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Chapter III Part 2

The Igneous Rocks of the Mogok Stone Tract

 

A Ph.D. Dissertation by Kyaw Thu

Department of Geology, University of Yangon, Myanmar
Copyright © Kyaw Thu, 2007

 

 

Contents

See complete table of contents here.
 

Chapter III Petrography

3.1 Ultramafic and Mafic Rocks
3.2 Augite-biotite Granite
3.3 Leucogranite
3.4 Syenitic Rocks
3.5 Urtite Series
3.6 Kabaing Granite
3.7 Pegmatites and Aplites

References

 

List of Figures (images from Pala International are not listed)

XN = crossed Nicols (explained here); PPL = plane polarized light (explained here)

3.29 Anhedral aegirine augite (Aug) grain in quartz syenite, PPL, 10 X
3.30 Quartz syenite containing alkali feldspar (Or), aegirine augite (Aug), and quartz (Q), XN, 10 X
3.31 Anhedral garnet (Gar) grain, iron ore and biotite (Bio) showing foliation in quartz syenite, PPL, 10 X
3.32 Anhedral and basal section of scapolite grains in quartz syenite from contact zone, XN, 10 X
3.33 Myrmekitic intergrowth of quartz and plagioclase feldspar in quartz syenite, XN, 25 X
3.34 Myrmekitic (m-q) intergrowth of quartz and feldspar, and biotite (Bio) in alkali-feldspar syenite, XN, 25 X
3.35 K-feldspar showing grid twinning with patch perthite (pe) in alkali-feldspar syenite XN, 25 X
3.36 Subhedral nepheline (ne) crystal in foid-bearing alkali-feldspar syenite, XN, 10 X
3.37 K-feldspar with perthitic (pe) veinlets and opaque mineral in quartz syenite, XN, 25 X
3.38 Euhedral sphene crystals in alkali-feldspar of foid-bearing alkali-feldspar syenite, PPL, 25 X
3.39 Euhedral plagioclase (Pgl) crystal showing polysynthetic twinning in alkali syenite pegmatite, On-dan, XN, 25 X
3.40 K-feldspar with flame perthite (pe) enclosing zircon crystal and corundum grain, On-dan, XN, 25 X
3.41 K-feldspar (Or) with flake perthite (fl-pe) in alkali-syenite pegmatite, XN, 25 X
3.42 Garnet (Gar) and biotite (Bio) flakes in alkali-syenite pegmatite, Lay-thar, PPL, 10 X
3.43 Corundum (C) crystal enclosed in spinel (Sp), alkali-syenite pegmatite, Lay-thar, PPL, 10 X
3.44 Corundum showing typical three sets of short rutile (Ru) needles and small zircon (Zr) inclusions in alkali-syenite pegmatite, PPL, 25 X
3.45 Plagioclase (Pgl) showing pericline twinning in quartz monzonite, XN, 10 X
3.46 Complex polysynthetic twinning in plagioclase (Pgl) and augite in quartz monzonite, XN, 10 X
3.47 Hornblende and pyroxene and the rest white colour is nepheline in urtite, PPL, 25 X
3.48 Pyroxene is partly altered to hornblende in urtite rock, PPL, 25 X
3.49 Calcite showing twinning and amphibole in urtite, XN, 25 X
3.50 Euhedral biotite crystal in urtite, XN, 25 X
3.51 Amphibole showing simple twin in urtite, XN, 10 X
3.52 Rectangular shaped nepheline, hornblende and calcite in urtite, XN, 10 X
3.53 Anhedral deep yellowish brown, titanium rich melanite garnet and augite in urtite, PPL, 10 X
3.54 Mineral constituents of the ijolite containing hornblende, augite, nepheline, iron ore and plagioclase feldspar, XN, 10 X
3.55 Feldspar showing combination of Carlsbad and albite twin, hornblende and nepheline in ijolite, XN, 10 X
3.56 Euhedral zircon crystal in nepheline, PPL, 25 X
3.57 Small corundum grains in ijolite, Thurein Taung, PPL, 25 X
3.58 Mineral constituents of jacupriangite include hornblende, pyroxene, and plagioclase showing pericline twinning, XN, 10 X
3.59 Basal section of hornblende in jacupriangite, XN, 25 X
3.60 Kabaing granite intruded into the dunite body in Pyaung-gaung, 23° 00' 18.4" N, 96° 27' 8.5" E
3.61 Mineral constituents of biotite microgranite includes quartz, feldspar and biotite mica, XN, 10 X
3.62 Euhedral alkali-feldspar shows zoning with some biotite in Kabaing granite, XN, 10 X
3.63 Chlorite, alteration product of biotite in Kabaing Granite, PPL, 25 X
3.64 Elongated bioite crystal showing bent nature due to deformation, 52 XN, 25X
3.65 Granophyric quartz (Q) in K-feldspar, XN, 10 X
3.66 Graphic quartz (Q) in k-feldspar and the quartz protrudes in the orthoclase (Or) by forming indentations, XN, 10 X
3.67 Graphic quartz (Q) with extinctions into the k-feldspar, XN, 10 X
3.68 K-feldspar with perthite. Both the high proportion of perthite to k-feldspar and the irregular shapes rather suggest a metasomatic origin for the plagioclase. K-feldspar in the extinction position, XN, 10 X
3.69 K-feldspar showing rare sector twin and intergrowth with quartz, XN, 25X
3.70 Chalcedonic silica vein in k-feldspar in aplite, XN, 25 X
3.71 Plotted data of the igneous rocks in the Mogok Stone Tract, except ultramafic rocks (source; IUGS Classification, 2006)
3.72 Modal classification of ultramafic rocks based on the proportions of olivine (Ol), orthopyroxene (Opx), clinopyroxene (Cpx), pyroxene (Px) and hornblende (Hbl) (source: IUGS Classification, 2006)

 

List of Tables

3.1 Classification of pegmatites of the rare-element class for Sakhan-gyi and Pan-taw area, after Cerny (1991)

Chapter III Petrography – Part 2

3.4 Syenitic Rocks

The syenitic rocks have been found to vary in texture and composition from place to place from a quartz syenite to a monzonite.

According to IUGS classification, it can be classified into the following subunits:

  1. Quartz-syenite
  2. Alkali-feldspar syenite
  3. Foid-bearing alkali-feldspar syenite
  4. Alkali-feldspar syenite pegmatite
  5. Quartz monzonite

Megascopic Study

It is whitish to brownish yellow coloured, phaneritic rock. It shows medium to coarse-grained, sometimes pegmatitic texture. Feldspar is the most predominant mineral, and it is orthoclase with moonstone schiller and with a subordinate quantity of pyroxene and a little quartz. Sapphire, garnet, spinel, biotite, tourmaline, graphite and iron ore are occasionally present.

  1. Quartz-syenite

Microscopic Study

It is medium to coarse-grained, hypidiomorphic granular texture, and principally composed of alkali-feldspar, quartz, a little plagioclase, aegirine augite, and common hornblende. Subordinate amount of garnet, biotite, apatite, zircon, sphene and iron ore are common. Alkali-feldspars are usually perthitic intergrowth, mostly cryptoperthite to microperthite and which are higher % than plagioclase.

Flame, patch, film and string perthites are common and perthitisation is simply an exsolution phenomenon in alkali-feldspars that operates within the feldspar alone, it records to cooling history after crystallization. Sometimes perthitisation are also due to metasomatic, replacement and infiltration processes. Plagioclase (An30–An35) shows subhedral crystal with combination of Albite and Carlsbad twinning and saussurited along the cracks. Quartz is usually anhedral and contains about 6–8% of total volume. The principle mafic minerals are aegirine augite, biotite with some hornblende.

  1. Alkali-feldspar syenite

Microscopic Study

It shows medium to coarse-grained, holocrystalline, hypidiomorphic granular texture. It is principally composed of alkali-feldspar, pyroxene and very small amount of quartz grains. Plagioclase (An28–An30) also contains a subordinate amount [of] accessory minerals include zircon, apatite, sphene and opaque minerals. Alkali-feldspar occurs as micro and cryptoperthites, and includes patch and flame perthites.

Quartz occurs as anhedral smaller grains and formed as intersticial between alkali-feldspar grains. Aegirine augite shows green to greenish colour in PPL with pyroxene cleavages in transverse sections. Sericite occurs in the fractures and grain boundaries of the alkali-feldspar.

  1. Foid-bearing alkali-feldspar syenite

Microscopic Study

It shows medium to coarse-grained, holocrystalline, hypidiomorphic granular texture. It is principally made up of alkali-feldspar, nepheline, cancrinite, pyroxene and a small amount of plagioclase. Accessory minerals include zircon, sphene, apatite, iron ore and sometimes include corundum grains. Alkali-feldspar occurs as micro and cryptoperthites, and includs string, patch, film and flame perthites. Subhedral plagioclase shows polysynthetic twinning is within the oligoclase-andesine range in composition. Nepheline occurs as subhedral grains and contains up to 8–10% of the rock and cancrinite is observed in some sections.

Anhedral Aegirine Augite Grain photomicrograph image
Fig. 3.29. Anhedral aegirine augite (Aug) grain in quartz syenite, PPL, 10 X.
Quartz Syenite Containing Alkali Feldspar, Aegirine Augite and Quartz photomicrograph image
Fig. 3.30. Quartz syenite containing alkali feldspar (Or), aegirine augite (Aug) and quartz (Q), XN, 10 X.
Anhedral Garnet Grain photomicrogaph image
Fig. 3.31. Anhedral garnet (Gar) grain, iron ore and biotite (Bio) showing foliation in quartz syenite, PPL, 10 X.
Anhedral And Basal Section Of Scapolite Grains photomicrogaph image
Fig. 3.32. Anhedral and basal section of scapolite grains in quartz syenite from contact zone, XN, 10 X. 
Myrmekitic Intergrowth Of Quartz And Plagioclase Feldspar photomicrogaph image
Fig. 3.33. Myrmekitic intergrowth of quartz and plagioclase feldspar in quartz syenite, XN, 25 X.
Myrmekitic Intergrowth Of Quartz And Feldspar photomicrogaph image
Fig. 3.34. Myrmekitic (m-q) intergrowth of quartz, feldspar and biotite (Bio) in alkali-feldspar syenite, XN, 25 X.
  1. Alkali-feldspar syenite pegmatite

Microscopic Study

This rock is rare in nature and uncommon in the geological and petrographic literature. The old name “plumasite” (after Plumas County, California where it was first discovered) was proposed by A. C. Lawson (Bulletin, Dept. of Geo., Univ. California, 1903, pp. 219–220), for a plagioclase-corundum rock forming a dyke in serpentinite. The alkali-feldspar syenite pegmatite observed in Mogok area carries more alkali-feldspar and appears similar to alkali-feldspar syenite pegmatite found in Nezametnoye deposit (Primorye region, Far East Russia). The alkali-feldspar syenite pegmatite from the Ondan, Thurein Taung, Chaung-gyi area is found intruded into marble, whereas in Laythar area itis found intruded into calc-silicate rocks.

It is coarse to very coarse-grained, holocrystalline hypidiomorphic granular texture. The rock is principally composed of alkali-feldspar, about 75% of the total volume and subordinate amount of plagioclase. Corundum, mainly sapphire is the principal accessory mineral, highly fractured, often with feldspar, rutile and zircon inclusions, and also quartz is totally absent. Sometimes, sapphire grains are closely bounded by spinel grains in alkali-feldspar groundmass.

Plagioclase also contain as subhedral grains and composition range from An45–An50 (Lay-thar, On-dan, Chaung-gyi) to An60–An65 (Thurein-taung), andesine to labradorite range. Alkali-feldspar shows colourless, cloudy, often altered to clay, as micro and cryptoperthites, showing string, patch, flake and flame perthiles. Accessory minerals include garnet, spinel, biotite, black tourmaline, zircon, scapolite, sphene and graphite flakes.

K-Feldspar Showing Grid Twinning With Patch Perthite photomicrograph image
Fig. 3.35. K-feldspar showing grid twinning with patch perthite (pe) in alkali-feldspar syenite XN, 25 X.
Subhedral Nepheline Crystal In Foid-Bearing Alkali-Feldspar Syenite photomicrograph image
Fig. 3.36. Subhedral nepheline (ne) crystal in foid-bearing alkali-feldspar syenite, XN, 10 X.
K-Feldspar With Perthitic Veinlets And Opaque Mineral In Quartz Syenite photomicrogaph image
Fig. 3.37. K-feldspar with perthitic (pe) veinlets and opaque mineral in quartz syenite, XN, 25 X.
Euhedral Sphene Crystals In Alkali-Feldspar photomicrogaph image
Fig. 3.38. Euhedral sphene crystals in alkali-feldspar of foid-bearing alkali-feldspar syenite, PPL, 25 X. 
Euhedral Plagioclase Crystal photomicrogaph image
Fig. 3.39. Euhedral plagioclase (Pgl) crystal showing polysynthetic twinning in alkali syenite pegmatite, On-dan, XN, 25 X.
K-Feldspar With Flame Perthite Enclosing Zircon Crystal And Corundum Grain photomicrogaph image
Fig. 3.40. K-feldspar with flame perthite (pe) enclosing zircon crystal and corundum grain, On-dan, XN, 25 X.

K-Feldspar With Flake Perthite photomicrograph image
Fig. 3.41. K-feldspar (Or) with flake perthite (fl-pe) in alkali-syenite pegmatite, XN, 25 X.
Garnet And Biotite Flakes In Alkali-Syenite Pegmatite photomicrograph image
Fig. 3.42. Garnet (Gar) and biotite (Bio) flakes in alkali-syenite pegmatite, Lay-thar, PPL, 10 X.
Corundum Crystal Enclosed In Spinel photomicrogaph image
Fig. 3.43. Corundum (C) crystal enclosed in spinel (Sp), alkali-syenite pegmatite, Lay-thar, PPL, 10 X.
Corundum Showing Typical Three Sets Of Short Rutile Needles And Small Zirzon Inclusions photomicrogaph image
Fig. 3.44. Corundum showing typical three sets of short rutile (Ru) needles and small zircon (Zr) inclusions in alkali-syenite pegmatite, PPL, 25 X. 
Plagioclase Showing Pericline Twinning photomicrogaph image
Fig. 3.45. Plagioclase (Pgl) showing pericline twinning in quartz monzonite, XN, 10 X.
Complex Polysynthetic Twinning In Plagioclase And Augite photomicrogaph image
Fig. 3.46. Complex polysynthetic twinning in plagioclase (Pgl) and augite in quartz monzonite, XN, 10 X.
  1. Quartz monzonite

Microscopic Study

It is coarse-grained, hypidiomorphic granular texture. It contains alkali-feldspar, plagioclase, quartz and pyroxene and biotite, sphene and opaque minerals as accessories. Perthite exhibits string, flame, patch and film types and are suturally contacted with the surrounding minerals. Plagioclase shows subhedral crystal with polysynthetic twinning, composition ranging from An45–An50' andesine range.

Quartz occurs as anhedral grains made up of about 10–15% of the total volume. It is included within the perthite, plagioclase and orthoclase by suture contact.

3.5 Urtite Series

This suite of rocks was first discovered by Prof. Adams (1926) in Mogok area, near Sin-khwa. Similar rocks associated with marbles were described by Mr. Banerji (Fermor, 1932) near Chaung-gyi-li-shu. It is mostly mafic, but variable as to colour index, all with SiO2 < 45%, that typically lack feldspar and whose principal constituents are a pyroxene and feldspathoid (usually nepheline) (William, et al., 1982). These rocks exhibit a certain amount of variation in the mineral constituents and texture in place to place. It shows a gneissic banding, and varies in the amount of their salic and femic constituents. The constituent minerals are usually a dark brown hornblende, a dark green pyroxene, nepheline, calcite, and a little iron-ore. These rocks are typically found in Thurein-Taung, Kyauk-pyat-that area, Sin-khwa and northeast of Bernard-myo. There is a complex terminology based on colour index. In the study area, it can be classified based on mineral constituent and colour index as follows:

  1. Urtite
  2. Ijolite
  3. Jacupriangite
  1. Urtite

Megascopic Study

These rocks, first seen near Sin-hkwa, have been found to extend west and south-west towards Lon-sho, Kyauk-pyat-that and Yadanar-kaday-kadar mine site. It seems to be intrusive in the granites and marble units, forming sills and dykes. These rocks occur as a broad patch in Thurein Taung with marble bands on it, and also contact with marble in Yadanar-kaday-kadar mine. It shows gneissose banding in surface of outcrop in Yadanar-kaday-kadar mine and Lon-sho mine sites. It is medium to coarse-grained granulitic rocks mostly made up of brown hornblende, aegirine augite, interstitial feldspar, nepheline and calcite.

 

Microscopic Study

It is medium to coarse-grained, hypidiomorphic granular texture. It principally composed of nepheline, about 70% of total volume, yellowish brown hornblende, subordinate amount of potash feldspar, augite and calcite. Accessory minerals are biotite, melanite, zircon and pyrite. Nepheline occurs as subhedral crystal and sometimes show basal section and parallel extinction observed under cross polar. Amphibole, mostly yellowish brown hornblende, occurs as anhedral to subhedral grains and some sections show typical amphibole cleavage. Some augite are partly altered to hornblende and calcite show polysynthetic twinning. Biotite flakes and deep yellowish brown melanite garnet are also observed.

Hornblende, Pyroxene And Nepheline In Urtite photomicrograph image
Fig. 3.47. Hornblende and pyroxene and the rest white colour is nepheline in urtite, PPL, 25 X.
Pyroxene Partly Altered To Hornblende photomicrograph image
Fig. 3.48. Pyroxene is partly altered to hornblende in urtite rock, PPL, 25 X.
Calcite Showing Twinning And Amphibole photomicrogaph image
Fig. 3.49. Calcite showing twinning and amphibole in urtite, XN, 25 X.
Euhedral Biotite Crystal photomicrogaph image
Fig. 3.50. Euhedral biotite crystal in urtite, XN, 25 X. 
Amphibole Showing Simple Twin photomicrogaph image
Fig. 3.51. Amphibole showing simple twin in urtite, XN, 10 X.
Rectangular Shaped Nepheline, Hornblende And Calcite photomicrogaph image
Fig. 3.52. Rectangular shape nepheline, hornblende and calcite in urtite, XN, 10 X.
  1. Ijolite

Megascopic Study

These rocks are seen in abundance at west of Kyauk-pyat-that and Thurein-Taung in contact with marble units. The rocks contain dominant hornblende, with subordinate light colour pyroxene and some basic plagioclase. The texture varies also from fine to coarse-grained and slightly banded. In Thurein-Taung, sapphire bearing alkali-feldspar syenite are mined between marble unit and ijolite rocks.

 

Microscopic Study

It is medium 10 coarse-grained, hypidiomorphic granular texture. It is principally composed of nepheline and plagioclase about 50% of total volume, yellowish brown hornblende, subordinate amount of augite, scapolite, zircon and pyrite. Plagioclase shows polysynthetic twinning and composition range from An60–An70, within the labradorite range. Hornblende occurs [as] subhedral grain, yellowish brown colour in PPL and showing amphibole cleavage and distinct pleochroism of yellowish brown to yellow. Nepheline occurs [as] subhedral rectangular section and showing parallel extinction under cross polar. Small zircon inclusions are found in nepheline.

Anhedral Melanite Garnet And Augite photomicrograph image
Fig. 3.53. Anhedral deep yellowish brown, titanium rich melanite garnet and augite in urtite, PPL, 10 X.
Mineral Constituents Of The Ijolite photomicrograph image
Fig. 3.54. Mineral constituents of the ijolite containing hornblende, augite, nepheline, iron ore and plagioclase feldspar, XN, 10 X.
Feldspar Showing Combination Of Carlsbad And Albite Twin, Hornblende And Nephrite In Ijolite photomicrogaph image
Fig. 3.55. Feldspar showing combination of Carlsbad and albite twin, hornblende and nepheline in ijolite, XN, 10 X.
Euhedral Biotite Crystal photomicrogaph image
Fig. 3.56. Euhedral zircon crystal in nepheline, PPL, 25 X. 
Small Corundum Grains In Ijolite photomicrogaph image
Fig. 3.57. Small corundum grains in ijolite, Thurein Taung, PPL, 25 X.
Mineral Constituents Of Jacupriangite photomicrogaph image
Fig. 3.58. Mineral constituents of jacupriangite include hornblende, pyroxene, and plagioclase showing pericline twinning, XN, 10 X.
  1. Jacupriangite

Megascopic Study

These rocks found at the base of Kyauk-pyat-that pagoda hill, near Kyauk-pyat-that -Thurein Taung road, and also base of the Thurein Taung, seems to be the most basic of the series. The rock is medium-grained, melanocratic, made up mostly of hornblende, titan-augite with subordinate amount of basic plagioclase and nepheline.

 

Microscopic Study

It is medium-grained, hypidiomorphic granular texture. A melanocratic rock made up mostly of brown hornblende and titan augite, about 70% of total volume, subordinate amount of basic plagioclase, nepheline,scapolite, fosterite, zircon and pyrite. Plagioclase shows polysynthetic twinning and basic composition range from An60–An70, in the labradorite range.

Hornblende occurs subhedral grain, yellowish brown colour in PPL and showing amphibole cleavage and distinct pleochroism of yellowish brown to yellow. Some hornblende shows twinning. Nepheline occurs [as] subhedral rectangular section and shows parallel extinction between cross polar. Fosterite, usually occur as small rounded grains with antigorite along the fracture. Small zircon inclusions are found in nepheline.

3.6 Kabaing Granite

Megascopic Study

It is medium-grained, equigranular rock. Weathered surface is dark grey in colour, and bluish white to greyish colour in the fresh surface. It is mainly composed of quartz and feldspar as principal felsic minerals and biotite and subordinate amount of hornblende as mafic minerals. Various size of quartz veins intruded into it.

 

Microscopic Study

Biotite microgranite is the holocrystalline, medium-grained, hypidiomorphic granular texture. It is essentially composed of quartz, alkali-feldspar, plagioclase, biotite and hornblende. Accessory minerals are sphene, apatite, rutile and zircon. Quartz occurs [as] anhedral grains in irregular aggregate and composed of 35–40% of the total volume. Orthoclase shows twinned and untwinned crystals, these subhedral orthoclase shows pronounced Carlsbad twin and sometimes show zoning.

Many untwinned orthoclase feldspar exhibit cloudy appearance due to alteration, kaolinization and sericitization. Myrmekitic texture of quartz and feldspar intergrowth is also observed. Plagioclase shows subhedral grains, zoning and polysynthetic twinning, and composition is approximately An30–An32. Biotite is chief mafic mineral in this rock, about 10-15% of the total volume. It is subhedral form and showing strong pleochroism of pale yellowish brown to dark brown and some altered to light green chlorite along the cleavage and margins. Inclusions of apatite and zircon [are] present in the quartz and feldspar grains.

3.7 Pegmatites and Aplites

Megascopic Study

This rock is not very prominent in the area. It has been found typically at Sakhan-gyi and Pan-taw, and as small dykes in Dattaw and Lay-thar. Pegmatite generally occurs as dykes in the marginal areas of the Kabaing Granite and also in the surrounding metamorphic rocks. It shows whitish colour on fresh surface and dark brown colour in weathered surface. There are several good exposures of pegmatite dykes, one at least 60 m wide in the south west of Sakhan-gyi and also about 100 m wide in Pan-taw. Outer margin of the pegmatite dykes are highly weathered and mostly kaoline. Aplite dykes about 2 m in thickness are intruded into the ultramafic rocks in Mya-sein Taung.

The major pegmatite bodies are Sakhan-gyi pegmatite and Pan-taw pegmatite and are found to be of complex type following Kerkvoot (1995); and of rare-element class, beryl type according to Cerny (1991) in Table 3.1.

Basal Section Of Hornblende In Jacupriangite photomicrograph image
Fig. 3.59. Basal section of hornblende in jacupriangite, XN, 25 X.
Kabaing Granite photo image
Fig. 3.60. Kabaing granite intruded into the dunite body in Pyaung-gaung, 23° 00' 18.4" N, 96° 27' 8.5" E.
Mineral Constituents Of Biotite Microgranite photomicrogaph image
Fig. 3.61. Mineral constituents of biotite microgranite includes quartz, feldspar and biotite mica, XN, 10 X.
Euhedral Alkali-Feldspar Shows Zoning With Some Biotite photomicrogaph image
Fig. 3.62. Euhedral alkali-feldspar shows zoning with some biotite in Kabaing granite, XN, 10 X. 
Chlorite, Alteration Product Of Biotite photomicrogaph image
Fig. 3.63. Chlorite, alteration product of biotite in Kabaing Granite, PPL, 25 X.
Elongated Biotite Crystal photomicrogaph image
Fig. 3.64. Elongated bioite crystal showing bent nature due to deformation, 52 XN, 25X.
Archaean Chart
Table 3.1. Classification of pegmatites of the rare-element class for Sakhan-gyi and Pan-taw area, after Cerny (1991).

Microscopic Study

It shows hypidiomorphic granular, pegmatitic texture, and sometime shows graphic texture. The rock contains only a few minerals which are usually in the form of extremely large crystals. Feldspar and quartz are visible in hand specimen. Most of the alkali-feldspars are weathered and kaolinized and also showing perthilic texture. Small amount of muscovite and lepidolite micas are observed. Sericitization are observed along the cleavage and fracture of alkali-feldspar and some alkali-feldspar showing like a sector twinning.

In aplite, mainly composed of anhedral granular grains of quartz, orthoclase and plagioclase, and apatite, zircon, biotite occurs as accessories. Chalcedonic silica vein and feathery appearing chalcedonic silica cementing quartz and feldspar are also observed.

Granophyric Quartz In K-Feldspar photomicrograph image
Fig. 3.65. Granophyric quartz (Q) in K-feldspar, XN, 10 X.
Graphic Quartz In K-Feldspar photomicrograph image
Fig. 3.66. Graphic quartz (Q) in k-feldspar and the quartz protrudes in the orthoclase (Or) by forming indentations, XN, 10 X.
Graphic Quartz With Extinctions Into The K-Feldspar photomicrogaph image
Fig. 3.67. Graphic quartz (Q) with extinctions into the k-feldspar, XN, 10 X.
K-Feldspar With Perthite photomicrogaph image
Fig. 3.68. K-feldspar with perthite. Both the high proportion of perthite to k-feldspar and the irregular shapes rather suggest a metasomatic origin for the plagioclase. K-feldspar in the extinction position, XN, 10 X. 
K-Feldspar Showing Rare Sector Twin And Intergrowth With Quartz photomicrogaph image
Fig. 3.69. K-feldspar showing rare sector twin and intergrowth with quartz, XN, 25X.
Chalcedonic Silica Vein In K-Feldspar In Alpite photomicrogaph image
Fig. 3.70. Chalcedonic silica vein in k-feldspar in aplite, XN, 25 X.
Plotted Data Of The Igneous Rocks In Mogok
Fig 3.71. Plotted data of the igneous rocks in the Mogok Stone Tract, except ultramafic rocks (Source: IUGS Classification, 2006). Click to enlarge (original image is muddy).
Modal Classification Of Ultramafic Rocks
Fig 3.72. Modal classification of ultramafic rocks based on the proportions of olivine (Ol), orthopyroxene (Opx), clinopyroxene (Cpx), pyroxene (Px) and hornblende (Hbl). (Source: IUGS Classification, 2006)

 

Thus concludes Chapter III. Access Chapter IV here.
References follow.

References

Adams, E.D., 1926. A visit to the gem districts of Ceylon and Burma. Bull. Can. Inst. Min. Met. v. 166, pp. 213–246.

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