Gabbroic Magma Research Articles

Gravity Interpretation of the Laramie Anorthosite Complex, Wyoming

Research Article| April 01, 1973 Gravity Interpretation of the Laramie Anorthosite Complex, Wyoming D. S. HODGE; D. S. HODGE 1Department of Geological Sciences, State University of New York, Buffalo, New York 14222 Search for other works by this author on: GSW Google Scholar L. B. OWEN; L. B. OWEN 2Department of Geology, University of Wyoming, Laramie, Wyoming 82070 Search for other works by this author on: GSW Google Scholar SCOTT B. SMITHSON SCOTT B. SMITHSON 3Gravity Interpretation of the Laramie Anorthosite Complex, Wyoming Search for other works by this author on: GSW Google Scholar Author and Article Information D. S. HODGE 1Department of Geological Sciences, State University of New York, Buffalo, New York 14222 L. B. OWEN 2Department of Geology, University of Wyoming, Laramie, Wyoming 82070 SCOTT B. SMITHSON 3Gravity Interpretation of the Laramie Anorthosite Complex, Wyoming Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1973) 84 (4): 1451–1464. https://doi.org/10.1130/0016-7606(1973)84<1451:GIOTLA>2.0.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation D. S. HODGE, L. B. OWEN, SCOTT B. SMITHSON; Gravity Interpretation of the Laramie Anorthosite Complex, Wyoming. GSA Bulletin 1973;; 84 (4): 1451–1464. doi: https://doi.org/10.1130/0016-7606(1973)84<1451:GIOTLA>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Interpretation of a gravity study of the Laramie anorthosite complex limits petrogenetic models. Bouguer gravity anomalies within the anorthosite average about −155 mgal, decrease by 12 mgal over younger granite, and increase up to 20 mgal over bordering syenite bodies. The gravity field over the anorthosite is low with respect to the metamorphic rocks on the northern border. Computed models indicate that the anorthosite mass is a plate 4 km thick with no apparent feeder pipes and is not underlain by a high-density mass. High gravity gradients and ubiquitous surface association of gravity highs over norite-syenite suggest that norite floors these syenite bodies at a depth as shallow as 0.5 km. Computed thickness of norite is about 4 km. From the gravity data, the norite-syenite seems to rim the anorthosite, except where younger intrusions may have disturbed this relation. Minimum calculated mass for respective rock types are: anorthosite, 9.7 × 1018 g; norite, 2.3 × 1018 g; syenites, 6.0 × 1017 g. The parent magma is estimated to have been of noritic anorthosite composition. Petrogenetic models derived from these data suggest that (1) in situ differentiation of gabbroic magma is unlikely, and (2) anorthosite initially differentiated from the parent magma followed by norite and syenites, then this sequence rose into the present domal configuration as a partly consolidated mush. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Rare earth element distributions in a proto-stratiform ultramafic intrusion

Rare earth element (REE) abundances are reported for ten whole rock and eight mineral samples from the Preacher Creek ultramafic intrusion of southeastern Wyoming. Chondrite-normalized distribution patterns for the whole rocks exhibit a broad maximum between Sm and Gd and reflect the REE pattern of clinopyroxene, the major REE-bearing phase. Alteration of the primary mineral assemblages to actinolite and chlorite, which is generally minor, does not appear to have significantly affected the REE distributions. Absolute abundances of the REE in the rocks and constituent minerals increase as a function of differentiation, and relative abundances suggest an accompanying light REE enrichment. Trapped-liquid phases, which may be relatively enriched in REE, possibly account for some or all of the observed REE trends. The REE data, interpreted in terms of crystal-melt fractionation, suggest derivation of the intrusion by crystallization from a gabbroic magma having a REE distribution pattern similar to the parent magma of the Skaergaard stratiform complex. The results of this study are in accord with and complement a previous proposal that the Preacher Creek body formed in a manner analogous to major stratiform intrusions.

Immiscibility between Feldspathic and Gabbroic Magmas

Although laboratory experiments in the past have not encouraged hopes of finding terrestrial rocks showing textures resulting from liquid immiscibility, this article describes work suggesting such an origin for features in some alkaline rocks.

Oxygen isotope study of the Koyama intrusive complex, Yamaguchi-Prefecture, Southwest Japan

The Koyama intrusive complex consists of varying rock-types from olivine pyroxenite to aplite as a consequence of differentiation of gabbroic magma. The whole-rock δ18O values of the main differentiates are +6.2 to +6.6‰ (in SMOW) for olivine rich members and are invariably around +7.8‰, from quartz gabbro to aplite. It is suggested that the Koyama magma differentiated without any significant isotopic fractionation. The isotopic difference between the two groups is largely due to the concentration into the former of olivine and pyroxenes which are isotopically lighter than plagioclase. The results support the view that the major rock-types are formed by the gravity sorting and accumulation of olivine, pyroxenes and plagioclase. Among the minor members of the complex, “granular” gabbro and a quartz diorite dyke are depleted in 18O by 2 to 3‰ compared to the major rock-types, whereas the veins of augite and of plagioclase-hornblende exhibit higher 18O content. It is suggested that a simple contamination of the residual magma by wall rock basalt can not produce such isotopic anomalies.

Relations of Granitic and Gabbroic Rocks, Northern Santa Lucia Range, California

The plutonic rocks of the northern Santa Lucia Range arc calc-alkaline and consist of hornblende gabbro, tonalite-granodiorite, and quartz monzonite of probable Cretaceous age. They were emplaced as concordant plutons during regional folding and metamorphism at temperatures above 650° C and at depths of 15 to 20 km. The country rocks consist predominantly of quartzofeldspathic meta-sedimentary rocks (Sur series) of probable Paleozoic age. The earliest major intrusions were of hornblende gabbro and leucocratic quartz monzonite. These formed homogeneous, mappable plutons and heterogeneous mixtures of gabbro and quartz monzonite with locally abundant hybrid rock. Tonahte-granodionte plutons were emplaced next and locally intruded the heterogeneous gabbro-quartz monzonite terrane. The last major intrusions were of quartz monzonite. Contacts between quartz monzonite and hornblende gabbro vary Irom sharp to gradational, and hybrid rocks are locally developed. Most contact relations indicate that gabbro was solid when quartz monzonite magma intruded it. Structural evidence of the broad contemporaneity of quartz monzonitic and gabbroic magmas allows the possibility that some mixing of magmas furthered the development of hybrid rocks and gradational contacts. Tonalite-granodiorite plutons emplaced somewhat later may have formed by hybridization of gabbro and quartz monzonite.

Rheomorphism of Granite by Gabbro Intrusions on the Coast of South-Central Labrador

Excellent exposures along the coast of Labrador for 150 km. reveal a consistent pattern of thick gabbro sheets overlying migmatic granite gneisses. The relationship between the two rocks is complex, and in many places each appears to have intruded the other. However, mineralogical, textural, and structural criteria may be interpreted as showing that the gabbros are younger than the gneisses. An exceptional example illustrating these relations is described from Spotted Island. It is suggested that the heat of the intruding gabbro magma caused widespread mobilization of the granitic basement rocks.

Strontium Isotope Studies related to Petrogenesis in the Caledonian Basic Igneous Province of NE. Scotland

Rubidium-strontium isotope analyses are reported for 90 whole-rocks from the Newer Gabbros and associated rocks of NE. Scotland. The Sr87/Sr86 ratio of the regional gabbro magma at Insch and Belhelvie underwent a progressive increase, in the former case from 0·703 to 0·712 during differentiation to syenogabbros and syenites. This is ascribed to a process of isotopic equilibration with country rocks of the Dalradian Series. The wider implications of a process by which the Sr87/Sr86 ratio of a basic magma can be increased in the crust without any other signs of petrological contamination are considered. Cordierite-norites and related rocks of the Haddo House and Arnage masses have initial Sr87sol;Sr86 ratios of 0·720–0·730, essentially the same range as for the country rocks at the time of intrusion. This is interpreted as support for the hypothesis that such rocks were formed by the partial fusion of pelitic sediments adjacent to the basic igneous sheet.

Experimental Studies of Igneous Rock Series: A Zoned Pluton in the Wallowa Batholith, Oregon

The phase relationships of five analyzed granodiorites and tonalites from the zoned Needle Point Pluton of the Wallowa Batholith, Oregon, supplied by W. H. Taubeneck, have been determined in the presence of 15-25 weight per cent H_2O at pressures to 3 kb. Pressure-temperature curves were located for the beginning of melting and for the disappearance of K-feldspar, quartz, plagioclase, biotite, and hornblende. For two specimens, a granodiorite and a tonalite, the melting behavior at 2 kb. pressure was determined in more detail with approximate point counts of crushed fragments. Representative results are: granodiorite (Differentiation Index [D.I.] 76) melting begins at 705° C., K-feldspar disappears at 710° C., quartz disappears at 735° C.; tonalite (D.I. 55) melting begins at 725° C., the trace of K-feldspar disappears at the same temperature, quartz disappears at 755° C. In both rocks, calcic plagioclase, amphibole, and opaque minerals remain at 900° C. The hornblende becomes unstable at a slightly higher temperature and passes into the melt phase. Forty-seven per cent of the granodiorite melts between 705° and 730° C., and 55 per cent melts at 900° C. Twenty-nine per cent of the tonalite melts between 730° and 800° C., and 32 per cent melts at 900° C. At 900° C., the approximate plagioclase content of the granodiorite is 27 per cent, and of the tonalite 40 per cent; the glass in each rock has a refractive index of 1.510 ± 0.002. The experimental conditions (excess H_2O; f_(O_2) not controlled) prohibit detailed comparison with the natural occurrences. However, the persistence of calcic plagioclase and hornblende in abundance at 900° C. at 2 kb. pressure indicates that if these rocks were formed by anatexis, then either high temperatures are required at the base of the crust or the magmas consisted of a eutectic-type granite liquid with suspended crystals. The high liquidus temperatures suggest that gabbroic magma may be involved in the development of magmas of intermediate composition in batholiths.

The Net-Veined Complex of the Austurhorn Intrusion, Southeastern Iceland

The Austurhorn intrusion, one of a number of larger gabbro and granophyre intrusions that occur within the Tertiary lava pile of southeastern Iceland, is a composite, stocklike body cropping out over about $$11 km.^{2}$$. The eastern part of this intrusion consists of a net-veined complex, in which acid, basic, and hybrid intermediate rocks are intimately associated. Within the net-veined complex the basic and hybrid rocks occur as rounded to angular masses enclosed in and veined by granophyre. Many of these inclusions have pillow-like forms, with finer-grained margins and highly regular contacts, and most other inclusions appear to be pieces of fragmented pillows. Commonly the pillows occur in groups in which the individual pillows, all of the same composition, are separated from one another by thin layers of granophyre. There are, however, many examples of pillows of different compositions occurring alongside one another. Evidence concerning the formation of the net-veined complex is discussed, and it is concluded that the pillows represent intrusions of basic and hybrid magmas into acid magma. The complex is considered to have been formed in situ shortly after the emplacement of, first, the granophyre magma, which displaced the country rocks by stoping, and, second, the gabbro magma, which was intruded into the granophyre magma before the latter had completely solidified.

The hypersthene-gabbro of the Insch Complex, Aberdeenshire

A geological and geochemical account is given of the third great gabbro unit of the Insch Igneous Complex, the hypersthene-gabbro, and its origin is discussed. It is suggested that the hypersthene-gabbro and associated noritic rocks may have been derived from the regional gabbro magma of the province by contamination at depth. After this contaminated magma had risen into the dunite-syenite differentiated series that makes the main part of the Insch complex, it suffered a further local marginal contamination with the formation of norites and cordierite-norites associated with silica-poor xenoliths carrying corundum and spinel.

La Cueva Intrusive Complex and Dome, Northern Coahuila, Mexico

Research Article| December 01, 1963 La Cueva Intrusive Complex and Dome, Northern Coahuila, Mexico FRANKLIN W DAUGHERTY FRANKLIN W DAUGHERTY Dept. Geology, West Texas State University, Canyon, Texas Search for other works by this author on: GSW Google Scholar Author and Article Information FRANKLIN W DAUGHERTY Dept. Geology, West Texas State University, Canyon, Texas Publisher: Geological Society of America Received: 02 May 1963 First Online: 02 Mar 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Copyright © 1963, The Geological Society of America, Inc. Copyright is not claimed on any material prepared by U.S. government employees within the scope of their employment. GSA Bulletin (1963) 74 (12): 1429–1438. https://doi.org/10.1130/0016-7606(1963)74[1429:LCICAD]2.0.CO;2 Article history Received: 02 May 1963 First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation FRANKLIN W DAUGHERTY; La Cueva Intrusive Complex and Dome, Northern Coahuila, Mexico. GSA Bulletin 1963;; 74 (12): 1429–1438. doi: https://doi.org/10.1130/0016-7606(1963)74[1429:LCICAD]2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract La Cueva intrusive complex shows a remarkable sequence of sodic igneous rocks. The major rocks, in order of intrusion, are: (1) quartz monzonite, (2) sodic syenite and gabbro of the differentiated series, and (3) sodic microsyenite. Most of the other igneous rocks exposed are part of a younger series of hypabyssal rhyolitic intrusions.La Cueva intrusive complex is within La Cueva dome. Striking changes in dips along the flanks of the dome result largely from multiple intrusion and variation in the shape of individual intrusive masses.Field relationships indicate forceful emplacement of the main mass of the complex in three stages: (1) early laccolithic intrusion of quartz monzonite, (2) intrusion of the differentiated series which tilted and broke through the laccolith, thrusting aside the country rock, and (3) intrusion of a pluglike body of microsyenite through the center of the differentiated series.Field associations suggest that the rocks may have been derived from the same regional magma, but the proportion of felsic to mafic rocks is unusually high. The large volume of rhyolitic rock may be the result of dilution of rising gabbroic magma by assimilation of constituents of the basement rocks that melt at low temperatures. The silica-deficient rocks may have been isolated in a subsidiary magma chamber, where they underwent a divergent trend of differentiation. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Differentiation in the Olivine-Gabbro of the Insch Mass, Aberdeenshire

Geological, petrographic and geochemical accounts of two of the basic units of the Insch Igneous Mass are provided. One, the Insch Differentiated Suite, consists of iron-enriched olivine-gabbro with up to 24% total iron-oxides, that passes upwards through syeno-gabbro into syenite, with over 7% potash. This suite is inter-preted as resulting from the seeping of low-temperature constituents towards the roof of the crystallising olivine-gabbro magma which, itself, may have been enriched in iron by the abstraction of refractory magnesium-rich phases from the regional gabbro magma. The second basic unit dealt with is the Bourtie Series of dunites, peridotites and troctolitic gabbros, all rich in magnesia, and interpreted as early cumulates. The possibility of the geological connection of the two units to make a wider differentiated series ranging from dunite to syenite is discussed. Whilst this interpretation is favoured, it is agreed that special geochemical and mineralogical investigations are necessary to test its validity. Thirty-seven new analyses are used in the petrogenetic discussions. A revised geological map of the whole Insch Igneous Mass is presented.

Noritization and hyperitization accompanied with biotite migmatite, at the mid-stream of Horoman river in southern Hidaka metamorphic terrain (II)

At the contact between hornblende tonalite and its xenolith, biotite nortite was formed, while at that of biotite migmatite and amphibolite, biotite hyperite was produced. From the careful observation in the field and microscope the writer attained the conclusion that the norite or hyperite is not originated from gabbroic magma and the inclusion is not ascribed to the forerunner of magma, nor to the amphibolite. Feature of mechanical movement in norite or hyperite is considered to have taken place at the time of formation of biotite migmatite or amphibolite. Metamorphic aspect in biotite migmatite is a kind of “metablastesis”. Dispersion of biotite into matrix, and recrystallization of biotite into pyroxene or hornblende are the general behaviors in this district.

ORIGIN OF THE ULTRAMAFIC COMPLEX AT UNION BAY, SOUTHEASTERN ALASKA

An ultramafic complex at Union Bay, in southeastern Alaska, is an exceptionally well-exposed example of a series of similar complexes forming a belt parallel to the Coast batholith. A body of gabbro, approximately circular in plan and about 6 miles in diameter, intrudes folded sedimentary rocks of probably Triassic and Cretaceous age. A moderately low grade of regional metamorphism in the sedimentary rocks is increased to almandite-zone grade adjacent to the gabbroic contact. Intrusive into the gabbro is a remarkable ultramafic complex which comprises a vertical pipe approximately 1 mile in diameter, to which is attached a lopolithic offshoot approximately 5 miles long and 3 miles wide. The ultramafic units range through hornblende pyroxenite, pyroxenite, olivine pyroxenite, peridotite, and dunite, and both the pipe and the lopolith show a well-developed concentric zoning with dunite in the center and pyroxenite or hornblende pyroxenite on the periphery. Magnetite is a primary constituent of the pyroxenite unit. The most plausible explanation of the structure and petrology of the ultramafic complex is a hypothesis of successive injections of an ultramafic magma changing in composition with time and intruded in reverse of the order of crystallization of the component minerals. It has not been possible to explain the gabbro (which is olivine-free) by the same mechanism, yet the space and time relationships here, as elsewhere in the world, are probably not accidental. It is proposed, therefore, that a body of gabbro was emplaced, and into this was injected a plug and associated lopolith first of diopside-magnetite magma with minor olivine, then diopside-olivine magma, and finally olivine magma, each new magma intruding the central, incompletely solidified portion of the earlier magma. Minor mixing and melting explain the composition and distribution of the intermediate units of the complex. Lack of contact metamorphism of grade that might be expected to accompany intrusions of such high temperature is explained first by the fact that the ultramafic magmas were almost everywhere in contact with gabbro, not sedimentary rocks, and second by the fact that, being anhydrous, the ultramafic magmas absorbed, rather than expelled, volatiles. The only likely source for magmas of these compositions would be melting portions of the mantle, but no proposed mechanism for supplying the energy for this process is suggested herein. With an available source of energy, however, it can be postulated that magmas would be generated in the succession proposed. The gabbroic magma probably would originate as a result of melting at a shallower level.

Layered basic xenoliths in some Norwegian gabbros

Xenolithic inclusions in gabbros exhibiting a layered structure and a gabbroic suite of minerals are shown to represent calcareous sediments that have been basified by the gabbroic magma. Mg, Fe and Ca have been added to the xenoliths; Si, Al and alkalies are subtracted.

PETROGENESIS OF A GABBRO-GRANOPHYRE COMPLEX IN NORTHERN WISCONSIN

Field and laboratory investigations suggest that a granophyre-gabbro complex in northern Wisconsin is similar to other acidic and basic complexes so common throughout the world. The Wisconsin complex consists of two distinct belts of gabbro and granophyre split by a narrow belt of basic extrusives, Keweenawan in age, with flows the oldest, gabbro next, and granophyre the youngest. Still younger are basic dikes that show preferred orientation. The complex is on the steeply dipping south limb of the Lake Superior geosyncline. The gabbro was intruded between flow units along planes of shear that resulted during formation of the geosyncline. Some gabbroic magma followed thrust faults. Two structures in the gabbro—fluxion structure and rhythmic layering—appear to be unrelated in origin. Fluxion structure is believed to be the result of shear or flow occurring parallel to the walls of the intrusion as the mass crystallized, whereas the layering appears to be the result of crystal settling. A study of mineral compositions in the gabbro reveals simultaneous iron enrichment in coexisting pyroxenes and olivines, and soda enrichment in the plagioclase. The granophyre is believed to be the result of crystallization of a granitic melt which was intruded following solidification of the gabbro. Criteria for a replacement (metasomatic) origin of the granophyre are inconclusive. It is postulated that the granitic melt originated at depth through either (1) partial remelting of the sial during downwarping of the Lake Superior geosyncline or (2) fractional crystallization. This melt was then squeezed into its present position during further deformation of the geosyncline and followed essentially the same zones of weakness taken by the gabbro magma. The minerals of the granophyre could be the residua of fractional crystallization or the first liquid formed in remelting. The type of granophyric texture in the red rock (granophyre) is largely dependent upon the anorthite content of the microperthite. In addition, the granophyric texture cannot be the result of eutectic crystallization, for this type of crystallization implies that the melt crystallizes with no resulting change in composition; however, in the red rock, a change in composition is reflected in the anorthite content of the microperthite intergrown with quartz. Hence, the granophyric texture must be due either to simultaneous crystallization without a eutectic or to some secondary process. Between the gabbro and the granophyre (red rock) is a narrow zone (maximum 200 feet) of “intermediate rock” which exhibits the peculiar mineralogical and textural characters of an alteration zone. Geochemical culminations of FeO, TiO 2 , MnO, and P 2 O 5 exist in this zone between gabbro and granophyre. In addition, the intermediate zone has been enriched in the O 18 relative to the O 16 isotope. The peculiar chemical, mineralogical, and textural features of the “intermediate rock” are the result of contact metasomatism caused by the action of a granophyric melt on gabbro. Also the alteration of the basic flows in the area probably resulted from solutions emanating from the granophyre.

The ilmenite deposit of Abu Ghalqa, Egypt

Bands and lenses of ilmenite ore in the upper part of a small body of titaniferous gabbro in the Abu Ghalqa area of the Eastern Desert of Egypt were formed by gravitative accumulation of ilmenite-rich residual fluids during late stages in the consolidation of the gabbroic magma.

The geology of the Arnage district (Aberdeen-Shire) : a reinterpretation

Summary In 1923, Read considered that in the Arnage district an initial gabbroic magma had been contaminated by country-rock material to produce on consolidation quartz-biotite-oligoclase-cordierite rocks (Arnage Type), pseudo-dioritic rocks (Kinharrachie Type) and finally granitic rocks (Ardlethen Type). Detailed field investigation leads to a new interpretation. The continuous magmatic series proposed by Read has been broken into two parts :— 1. An older part, consisting of the Arnage-Kinharrachie-Ardlethen types, produced by migmatization and comparable with migmatitic complexes of “ Older Granite ” age developed elsewhere in Aberdeenshire. 2. A newer part, gabbroic in character and belonging to the basic phase of the “ Newer Granite ” igneous activity of North-East Scotland. The “ Older Granite ” migmatites of Arnage have characters, such as the hornfelsic nature of their enclaves and their richness in cordierite, that are appropriate to their setting in the metamorphic zones of North-East Scotland. They present a contrast to the Cromar type of migmatites set in the Barrovian metamorphic zones of the South-East Highlands.

Olivine-bearing and other Basic Hornfelses around the Insch Igneous Mass, Aberdeenshire

AbstractBasic hornfelses occurring at the contact of slates and olivinenorite are ascribed to influx of Ca, Mg, Fe, Ti, Mn, and P from the gabbro magma into the sedimentary rocks. This kind of basic front, unlike those postulated for acid bodies, has a basic back.

Preliminary observations on the distribution of trace elements in the rocks of the Skaergaard intrusion, Greenland

The Skaergaard intrusion of Kangerdlugssuak, East Greenland (68° 10′ N., 31° 40′ W.) contains a series of gabbros and related rocks—the layered series—which has been interpreted as the result of fractional crystallization of a fairly normal gabbro magma (Wager and Deer, 1939). Analysis of these rocks and of some of the constituent minerals by Dr. W. A. Deer has shown a considerable and systematic variation in the major constituents (1939, table XXXVI and pp. 67–84) and it was considered therefore that it would be of value to determine the variation in some of the rarer elements.The work of Goldschmidt and his collaborators on the distribution of trace elements in minerals (summarized in Goldschmidt, 1937) combined with our recently acquired knowledge of many crystal structures has made clearer the factors controlling the distribution of the trace constituents in minerals.