysicalSciences,Uniuersity ol Chicago,
Chicago,Illircis 60637
Abstract
A general introduction emphasizes the strolg interactions between mineralogy and related
sciences,and the need for cooperative programs among the various mineralogical groups.
To provide maximum access to the literature, the bulk of lunar rock types are classified
into ANT, FETI, and KREEP groups. The Anorthositic-Noritic-Troctolitic compositions mostly
occur as metabreccias, and are believed to come from the crust. The basalts rich in Fe
and Ti
result from late lava flows in mare basins. The KREEP suite of basaltic compositions occurs
mostly as glassesand metabreccias rich in K, REE, P, and other large-ion elements; it probably results from a late liquid in primary differentiation of the Moon, or partial melting of
ANT rocks, or both. Some lunar rocks are hybrids formed by melting of soils, Metamorphism
is widespread. Minor rock types include peridotite, spinel allivalite and troctolite, and
other
fragments containing Mg-rich minerals. Granitic material occurs rarely. Minor meteoritic
fragments occur.
Major mineral groups are pyroxene, feldspar, olivine, spinel, pseudobrookite (armalcolites),
silica (quartz, tridymite, cristobalite), Fe-rich types (iron, troilite, ilmenite), phosphates
(apatite, whitlockite), and Zr-minerals (zircon, baddeleyite). Minor silicates
include pyroxferroite, amphibole' garnet' and tranquillityite. Minor sulfides are mackinawite, pentlandite,
cubanite, chalcopyrite, and sphalerite. Minor oxides are rutile and corundum. Minor metals
include copper, brass, and tin. A Zr-rich mineral is either zirkelite or zirconolite. Schreibersite,
cohenite, and niningerite probably derive from iron meteorites. Rusty areas probably consist
of goethite perhaps associated with hematite and magnetite; presence of Cl suggestspossible
terrestrial oxidation of meteoritic lawrencite.
Important features of feldspars are: mostly calcic plagioclase, but rare K,Ba-feldspar in
residua; correlation of Mg,Fe,K, and Na with rock type giving test of differentiation models;
Eu anomaly depends on temperature, oxidation state, etc; solid-solution of ca(Mg,Fe)sLo, id
FETI plagioclase; exsolution of pyroxene, silica, and Fe in ANT plagioclase.
Important features of olivines are: correlation of mg [- atomic Mg
/(Mg + Fe)] with rock
type giving test of differentiation models; Ca may distinguish volcanic from plutonic environment; cr enters as divalent ion, and may provide guide to oxidation state; Mg,Fe ordering
of some olivines indicates long annealing; melt inclusions give magma composition; high Al
in some olivines may correlate with spinel exsolution; melt inclusions give magma cornporitioo.
Introduction
presentation many relerencesore giuen, but these
are
highly selectiue and chosen for conuenienceand
This written address differs considerably lrom the
not
oral presentation. Because ol time limitations, the
lor priority or prestige. In particular, relerences
general introduction was omitted. Seueral handred
tend to concentrate on controaersial items and on
papers submitted lor publication, Matters of commin:erdogists and peffologists worked o,n the lunar
program, and most ideas were discouered almo,st mon krwwledge ffe not fally documented.
simultaneously by seaeral persons or groups. In the
The PresidentialAddressprovides an opportunity
oral presentation, relerences to indiuidual scientists for the Retiring Presidentto philosophizeor to rv
were deliberately omitted to auoid tedious repetition
view some aspectof his work or of mineralogy in
and inuidious selection of names. In this written
general. I shall briefly pontificate, and then use a
l PresidentialAddress,
MineralogicalSocietyof America. review of lunar mineralogyas an implicit illustration
Delivered at the 54th Annual Meeting of the Society, 13 of my main theme: that the distinctionsbetween
November1973.
mineralogr and allied sciences(including geochemis231
IOSEPH V. SMITH
try, geophysics,petrology, and crystallography) are
blurring so rapidly that research mineralogistsshould
become competent in as many areas as possible, and
should collaborate closely with scientists lvith related
and overlapping interests.
It is quite impossible to understand the phase
equilibria of (say) plagioclase feldspars without information on the following: (1) controlled laboratory syntheses at high temperature with X-ray diffraction and electron-optical characterization of the
products; (2) chemical, X-ray diffraction, and electron-optical characterization of plagioclases from
natural rocks ranging from volcanic to metamorphic; (3) heating studies of natural specimens. In
addition, calorimetry and resonancetechniques, plus
many others, provide further knowledge. A crystallographer without a thermodynamic background
might misinterpret the phase relations, while a geochemist might overlook the importance of domain
texture in the occurrence of trace elements. The
electrical conductivity and diffusion coefficients of
ferromagnesian minerals depend strongly on structural defects and the oxidation state, thereby having
a profound effect on geophysical interpretation of
phenomena such as creep in the mantle and the temperature profile of the Moon. The routine use of the
electron microprobe (hopefully to be joined by the
ion microprobe for trace elements and isotopic ratios) brings geochemical data within the immediate
control of the mineralogist and petrologist. The organization of research is beginning to utilize the
power of coordinated studies, as in the mineralogypetrology-geochemistry consortia for lunar specimens. Many mineralogical problems must be studied
by groups of investigators, often geographically dispersed; such investigators should be diplomats as
well as scientists.
Unquestionably,it is important to develop strong
cooperativeprogramsamong the various mineralogical groups (and also among the various geological
societies); otherwise prolonged isolation leads to
distrust and antagonism.It would be tragic if the
world of mineralogy split up into separatecamps,
each fearful and contemptuousof the others.
Finally, I want to mention the practical uses of
minerals.Agriculture dependson the proper interaction between the inorganic minerals and the organiccontentof soils.Many chemicaland all metallurgical industriesdependon extraction of minerals,
some of which are becoming scarce.Novel uses of
minerals, such as the employmentof zeolites as
catalystsin the petroleum industry and the applieation of mineral analogs as lasers and solid-state
devices,should attain increasingimportance; for
example,severalrare mineralswith strong pleochroism may havepotentialelectronicuses'For this reason, mineralogicalsocietiesshould also collaborate
closely with chemical and physical societies.
No mineralogistcan rest on the knorvledgegained
in his formal education.One of the greatadvantages
of the lunar program was the opportunity for the
scientiststo meet closely with practitionersin other
disciplines. Some mineralogists gained a working
knowledgeof such apparentesotericaas a europium
deficiencyand the time relation of BABI and ADOR.
It is extremely important to provide opportunities
for mineralogiststo expandtheir knowledge,and the
Mineralogical Societyof America shouldbecomeactive in organizing short coursesand symposia,and
in publishing critical monographsand treatises'
So much for the pontificating;now for lunar mineralogy with its illustration of the strong interconnectionsbetweenmineralogy,petrology, geochemistry, and geophysics.
Fortunately,not all mineralogyrequires the big
Lunar Mineralogy
battalionswith the kilo- or mega-buck.A perennial
joy of mineralogyis the scopefor the collectorand
Introduction
the artist, for the field man and the museumcurator.
The completionof the Apollo missionsprovidesa
Somemodern artistsexaminethe world for existing
natural beauty; collectors of minerals have been suitabletime to evaluatethe statusof lunar minerdoing this foi centuries.Mining operationscontin- alogy.An article"How the Apollo programchanged
ually reveal new mineral deposiisof spectacularin- the geology of the Moon: suggestionsfor future
terest,such as the one at St. Hilaire. Somemineral studies"by Smith and Steele(1973) givesa general
museumsexpand their collectionsand put on mag- background.Briefly, the Moon was in orbit around
nificent dispiays;others deteriorate.Probably most the Earth at least 4 giga-yearsago; it underwent
recognizethe value of a pluralisticap- major chemicaldifferentiationat least3'9 giga-years
mineralogists
proach,*a uppr.iiute the mutual interestsand dif- ago (and perhapsas early as 4.5 giga-yearsago)
ierent styles of the various types of mineralogists. producinga crust, a mantle,and perhapsa core; in-
LUNAR MINERALOGY
tensebombardmentdevastatedthe surfaceto depths
of severaltens of kilometers,perhapswith significant
late accretionof the incoming projectiles;lava flows
coveredimpact basinswith the last activity occurring
near 3 giga-years ago (though some unexplored
areasof the Moon may have later extrusions);and
continuing bombardmentby relatively small projectilesproduceda surfacecoveredby debris(regolith).
Geophysical and geological data show that the
presentcrust of the Moon consistsof a looseregolith
some meters thick lying on some tens of kilometers
of material with low seismic velocity. In the mare
regions, the upper parts are dominated by basalts
rich in Fe and Ti, whereasin the highland regions,
the dominant rocks are probably rich in calbic
plagioclasewith subordinate pyroxene and olivine.
The nature of the hypothetical mantle is controversial, but many workers believe that it is dominated
by magnesianolivines and pyroxenes.Figure I, a
model of the Moon proposed after the Apollo 11
mission,may provide a crude description for a time
prior to 4 giga-yearsago. There are some very difThin crusl (5-2Okm)
u n d e r l o i nb y l i q u i d
Impocls creote
lorge lovolokes
P l o gi o c l o s e- r i c h
rock ond bosolfs
=2.8-l.t gm/cmJ
P
(seos
._ ei?,r'
-
Liquid feirobosotl
p = 3.3 gm/cm3
( d r o w nt o n e o r s i d e
by Eorth's grovity
due to highdensily
Forside
ick Crust,
liftle or no
underlying
liquid
Impocls creote
deep crolers
w i l h l i ff l e " o r
no lovo.
t ' t h i n l o y e ro f s u l f i d e
o n d i l m e n i i e- r i c h
r o c k .p : 5
comporedlo overlyingcrusf')
Ftc. 1. A possiblemodel for the Moon during primary
differentiationprior to 4 giga-yearsago. This m9!-e] was
proposedin l97o beforedetailedcharacterizatio"
rocks. Major reworkingof the outer part of "l-*fP!"
the Moon
would occur in the neit giga-year,and the FEn-(;;.;;
basaltswould appearin this period.The ANT groupmight
consistof the '"plagioclase-rich
rock and basalts"plus further differentiatesfrom the ferrobasalt.The KREEp group
might arise from both the late "ferrobasalt"and reme.lting
of the ANT group.The FETI basaltsmight result from
remelting of cumulatesof pyroxene and oxide ;";;;
followed by complexfractional crystallization.Minor ultrabasicrocksmight be cumulatesfrom the uppermantleand
crust.From Smitheral (1970).
233
ficult geochemicaland geophysicalproblems, especially concerning(1) the sourcesof energyand the
timing of igneouseventson the Moon, and (2) the
ultimate origin of the bulk chemistry of the Moon.
The mineralogy and petrology of the Moon provides a fascinatingchallengeto the lunar detective.
Few samplesreturned to Earth are simple igneous
rocks. Most consist of complex breccias or finegrainedsoils.In spite of careful selectionof prospecting sites and detaileddocumentation(too detailed
for the temper of many lunar investigatorsfacedwith
sampledescriptionforms!), it is very difficult to pin
down the actual starting point of most lunar samples.
Fortunately the mineralogy of the Moon is rather
simple compared to that on the Earth, comprising
only some few dozens of speciescomparedto several thousand (ultimately, of course, the full range
of meteoritic minerals should be found on the
Moon). There are.few rock types (though one
might not think so from the confusingnomenclatures
in the literature!). In general,it is possibleto arrange most of the information on lunar rocks and
minerals into plausible sequencesconsistent with
crystal-liquiddifterentiation,metamorphism,hybridization.The major uncertaintiesrelateto geochemical
and geophysicalproblems: what were the time sequencesof crystal-liquid fractionation; what was the
extent of late accretion; how important were remelting and metamorphism;what are the implications for the internal structure and the bulk composition of the Moon? In attemptingto answerthese
questions, the mineralogic detective must bear in
mind that the regolith is dominated by crustal material,and that deepseatedmaterialshouldbe sparse
and found mainly as fragments in breccias from
major impacts.
Generalstatementon lunar rock types
The nomenclature of the lunar rocks is in a shambles. Unfortunately many lunar rocks are actually
metamorphosed breccias or remelted regolith, ani
,o,n" ,ru.".
are given on the basis of bulk chemistry
"".:'":"'rather than the petrology and mineralogy' Furthermore, the chemistry of the minerals is biased towards
alkali-poor compositions and reduced states of transition metals with respect to terrestrial analogs, thereby
causing
'* tf,oUt"11,
when terrestrial names are applied
"
--l;
to lunar rocks.
The following nomenclature and description of
the major rock types are chosen to maximize initial
accessto the literature:
234
IOSEPH V. SMITH
ANT suite: (name from Anorthositic-NoriticTroctolitic; Keil el aI, 1972) comprised mainly of
calcicplagioclase,
orthopyroxene,and olivine; ranging from anorthositic to noritic and troctolitic in
bulk composition.Mostly occurs as metamorphosed
breccias (often monomict), fragments,or glasses.
Commonlyassignedto crust.Believedto resultfrom
early differentiationwith gravity separationof crystals and liquid. Terms like gabbro,anorthositicgabbro, and highlandbasalt are also used.
FETI suite: (new name referring to high Fe and
Ti content) comprisedof basaltsrich in Fe and Ti;
dominated by zoned pyroxenes (mostly augitic),
calcicplagioclase,
and ilmenite.Olivine may or may
not occur. The final residuumcontainsseveralrare
mineralsand has a granitic component.Mostly occurs as rock fragments,but some glassesoccur. Interpreted as the result of late lava flows in mare
basins,probably mostly frorn the tops of lava columns severalkilometersthick.' Origin controversial
but perhaps from remelting of cumulates rich in
pyroxenesand opaque minerals, modified by nearsurfacedifferentiationin lava suites.Usually called
marebasaltin literature.
KREEP suite: (name is acronym given by Hubbard et aI, l97la) a description introduced for basalticcompositions
rich in K, REE, P,Zr,Ba, U, and
Th. Mostly occursas glassesand meta-breccias
but
a few crystalline rocks with basaltic texture occur.
Dominated by plagioclase,low-Ca pyroxene and
mineralssuch as apatite and zircon which contain
the abovecharacteristicelements.Origin controversial but perhapsresults from either liquid produced
at the crust-mantleinterface during primary difterentiation, or liquid from partial melting of ANT
crustal rocks, or both. Other terms includinghighaluminabasalt are also used.
There are some problems with this three-fold
nomenclature:(1) There is evidenceof both lowand high-K anorthosites (e.9., Hubbard et al.,
l97lb); perhapsthe former wereproducedby flotation of plagioclase
duringformationof the crust,and
the latter from crystal-liquid differentiationof a
K-rich liquid produced by partial melting of the
crust; if so, the former might be assignedto ANT,
and the latter to KREEP. (2) Somerocks such as
14310 almostcertainlywere producedby remelting
of regolith (e.g.,Denceand Plant, 1972), and therefore have a bulk compositionderived from many
componentsincluding meteoritic infall (Morgan
et aI, 7972); many brecciasshow metamorphictex-
tures (e.g.,Andersonet al, 1972;Chao,Minkin, and
Best, 1972 Warner, 1972), and it is not always
obvious what rocks and minerals composedthe
original breccia. (3) The applicabilityof the terms
high-aluminabasalt and very high-aluminabasalt
(e.g.,Bansalet al, 1973) with respectto other rock
descriptionsis not fully clear at the moment (see
later); whethersuch rocks representa distinctigneous suite, or whether they are hybrids formed by
meltingof regolith,or both, is unclear.
On the whole the abovethree-foldgroupingcovers
many of the lunar rocks, correlatesfairly well with
the chemistry of the component minerals, and perhaps also correlateswith chronologicaldevelopment
of lunar rocks. It certainlyallows a preliminaryexamination of the petrogeneticsignificanceof mineralogicalproperties.
The nature and extentof the minor rock typesis
rather uncertain. Crystal-liquid differentiation of
basalticcompositionsleavesa final immiscibleliquid
with granitic affinity (e.g., Roedder and Weiblen,
1970). Minor glassyor crystallinerepresentatives
were found in the lunar samples,but whetherlarge
rock bodies of granitic composition occur is uncertain. Grains of olivine and pyroxene with very Mgrich compositionsoccur as a minor constituentin all
the lunar soils (seemany papers),and spinelallivalite and troctolite samples(e.g., Agrell et al, 1973;
Prinz et aI, 1973a), a peridotite clast (Anderson,
1.973) and ultrabasic fragments (e.9., Steele and
Smith, 7972\ have been described.All thesematerials are more Mg-rich than equivalents from the
broad ANT suite. A possible source is the upper
mantle, but crustal processesor accretionmust be
considered.Rare fragmentsof iron meteoriteshave
been found with characteristictextures, and other
iron specimensare believedto repfesentmelted or
metamorphosedmeteorites on chemical evidence.
Contributions of stony meteoritesto the lunar fines
have been recognizedfrom bulk chemistry (e.9.'
Ganapathyet al, l97O), and an enstatitechondrite
has been found (HaggefiY, 1972a).
Types ol Lunqr Minerals
The major mineralsto be describedin detail later
are:
Pyroxene
Feldspar
(Mg, Ca, Fe, Al, Ti, Cr, etc)
(Si, elc)O3.Very low in Na and K.
Mostly anorthiteand bytownite,but
rare K, Ba-feldsparin residua.
LUNAR MINERALOGY
235
rocks, e.g. Taylor, Mao, and Bell, 1973), and magnetite?(in "rusty" rocks).
Hydroxylated mineral: "rusty" areas on some
rocks contain goethite? or another iron oxide hydroxide (Agrell et al, 1972), perhapswith hematite
and magnetite;presenceof Cl suggestspossibility of
terrestrial oxidation of lawrencite,originally derived
from cometary or meteoritic contamination on the
Moon (El Goresy et al, 1973; Taylor, Mao, and
Bell, 1973), but lead isotopedata suggestthat the
material is indigenousto the Moon (Nunes and
Tatsumoto, 1973).
Minor minerals,probably largely from iron meteorites, are: schreibersite,
cohenite,and niningerite.
The nature of the lunar mineral variously compared with terrestrial zirconolite or zirkelite is controversial, and X-ray structural data are neededto
provide a control for assignmentof atoms: either
(Ca, Fe, etc) (Zr, etc)zOzfor zirconoliteor 1:1:5
Minor silicatemineralsare: pyroxferroite(analog for zirkelite (Wark et aI, 1973; Buscheet al, 1972;
of pyroxmangite, common in residua of FETI ba- Peckett, Phillips, and Brown, 1972; Brown et aI,
salts;Chaoet al,1970; Burnham,1971), amphiboles 1972). Other Zr, Ti-rich mineralsmay occur (e.9.,
(single occurrencesof alumino-tschermakite(Dence Zl: Haggerty.1972b).
et al, l97l), and magnesio-arfvedsonite-richterite Minor metallic mineralsare: copper (tiny grains
(Gay, Bancroft,and Bown, 1970) in FETI basalts; near Fe metal, ilmenite,or troilite), brass (variable
high in halogens), garnet (almandine-richgrains in compositionextendingoutsideterrestrialrange), tin
a FETI basalt; Traill, Plant, and Douglas, l97O), (embeddedin Fe metal), and indium (contaminant
tranquillityite (residua of basalts; Lovering et al, from rock box).
I97l), melilite? (uniaxial grainsin fines,no X-ray
Aragonite(Gay, Bancroft,and Bown, 1970), SiC
or chemicaldata; Massonet al, 1972), phyllosili (Jedwab,t973), and graphitemay be contaminants.
cates??(in fines,probably terrestrialcontaminants;
Dreveret al, l97O), sphene??(perhapsin devitrified Important Aspectsol Feldspars
spherule,tentative X-ray powder data; Gay, BanIn this and succeedingsections,some aspectsof
croft, and Bown, l97O\.
the major mineral groups important for understandMinor sulfide minerals are: mackinawite,pent- ing the genesisof lunar rocks and minerals are outlandite, cubanite,chalcopyrite(all associatedwith lined.
troilite at margins or as inclusionsconsistentwith
The great majority of lunar feldsparsare calcic
exsolution or inversion; optical and chemical iden- plagioclases.
K,Ba-feldspars
crystallizein the residua
tification but no X-ray data; e.g., Taylor and Wil- of FETI basaltsand occur in KREEP basalts;they
liams, 1973), sphalerite(e.g., El Goresy,Taylor, may occur with silica minerals as a granitic rock
and Ramdohr,1973).
typ @.g., fragment of hedenbergitegranophyre,
Minor oxide mineralsare: rutile (associated
are very rare
with Mason et al, l97L). Sodicplagioclases
ilmenite as lamellae and inclusions indicative of and may resultfrom minor meteoriticcontamination.
exsolution,e.9., Haggerty,1972b; as distinct grains
The plagioclases
in the FETI basaltsrange from
in a peridotite,Anderson,1973; and as blue grains sector-zonedhollow tubes to anhedral grains interin soils, Jedwab, 1973), corundum (in fines as gro\rynwith pyroxene.Plagioclasesfrom ANT rocks
aggregates,Kleinmann and Ramdohr, 1971; and usually show textures ranging from fine-grained
as large crystal twinned on (0001), Christophe- "equilibrium" mosaicsto irregular shock textures;
Michel-Levy bt al, 1973; vapor deposit?,meteoritic? probably most textures result from one or more
or terrestrial contaminant?), hematite? (lamellae episodesof metamorphism(both thermaland shock
in ilmenite,and associated
with goethite?in "rusty" types) following an earlier igneousstage.
Olivine
Mostly Mg-rich but rare fayalite in
residua.Divalent Cr.
N MgAlrOn in Mg-rich rocks.
Spinel
Chromite zoned to ulviispinel in
FETI basalts.
Pseudobrookite Armalcolite group with divalent
iron. Low Cr and Zr in FETI
basalts and high Cr and Zr in
metamorphosedMg-rich rocks.
Silica
Tridymite, cristobalite,and quartz.
Fe-rich
Iron, troilite, and ilmenite occur,
especiallyin FETI basalts.
Phosphates
Apatite and whitlockite, especially
common in KREEP rocks. High in
REE, Cl, and F.
Zr-minerals
Zircon and baddeleyitetypical of
KREEP rocks; also in residua of
FETI basalts.
|OSEPH V. SMITH
Electronmicroprobeanalysesof ANT plagioclase
are consistentwith the MTaOs formula, but interpretation of analysesof FETI plagioclaseis unclear.
The location of the Mg and Fe atomsis uncertain,
and the closestfit to an MT4O8formula is obtained
if theseelementsare mostly in the tetrahedralsites.
Weill el ol (1971,) outlinedthe problem,and Wenk
and Wilde (1973\ coordinatedthe availabledata.
Mcissbauerand electron-spinresonancedata show
that a small part of the iron is ferric while most is
ferrous,and that the iron enterstwo structuralsites,
one of which is tetrahedral(e.9., Schiirmannand
Hafner, 1972a;Hafner, Niebuhr, and Zeka, 1973);
see also optical absorptiondata (Bell and Mao,
1973).
The compositionof the calcic plagioclasescorrelateswith the host rock. Figure 2 is a plot of wt
percentFe os mole percentAb inferred from the Na
content. The plagioclasesfrom the FETI basalts
have the highestFe contents,and there is a definite
tendencyfor the Fe contentto increasefrom about
0.5 wt percentat Ab5 to 1 wt percentnear Abrz.
The plagioclases
from the KREEP basaltstend to be
more sodic,mostly occurringin the rangefrom Abro
to Abzs: again the Fe content correlateswith Ab,
but the concentrationis lower than for FETI basalts.
that from ANT
The third main groupof plagioclases,
rocks, tendsto be highly calcic with mcst compos!
tions nearAbs. The compositionrangesof ANT and
t.2
r
- 0.8
KREEP
d-S
bosolls
=
------
z-
x
xxx
0
- 2 0 r 0 "0I
x
- ----
--\__Ilori--tlt
i--J-b-,x___)
m o l%
. Ab
Frc. 2. Fe us Na content of lunar plagioclases.Schematic
drawing showing principal rangesfor FETI basalts,KREEP
basalts, and ANT materials. P shows the plagioclase in
peridotite clast 15445,10 (Anderson, 1973) and PST that
for pink-spinel troctolite (Prinz et al, 1973a). The ranges
for plagioclasesfrom KREEP and ANT rocks probably
overlap. The crossesshow data for 67747 feldspathic basalt
which contains large poikilitic olivines enclosing feldspar
laths and intergranular pyroxenes (Steeleand Smith, 1973).
Data from many sources.Data for Mg, Mg/Fe, and K to be
publishedelsewhere.
KREEP plagioclases
appearto overlap.The Mg and
K contentsappearto be distinctivefor the threetypes
of plagioclase,
but the data are probablyconfusedby
relativelylow accuracyof electronmicroprobeanalySmith (1971,)notedthat the
sesfor theseelements.
compositionbands seemedto divergefrom a compositionnear Ab3; K 0.03,Fe 0.1, Mg 0.01 wt percent, and this turned out to be a commoncomposition for plagioclasein anorthosites(e.9., Steeleand
from a peridotite and a
Smith, l97l). Plagioclases
pink-spineltroctolite fall near the main concentration rangefor ANT rocks.As an illustrationthat the
situation is not simple, data points are shown in
Figure2 for a feldspathicbasaltwith igneoustexture
but Fe content evenlower than for KREEP basalts
(SteeleandSmith,1,973).
The well-knownpreferenceof Na for liquid over
coexistingplagioclase(e.9.,Kudo and Weill, 1970)
to be used
allowsthe Na contentof lunar plagioclase
low
Ab conmodels.
The
as a test of differentiation
their
origin
plagioclase
with
is consistent
tent of ANT
produced
later
as cumulatesfrom liquids which
KREEP plagioclase(note the very calcicplagioclase
in the pink-spineltroctolite); alternatively,partial
melting of ANT rocks could yield liquids which give
rise to KREEP plagioclase.If FETI basaltsresult
source
from partial melting of a plagioclase-bearing
material,that plagioclasewould be extremelylow in
Ab content, assumingthat equilibrium occurred.
Unfortunatelythere are few laboratory data on
the partition of Fe, Mg, K, etc betweenplagioclase
and basalticliquids,thoughone might expectthat the
substitutionof Mg and Fe in plagioclasewould decreasewith temperature,and that the Mg/Fe ratio
will dependboth on the oxidation state and the Mg/
Fe ratio of the coexistingliquid. Greenet al (1972)
and Akella and Boyd (1972) reported 0.3-0.6
MgO and 0.5-1.0 wt percent FeO in plagioclases
synthesizedin the presenceof liquid from lunar
basalticcompositions,which valuesare comparable
with those observedin FETI basalts. Systematic
data are badly neededto allow use of these elements for testing difierentiationmodels. The mg
ratio of the plagioclase(where mg : atomic Mg/
(Mg + Fe) ) may prove valuable in giving an
indication of the parent liquid (e.g., Walket et aI,
1973); sparsedata of low accuracysuggestthat
lng tends to be higher for KREEP than for FETI
plagioclase,as expectedby comparisonwith the
data for pyroxenes.Crawford (1973) found that
mg of zoned FETI plagioclasewas the same as
LUNAR MINERALOGY
for pyroxene crystallizing simultaneously.Detailed
measurements
of the alkali and alkaline-earthelements are badly neededas tests for differentiation
models; unfortunatelythe concentrationsof these
elements, except for Na and K, are too low for
accuratemeasurement
with an electronmicroprobe,
but hopefully detailed ion-microprobe data will be
forthcoming (see Andersen,Hinthorne, and Fredriksson, 1970, for indicativedata).
Interpretation of the minor element content of
plagioclases from ANT rocks, metamorphosed
breccias,and fines must take into accountthe effect
of solid-statereactionswhich change the content
of minor and trace elements.Steele and Smith
(1974) pointed out that the pyroxene and silica
minerals occurring as inclusionsinside ANT plagioclase,and as grains at plagioclaseboundaries(e.g.,
James, 1972), may result from solid-state expulsion from plagioclasewhich originally showed high
contentsof Fe and Mg, perhapscomparableto those
for FETT plagioclase. They proposed that with
falling temperaturethe componentCa(Fe,Mg)SisOe
broke down into pyroxene Ca(Fe,Mg)Si2O6and
silica SiOe. If this is true, the pyroxenesin some
lunar anorthositesconsist partly or perhapswholely
of material exsolvedfrom plagioclase.Nevertheless
the mg content of the pyroxene may provide a
guide to that of the liquid from which the plagio.
clase crystallized.The best evidencefor exsolution
is that of Lally et al (1972) who obtainedelectron
micrographsof pyroxenegrainslying in twin boundariesof plagioclase.Brett et al (1973), and others,
reported tiny oriented rods of FeeeNi, metal in
plagioclase grains from Luna 20, and preferred
sub-solidusreduction as the explanation.
The europium anomaly has figured in many geochemical discussionsof lunar rocks. Unfortunately
all accurate data obtained to date are on bulk
samplesusing neutron activationor mass spectrometric analyses.Under reducing conditions,europium becomesdivalent, whereas its neighboring
rare earths remain trivalent. In the divalent state.
europium tends to enter the feldspar structure
preferentially,thus giving a positive anomaly in a
feldspar and a negative anomaly in a coexisting
basalticliquid. Unfortunately,the europiumanomaly
dependsin a complex way on the temperature,
oxygenfugacity,and bulk composition(e.g., Weill
and Drake, 1973) and carefulstudy will be needed
before quantitativeevaluationof lunar rocks can
be made. Neverthelessit seemsreasonablvcertain
237
that the europium anomaliesin lunar rocks can be
ascribed largely to plagioclaseJiquidfractionation.
There are many electron-diffraction and X-ray
difiraction studiesof lunar plagioclaseswhich show
complex domain textures indicative of exsolution
and order-disorder. Unfortunately, the phenomena
depend in a complex way on the bulk chemical
composition and on the crystallization and postcrystallizationhistory. At the moment the data are
of more value in providing evidenceon the phase
relationsof plagioclasein generalthan on the specific problems of lunar mineralogy.
It is quite certain that lunar plagioclasecan
provide many clues to the discerninglunar detective.
Important Aspectsol Oliaines
The mg value of lunar olivines provides a valuable guide to crystal-liquiddifferentiation.Figure 3
summarizesthe principal ranges of mg for lunar
olivines togetherwith correspondingdata for pyroxenes (see next section).
The ANT group of rocks carries olivines mostly
in the range of mg = 0.85 - 0.65 whereasthe
FETI group carries olivines ranging mostly from
0.7 to 0.3 with rare fayalitesin the final residua.
KREEP
(mol.%)
,
En
Fo
r00
Fs
Fo
80
60
40
(v
Ftc. 3. Schematic drawing of major-element concentrations of lunar olivine and pyroxene. Note that considerable
zoning occurs and that the ranges cover the principal but
not the total ranges of composition. The pyroxenes of FETI
basalts show final zoning to pyroxferroite or Fe-rich
pyroxenes, while the olivines zone into fayalite. Both
KREEP and FETI pyroxenes give compositions which
tend to concentrateat the Mg-rich side of the ranges.Data
from many sources. The data for ultrabasic fragments
(filled circles) are from Steele and Smith (1972). P and
PST refer to peridotite and pink-spinel trocolite (see Fig.
2). The rectangle shows the composition range of large
crystals of orthopyroxene in Apollo 17 fines which may
result from a true norite with coarse texture: veins of
diopside have the same composition as the filled circle
(Irving et al,1974).
238
TOSEPH V. SMITH
Ultrabasic rocks and fragments carry olivines with
mg = 0.93 - 0.83; sevencompositionsfound by
Steeleand Smith (1972) for clasts and fragments
are shown in Figure 3 along with that of 0.91 for
the peridotite clast of Anderson (1973) and of
0.92 for the pink-spinel troctolite of Pinz et al
(1973a). Other data in the literature for fragments, clasts, and single grains fall in the same
range. Olivine is rare or absentin KREEP rocks.
Figure 4 is a simplified versionof Figure 6 of Steele
and Smith (1973\ which shows data on mg of
olivines, Ca-poor pyroxenes,and basaltic liquids.
Roeder and Emslie (1970) determinedthe fractionation of Mg between olivine and terrestrial
basalticliquids,and Medaris (1969) evaluatedthe
fractionation of Mg between olivine and low-Ca
pyroxene. Electron microprobe analysesof synthetic lunar samplesyield a single band displaced
to the left of the one for terrestrialsamples,and
there is no clear distinction betweenolivines and
low-Ca pyroxenes.At liquidus temperatures,mg is
similar for low-Ca and high-Ca pyroxene,but at
low temperature,mg is higher for the low-Ca pyroxene. In general,for crystalJiquid fractionationon
the Moon this diagram provides a good test of
differentiation models, irrespective of the precise
details of temperatureand pressure.
Consider the olivines of the ultrabasic group.
If theseare phenocrystswhich accumulatedfrom a
basaltic liquid, that liquid must have mg ranging
from about 0.6 to 0.8. This is actually the range
-^folivine
" ' e l l o w- C o p x
intered:
o li v i n e s !
0 n 0p x s :
of sourceI
rocks of :
tL
0.6
| | DOSdI$
0.8
olivinesofT
ultrobosic
l
frogments-
ANT--KREEP
FETI
mg liquid
Ftc. 4. Relation between mg for coexisting liquid and
either olivine or low-Ca pyroxene. The shaded area shows
the range of synthetic data for lunar basaltic compositions
obtained by various workers including Biggar et aI (1971),
Green et aI (1971, 1972) and Kushiro (1972). The ranges
below the abscissashow the bulk composition for the three
major rock types. For KREEP-type material, the mg value
drops to zero for extremely-differentiated residua. See text
for further explanation. [Modified from Steele and Smith
( 1 9 7 3F
, ie.6)1.
of mg for the bulk compositionof most ANT rocks.
If the FETI basalts result from partial melting of
a rock dominatedby olivines and pyroxenes,the
mg rangeof the latter should be about 0.6 to 0.8.
Partial meltingof the ANT suite shouldgive liquids
with mg mostly from 0.4 to 0.6, which overlaps
the rangesfor the bulk valuesof FETI and KREEP
basalts.
The minor elementsin olivinesvary considerably,
and have petrogeneticvalue. Smith (1977) cautioned about problemsarisingfrom inaccurateelectron-microprobeanalyses,especiallyfor secondary
fluorescenceof transition metals. Review of the
extensivedata in the literatureshowsthe following:
( 1) Mn correlatespositively with Fe and has
no petrogeneticsignificance(indeed major deviations from the correlation probably indicate an incorrect analysis).
(2) Ca tends to increasewith Fa content ranging from 0. 1-0.4 wt percent CaO near forsterite
to 0.4-0.9 wt percentfor the rare fayalitesin residua
of FETI basalts(Fig. 5). The rangesfor the FETI
and ANT groups overlap, but the latter tend to be
richer in Mg and poorer in Ca. Note that many
specimensin the ANT group are breccias, often
recrystallized.Simkin and Smith (1970) found that
the Ca-content provides a distinction between terrestrial olivines from plutonic rocks and those from
volcanic or hypabyssalrocks. In Figure 5 all the
olivines from FETI basalts lie in the region for
volcanic and hypabyssalterrestrial olivines; furthermore, the dominantrangeof FETI olivinesis close
to the data points found by several investigators
for olivines synthesizedin the presence of liquid
from lunar basalticcompositions.The ANT group
overlaps both the plutonic and the volcanic-hypabyssalrangessuggestingthat this group derivesboth
from volcanic and plutonic rocks. Smith (1971)
noted a distinction betweena high-Ca and a low-Ca
group on the basisof just Apollo 11 and 12 data,
but examinationof all available data shows that
there is a continuousgradation-e.9., the data by
Prinz et al (1973b) for ANT fragments in the
Luna 20 sample.The effect of metamorphismon
the Ca content of olivine has not been quantified
so far, but availabledata on diffusion rates (e.9.,
Buening and Buseck, 1973) suggestthat metamorphism in a thick ejecta blanket may cause significant migration of Ca from the olivine. Hence
a low Ca content may indicate either a plutonic
origin, or metamorphism,or both. The possibility
LUNAR MINERALOGY
r.0
FETI,,
0.8
o
o
O
he
E
nA
ANT
t^
A
IPST
l-/
0.2
n
Fo
|
\?
ltt
;/
lr/
A
A
0.4
---;;L-rffiyt__
_
\---__
XP |
80
- -'i
v o l c o n igc
|
60
40
m o l .% F o
20
Fo
FIc. 5. Relation between wt p€rcent CaO and mole percent Fo for lunar olivines. The two ringed areas show the
ranges of electron microprobe analyses for ANT and FETI
olivines. Most specimens fall in the regions of Fo marked
by the continuous lines. P and PST stand for a peridotite
and a pink-spinel troctolite (see Fig. 3). The dashed line
marks the boundary between terrestrial olivines from
plutonic us volcanic and hypabyssal rocks. The triangles
show olivines crystallized from liquids of lunar rock compositions (Biggar et al, l97l; Green er al, 1971, 1972:
Kushiro, 1972).
of metastablecrystallization with interaction between
olivine and other Ca-bearing minerals must be evaluated; furthermore the bulk Ca content of the host
rock must be considered. Butler (1972) gave detailed data on the zoning trends of four FETI
basalts, and found a correlation of Ca content with
texture indicating the effect of temperature of
crystallization or of cooling rate.
(3) Cr tends to correlate with Mg, but may
also correlate with the Cr content of the host liquid
and more importantly with the oxidation state. In
terrestrial olivines, the Cr content is undetectable
by normal electron microprobe methods presumably
because the Cr is trivalent and is not accepted
easily by the olivine structure. In lunar olivines, the
Cr content may rise to 0.n wt percent, presumably
because divalent Cr is similar to Mgz* and is accepted by the olivine structure. Haggerty et ol
(1970) reported optical absorption bands characteristic of Cr2*, but not of CrS-, in olivine from a FETI
basalt, but no comprehensivestudy of lunar olivines
has been reported. Figure 6 shows the ranges for
lunar olivines. The band for FETI olivines cor-
239
relatesnicely with data obtainedby severalworkers
for olivines synthesizedfrom melts of lunar basaltic
composition.The band for ANT olivinesis displaced
to higher Mg values and lower Cr concentrations.
Particularly noticeable are the low Cr concentrations for the peridotite and pink-spinel troctolite.
Currently there is no definitive explanationfor the
wide range of Cr-contents (when normalized for
the Mg correlation). A simple explanationmight
be that outgassing
of the Moon resultedin reduction
of Cr3* to Cr2*. Alternatively one might invoke
efiectsfrom the bulk compositionof the rock, from
the temperatureof equilibration,and from the effects
of simultaneouscrystallization of minerals such as
spinel and pyroxene (e.9., Butler, 1973). Dodd
(1973) reported Cr contentsin olivines from the
Sharps (H-3) chondrite. These occupy the range
from 0.02-0.2 wt percentCr2O3and 100-70 mole
percent Fo, without any correlation between Mg
and Cr. Dodd found a weak correlation between
Cr and Al. and favored the occurrenceof some of
the Cr as trivalent with a coupled substitution of
cr3* * Al3* - (Mg,Fe)r- + Si4..
(4) Al is a puzzle.Accurate analysisof Al by
electron microprobe methods is difficult because
n PFI
n
O
.6o
0.6
(J
Gj
O
d-e
0.4 bs
0.4
B
<j
=
0.2
U
Fo
BO
60
40
20
Fo
mol.% Fo
Frc. 6. Relation between wt percent CrrO" (or CrO) and
mole percent Fo for lunar olivines. The two ringed areas
show the ranges of electron microprobe analyses for ANT
and FETI olivines. The majority of the specimens fall in
the regions of Fo marked by the continuous lines. P and
PST stand for a peridotite and a pink-spinel troctolite (see
Fig. 3). The triangles show synthetic olivines crystallized
from liquids or lunar rock compositions (Biggar et a/,
1971, Green et al, 1971, 1972; Kushiro, 1972). PB and C
stand for olivine from two picritic basaltsand a clast (Gay
et al. 1972).
240
IOSEPH
V. SMITH
of the non-linear background. Gay, Bown, and
Muir (1972) reportedcurved threadsof tiny beads
of spinel from a clast (C) and a picrite basalt
xenolith (PB) in breccia 14321 (see Fig. 6 for
Cr content). The Al contents (Fig. 7) of the
olivines(1.12 wt percentin C, 0.38 and 0.19 in
PB) are unusually high, and should be much
greaterthan any possibleexperimentalerror. Simkin and Smith (1970) found no Al at the 0.01 wt
percentlevel in terrestrialolivines,but more recent
analysesby several investigatorsshow O.0n wt
percent.Dodd (1973) reportsup to 0.07 wt percent
Al2O3 in olivines from the Sharpschondrite. Figure
7 summarizesthe data for olivines from FETI
basaltsand ANT materials.Most of them fall below
0.2 wt percentbut a few are higher. The synthetic
olivine synthesizedby Kushiro et aI (1971) at 10
kbar and l29O"C has 0.07 wt percentAlzOg which
is near the averagefor all the olivines from FETI
basalts.Akella and Boyd (1972) found 0.04 and
0.43 wt percent AlzOs in olivines synthesizedat
I 100'C and I bar. and at 1350'C and 30 kbar.
Perhapsthe substitutionof Al increasesrapidly with
temperatures
over 1300'C. Attention is now focused
F E T Ib o s o l l
ANT
d o.a
N
oA
I= o.o
-Fo
rU a 1350'C
a
PBoo^
te.Sc .-.
t4310
o
on olivineswith high analysesof AlzO:r.The brecciated anorthositeand pink-spinel troctolite fragment from Apollo 16 samples(Prinz et aI, 1973a)
are coarse-grainedrocks for which there should
be no difficulty in obtainingan accurateanalysisof
Al. The 14301 microbrecciais fine-grainedbut the
analysesare very consistent(Prinz et al, 1973c).
Only one datum is given for an olivine from the
l43l0 basalt. A few scatteredanalysesof ANT
olivines lie above 0.2 wt percent AlzO;r. At this
time, there is no explanationfor these very high
analysesof AlzO:;.The analystshave excellentreputations but the data should be recheckedin other
laboratories.Perhapssomeproblemsariseif samples
are polished with alumina instead of preferred
diamond.
(5) Ti analysesare mostly in the range below
0.1 wt percent,but it is possiblethat carefulstudies
will show interestingfeatures.Unquestionably,
careful re-analysisof minor elementsin olivines is desirable by electron microprobe methods together
with data on trace elements(especiallytransition
metals) by other techniques.
Other interestingfeaturesof olivinesare: (1) the
M1 octahedralsite of somelunar olivinesis favored
by Fe over Mg atoms, suggestingprolonged annealing (see referencesin Brown and Prewitt,
1973); (2) late-stagereduction of fayalite yields
Fe metal * silica mineral * glass(e.s.,El Goresy,
Taylor, and Ramdohr, 1972); (3) occurrenceof
inclusions yields an estimateof the composition of
the parentmagmawhenhomogenized(e.9.,Roedder
and Weiblen, 1972\.
Acknowledgments
Sincere thanks go to A. T. Anderson and I. M. Steelefor
Fo
40
60
20
80
their creative collaboration on lunar studies,to many other
m o l%
. Fo
faculty members at Chicago for their advice, to I. Baltuska,
R. Banovich, O. Draughn, T. Solberg, and others for meetFra. 7. Relation between wt percent Al,O' and mole pering so many technical guidelines. An unpublished glossary
cent Fo for some lunar olivines. A representative selection
of lunar minerals by J. W. Frondel was very helpful. Techof data for FETI basaltsand ANT materials is shown: note
nical and financial help from the National Aeronautics and
that the former data actually should concentrate near FooSpace Administration is gratefully acknowledged; indeed, I
Fo"o, but that there is a tendency for tables of analysis to
have been amaznd at the general competence of the Apollo
quote unusual Fe-rich specimens.For the ANT specimens,
program, and not surprised at the occasional incompetence
some data points from the main cluster have been omitted.
wben compared to problems in my own laboratory. Many
The unusual data are: C and PB, clast and picritic basalt
thanks go to the National Science Foundation for providing
in 14321 breccia (Gay, Bown, and Muir, 1972); PST and A,
basic support which allowed establishment of necessary
pink-spinel troctolite and anorthosite (Prinz et al, 1973a);
equipment and know-how. I. M. Steele kindly reviewed the
14301, microbreccia (Prinz et al, l9'73c);14310, basalt
manuscript.
(Gancarz, Albee, and Chodos, 1971). The triangles are for
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