1. Intrusive Igneous Rocks, part 3
Syenite, Phonolite, Ijolite, Carbonatite, Ultramafic Rocks and Lamprophyre

2. IUGS Intrusive Igneous Rock Chart

3. General Intrusive Rocks Classification
Photo # igclas1.gif

4. Position on IUGS Diagram
Syenites are on the border between the QAP and the APF triangles
The remaining rocks are in the APF triangle
Phonolite is actually an extrusive or hypabyssal equivalent of nepheline syenite
The ultramafic rocks and lamprophyre are very rich in mafic minerals
Consequently they cannot be plotted on the main part of the main IUGS diagram
They plot on Figure2-2c (p. 27, Winter) in a separatetriangle

5. Syenite Intrusive igneous, plutonic Q = 0-5 P/(A+P) is 10-35
Name is for Syene (near Aswan), Egypt - Pliny the Elder named granite-like rocks from this area for the locality

6. Syenite Mineralogy
Major mineral is feldspar, with greater than 65% alkali feldspar (K-spar or albite)
K-spar is typically orthoclase, microcline or perthite - in dikes sanidine may be present
The ferromagnesian minerals are usually < 20%
Biotite is often present and is usually brown

7. Syenite Mineralogy
Plagioclase, if present, is generally subhedral and is often zoned (normal or oscillatory)
Small amounts of feldspathoids, like nepheline and sodalite, may be present - if more than 5% of these minerals are present, the rock is called nepheline (or sodalite) syenite
Accessory minerals: Titanite, apatite, ilmenite, magnetite, zircon, and monzonite

8. Syenite Photomicrograph
Process: Albite and K-spar are mutually soluble in the crystalline state to a limited extent
Mutual solubility is greater at high temperature
As a high-temperature K-rich alkali feldspar cools past the solvus into the two-feldspar domain, the dissolved albite exsolves to form irregular lamellae of albite within the potassium-feldspar host (perthite)
Perthite texture in twinned alkali feldspar (perthite) crystals in Syenite
[CN, medium power.]

9. Syenite Hand Specimen Photo
K-feldspar megacrysts
Pyroxene
Location: Brenner Pluton, Yukon
Photo: James Lang
Photo 3 jl_pos7m.jpg

10. Alkali Syenite
Intrusive igneous, plutonic
Q = 0-5
P/(A+P) is 0-10

11. Alkali Syenite Mineralogy
A syenite rich in sodium, this rock has strongly perthitic K-spars or anorthoclase
Plagioclase is albite to sodic oligoclase.
Mafic minerals include iron-rich biotite, iron or sodic amphiboles including hastingsite, arfvedsonite, or riebeckite, and pyroxene is either aegirine-augite or aegirine.
Accessory feldspathoids may be present

12. Nepheline Syenite Intrusive igneous, plutonic P/(A+P) is 10-35
F = 0-10
Foid syenite composed of granular aggregates, typically orthoclase, microcline, microperthite, cryptoperthite, or albite.
Other essential felsics: nepheline
Accessory feldspathoids: cancrinite, sodalite, hauyne, or nosean

13. Mafic Minerals in Nepheline Syenite
Mafics are soda-rich
Amphiboles arfvedsonite, or hastingsite
Pyroxenes aegirine-augite, aegirine, or titanaugite

14. Nepheline Syenite Photomicrograph
Large, simple-twinned crystal of perthite (alkali feldspar exhibiting wispy exsolution lamellae)
Equant, colorless crystals of nepheline. The turbid areas of the nepheline crystals are where it has been altered to sericite (fine-grained muscovite).
Slide also shows green alkali pyroxene (bottom left, interference color distorted by green body color) and altered biotite [CN, low power]

15. Nephelinite
Nephelinite (alkaline mafic lava) consisting of equant, low-birefringence crystals of nepheline and euhedral crystals of alkali pyroxene, often showing zoning, set in a glassy groundmass
Presence of the feldspathoid nepheline indicates that the magma was silica-undersaturated
CN, medium power

16. Nephelinite
Nephelinite (alkaline mafic lava) consisting of equant, sometimes 6-sided colorless crystals of nepheline and euhedral to subhedral greeny-brown crystals of alkali pyroxene
PP, medium power

17. Larvikite Intrusive igneous, plutonic
Pearl gray alkali syenite, sometimes opalescent, which grades into monzonite
Typically coarse textured
The name is for the locality, Larvik, Norway

18. Larvikite Mineralogy
Prominent crystals are rhombs of very finely twinned sodic-plagioclase and alkali feldspar
Mafics are diopsidic augite and titanaugite
Accessory apatite, ilmenite, titaniferous magnetite, nepheline, olivine, or biotite
Often used as a decorative stone because of a beautiful opalescence when polished

19. Larvikite Polished Section Photo
Polished surface on coarse-grained larvikite rock, a type of syenite from Norway - widely marketed under the trade name 'Blue Pearl'
Composed mainly of large crystals of alkali feldspar (K + Na), with lesser amounts of dark minerals
The large crystal in middle of photo is about 2 centimeters long, and shows a hint of concentric growth layers
The rock is particularly attractive because the feldspar cleavage surfaces (visible as bright areas in photo) show as pearly pale blue iridescent flashes
Location: Larvik district, southeast coast of Norway
Photo # cag121m.jpg
Photo: Carlo Giovanella

20. Ijolite Intrusive igneous, plutonic to hypabyssal F = 60-100 M = 30-70
Sodium > potassium
The name is for the locality parish Ijo, Finland

21. Ijolite Mineralogy
A feldspathoid-rich rock, containing essential nepheline (50-70%)
Essential: pyroxene, generally aegirine
Ijolites, carbonatites, and syenites are often associated, and are often rich in alkali elements
The silicate and carbonatite minerals have similar trace and minor element chemistries, including high values of Ba, Sr, Ce, La, Zr, P, and Cl
Frequent host rocks for rare-earth and apatite minerals

22. Carbonatite Intrusive igneous, hypabyssal
Igneous rock in which the carbonate minerals are primary
Apatite and pyrochlore are often considered essential
Accessory: Siderite, calcite, dolomite, or ankerite
Less common accessory minerals: Mg-rich magnetite, phlogopite, and pyrite

23. Carbonatite Occurrence
Carbonatites occur as plugs with surface areas of up to 8 km2.
They are enclosed in mafic alkaline rings.
Mafic rocks include biotite pyroxenite, ijolite, nephelinite (35% nepheline, 65% mafic on average), and jacupirangite (rocks consisting of pure magnetite, magnetite with accessory pyroxene, pyroxene with accessory magnetite, or pyroxene and nepheline with biotite and olivine).

24. Carbonatite Origin
The similarity between silicate and carbonate phase trace and minor elements has convinced most petrologists that the carbonatites must originate together with the silicates, possibly from the same source, and not from limestones, as early workers had suggested

25. Extrusive Rock Classification
Photo # igclas4.gif

26. Phonolite Igneous extrusive, hypabyssal?
The rock is usually a lava, but may occur in shallow dikes
Name, from the Gr. phone, meaning sound
Supposedly the rock rings when struck with a hammer
Other names are clinkstone and echodolite

27. Phonolite Mineralogy
An aphanitic rock composed of alkali feldspar, often anorthoclase or sanidine, nepheline, and mafics
Other feldspathoids may replace nepheline

28. Ultramafic Rocks Igneous intrusive, plutonic M  90
Examples: dunite, peridotite, and pyroxenite

29. Ultramafic Classification Diagram
Photo #igclas3.gif

30. Mineralogy of Ultramafic Rocks
Ultramafic rocks are composed of minerals near the top of Bowen’s Reaction series
These minerals, especially iron-rich varieties, are prone to weathering.
Serpentine minerals are common weathering products of olivine or pyroxene
Serpentine may be accompanied by talc, another weathering product, or by magnesite, formed by the precipitation of MgCO3 from sea-water after the release of Mg ion

31. Mineralogical Changes in Ultramafic Rocks
Magnetite is a very common accessory mineral in felsic to mafic rocks
In ultramafic rocks, almost all of the iron is associated with various mafic silicates minerals, so there is no iron oxide phase present
Opaque accessory mineral is often chromite
If enough chromite is present, the rock may become an ore of chromium
In hand specimen, chromite is mostly easily seen in fresh specimens of dunite

32. Mineralogical Changes in Ultramafic Rocks
Titanium is often present, and titanoaugites or other titanium rich minerals may be present

33. Pyroxenite Intrusive igneous, plutonic ol/(ol + cpx +opx) < 40%
M  90

34. Pyroxenite Mineralogy
Some pyroxenites contain more iron and are predominantly composed of opx - either hypersthene or bronzite
Bronzite is named for the noticeable bronze reflection (Schiller luster) that may be seen in hand specimen.
If clinopyroxene is present it is often the variety diallage which shows well-developed parting parallel to {100} in the augite-diopside composition range.
Ti-bearing augite or aegirine is also possible

35. Pyroxenite Mineralogy
Accessory minerals include:
Hornblende, generally brown, may be present
Chromite - Crystals may be diamond or square shaped - crystals in hand specimen are small, seldom showing octahedral habit

36. Clinopyroxenite This rock is dominated by the clinopyroxene augite
Location: West Point, GA
Photo #norite_X.jpg
Mistakeingly identified in UNC collection

37. Bronzitite Intrusive igneous, plutonic
A type of pyroxenite composed almost exclusively of bronzite
Accessory minerals may include olivine, picotite (chromian spinel), chromite, hornblende.
Secondary serpentine may be present
Also called bronzite

38. Bronzitite Photomicrograph
Almost all of the grains in this photomicrograph are hypersthene
This rock also contains some interstitial plagioclase, better seen in plane light.
Photo #bronzX.jpg
Location: Stillwater Complex, Montana

39. Hypersthene Photomicrograph
Orthopyroxenes are noted for having low, first-order interference colors
Also note the cleavages that intersect at about 90 degrees.
Another identifying characteristic of orthopyroxene is its parallel extinction.
Photo # opx_X.jpg

40. Peridotite Intrusive igneous, plutonic ol/(ol + cpx +opx) > 40%
M  90%
The mineralogy is similar to the pyroxenites, except that a mica, phlogopite, is often present
The rock is feldspar free
Name - from the French name for olivine, péridot

41. Dunite
Dunite is olivine peridotite
Ol > 90%

42. Dunite Photomicrograph
Almost all of the grains in this rock are olivine - note the high order interference colors of olivine
Secondary calcite occurs as veinlets through the sample
Photo # dunite.jpg

43. Harzburgite Intrusive igneous, plutonic
Harzburgite is a type of peridotite in which the major minerals are orthopyroxene (generally near enstatite) and Mg-rich olivine.
M  90%
ol/(ol+opx+cpx) = 40-90
cpx/(ol+opx+cpx) < 5%.

44. Harzburgite
Harzburgite is believed to represent depleted mantle material after the removal of a basaltic component
Name - for the locality, Harzburg, Germany

45. Chromitite Intrusive igneous, plutonic
Igneous rock composed of 95%+ of chromite
Similar rocks with larger percentages of ferromagnesian minerals may be named for the mineral, e.g. olivine chromitite
Rock is denser than most other ultramafic rocks

46. Hornblendite Intrusive igneous, plutonic M  90%
hb/(hb+px+ol) > 90%

47. Kimberlite
Potassium-rich peridotites which usually occur as “pipes” which originate in the mantle
Usually have an abundant volatile content, especially CO2 and H2O
Found in old (>2500 my), stable continental areas but are themselves much younger, often Mesozoic in age
Kimberlite magmas intrude along deep fractures, sometimes extensions of ocean fracture zones
Name - for the locality Kimberly, South Africa

48. Kimberlite Xenoliths
Kimberlites are most interesting scientifically because they carry a rich variety of xenoliths and xenocrysts
Many are these xenoliths are believed to be part of the mantle or deep crust
They are pieces that fell into the kimberlite magma during its passage to the surface
Xenoliths include garnet peridotites, lherzolites, and eclogites, which are believed to be samples of the mantle lithosphere and even the upper asthenosphere

49. Kimberlite Mineralogical Alteration
Kimberlites often show signs of alteration
Olivine is transformed to serpentine, usually antigorite
Phlogopite is transformed to chlorite, or vermiculite, with accompanying magnetite and/or calcite

50. Kimberlite breccia Kimberlites may also occur as breccia
This probably represents turbulent emplacement, where the magma is mixed with many different kinds of xenoliths
Xenoliths from depth are rounded, probably as the result of wear during the ascent, and the upper xenoliths are more angular

51. Kimberlite Composition
Intrusive igneous, dike or pipe
Kimberlite is a porphyritic peridotite or peridotite breccia
Essential: Olivine, usually altered to serpentine, and phlogopite (commonly chloritized)
Accessory: carbonates
Groundmass is usually calcite, serpentine, chlorite, phlogopite and accessory minerals such as chromian pyrope, ilmenite, magnetite, and perovskite

52. Lherzolite Photomicrograph
The picture shows olivine (right side of picture), orthopyroxene (lower left) and clinopyroxene (upper left)
Lherzolites contain over 60% olivine, and some clinopyroxene and orthopyroxene
Photo #r_4.gif
Lherzolite is a major constituent of the mantle and the source
of most basaltic magmas.

53. Lamprophyre
Lamprophyre chemical composition is very similar to that of alkaline olivine basalt
Lamprophyres are widespread but volumetrically insignificant rocks
Occurrence: As dikes, often cutting granitic batholiths or stocks - dikes may be radiating or may form a dike swarm of sub-parallel dikes
Dikes are small - a few meters, at most, thick, but may be miles long

54. Lamprophyre Composition
Fine-grained, dark-colored, often containing mafic phenocrysts, but never felsic phenocrysts
Groundmass is predominantly felsic and may include plagioclase, nepheline, sanidine, melalilite ([(Na,Ca)2(Mg,Al)(Al,Si)2O7], and analcite (NaAlSi2O6.H2O), a zeolite
Xenoliths of country rock are common

55. Lamprophyre Parent Rock
Parent material of lamprophyres is probably alkaline olivine basalt
Movement of alkaline olivine basalt magma toward the surface might induce decompressive melting, and the resulting magma could be injected into fractures resulting from compressive failure
Mafic phenocrysts would have formed early, before injection

56. Lamprophyre Composition
Intrusive igneous, diaschist
Lamprophyre rocks are melanocratic or mesotype hypabyssal rocks
Mafic phenocrysts occur in a fine-grained groundmass
Name, from the Greek lampros, meaning bright, in allusion to the bright reflection from biotite phenocrysts found in some lamprophyres

57. Lamprophyre Mineralogy
Mafic minerals are olivine (or serpentine), biotite, hornblende (usually green), and pyroxene (green diopside or Fe-Ti augite, which is purplish)
Felsic minerals are often alkaline, such as alkali feldspar (K-spar or albite) and Na-rich nepheline
Plagioclase is common, while quartz may be present
Often associated with carbonatites

58. Nepheline Diorite Lamprophyre
Intrusive igneous, diaschist
A dioritic lamprophyre is one in which the major feldspar is plagioclase
In addition this rock contains the feldspathoid nepheline

59. Lamprophyre Photomicrograph
Lamprophyre (alkaline mafic dike-rock) with microphenocrysts of biotite and pyroxene CN