visual features of kim

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VISUAL FEATURES OF KIM Kimberlitic Indicator Minerals

Maja Kiridzija

“Knowing the micro world leads to the macro discovery”

OBJECTIVE

At the end of today’s activities you will be able to identify and separate KIM

grains from the regular till samples in 30 minutes.

AGENDA 1. Introduction to KIM (30min)

Definition and list of all KIM KIM size fraction Procedure for KIM observation General visual features of KIM Importance of KIM visual features

2. PG, EG, IL, CR, CD and OL visual properties (30min) Visual features Morphologies and abrasion Kimberlitic vs Non Kimberlitic Similar minerals Sources of non kimberlitic Indicator minerals Review quizzes Break

3. Practicing KIM identification under the binocular microscope (60min) Kimberlite concentrates Teaching sets and teaching material Extraction of KIM from KIM concentrates Different exercises: separation of PG from EG, CR from IL, etc. Break

4. Microscopic observation of till concentrates spiked with KIM grains (60min) KIM observation (30 min) Recovery of spiked grains Conclusion Evaluation of the course

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Peridotitic Garnet

Eclogitic Garnet

Picroilmenite

Chromite

Chrome diopside Olivine

Peridotitic Garnet

Picroilmenite

Picroilmenite

Chromite

Name the KIM in the photo

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LIST OF KIM 1. Kimberlitic garnet

Peridotic (PG) Eclogitic (EG)

2. Kimberlitic Chromite (CR)

3. Picroilmenite (IL)

4. Chrome Diopside (CD)

5. Kimberlitic Olivine (OL)

OTHER KIM (not examined this time)

•  Kimberlitic orthopyroxene (enstatite)

•  Kimberlitic zircon

•  Diamond

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KIM DEFINITION

•  macrocrysts and megacrysts in kimberlite/lamproite

•  easily liberated from their host rock •  sufficiently dense to be concentrated by

gravity •  abundant compare to diamonds •  durable during transportation •  distinct by their visual and geochemical

features

SIZE FRACTIONS OF KIM

•  Most common in regional till sampling: –  0.25mm to 0.5mm

•  Why this size fraction? –  To ensure the abundance of KIM during till sampling –  To provide good range of KIM for geochemical studies

•  >0.5mm useful for studying KIM surface features (abrasion)

KIM OBSERVATION time consuming!

binocular microscope, tweezers or brushes, petri dishes, scratch pencil indicator collector, KIM observation skill and patience

“IF IN DOUBT PICK IT OUT!”

lines quarters spirals

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GENERAL VISUAL FEATURES OF KIM

•  Lack of crystal shape (with exceptions)

•  No inclusions (except of other mantle minerals)

•  Sculptured surface features: resorption, orange peel, coating, pitted surfaces, cockscomb, bumpy surface

•  Specific color: fire, bright emerald, bright pink/purple/red/orange

KIM SURFACE TEXTURES from M. B. MCCLENAGHAN AND B. A. KJARSGAARD

A) Cr-pyrope with dark green-grey adhering kimberlite (k);

B) Cr-pyrope with dark green-grey kelyphite rims (kp);

C) Cr-pyrope displaying sub-kelyphite orange peel texture on surfaces, outlined by dashed line;

D) Cr-pyrope displaying sub-kelyphite frosted surfaces;

E) Cr-pyrope displaying fresh, angular broken surfaces;

F) Cr-diopside displaying rounded (r ) to subrounded shape, adhering kimberlite (k), as well as broken angular surfaces (a);

G) and H) chromite displaying octahedral to suboctahedral (resorbed) shapes.

WHY IS IT IMPORTANT TO KNOW THE VISUAL FEATURES OF KIM? •  To be able to distinguish KIM among background and

other similar minerals in the core, till, stream or other concentrates

•  To be able to provide a quick and statistically viable selection of KIM for further geochemical analyses

•  To possibly determine: –  proximity of the kimberlitic source –  difference among indicator trains –  diamond potential of their source

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KIMBERLITIC GARNET

Pyrope, Peridotitic and/or Eclogitic garnet Mg, Fe,Ca, Al silicate with elevated Cr, Ti, Na

VISUAL FEATURES

•  Isotropic •  Purple/Orange color with “fire” •  Lack of crystal shape •  No polycrystalline aggregates •  Kelyphitic rim •  Resorption •  “Orange peel”

MORPHOLOGIES OF KIMBERLITIC GARNET

1. Kimberlitic garnets with kelyphitic rims

2. Fine resorption surfaces

3. Euhedral kimberlitic garnets (very rare)

4. Intensive surface “etching”

5. Frosted kimberlitic garnets

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Kelyphitic rim

Fine resorption

Euhedral

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Surface etching

Frosted

PERIDOTITIC vs ECLOGITIC GARNET

PERIDOTITIC •  Cr2O3 >2%

•  “purple garnets” pink-purple to blue and orange red with red fire

(“megacrystic garnet”)

ECLOGITIC •  Cr2O3<2%

•  “orange garnets” pale pink to pale orange and orange with orange fire

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“Megacrystic garnet” (elevated TiO2)

COLOR SCHEME

TYPICAL COLORS 1.  Blue-purple to deep purple

(Peridotitic)

2. Pink-purple (Peridotitic)

3. Red (Megacrystic)

4. Orange garnet (Eclogitic)

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UNUSUAL COLORS 1.  Green kimberlitic garnet (Dry Bones kimberlite, Canada)

2. “Dirty” kimberlitic garnet (Ranch Lake kimberlite, Canada)

3. Bright pink “G10” type kimberlitic garnet

4. White kimberlitic garnet (Dry Bones kimberlite, Canada)

KIMBERLITIC vs NON-KIMBERLITIC GARNETS

KIMBERLITIC NON-KIMBERLITIC no crystal shape often crystal shape (rounded and/or fractured)

no inclusions possible inclusions kelyphitic rim no kelyphitic rim (wholly or partially present)

sculptured surfaces no sculptured surfaces (resorption, etching, “orange peel”)

not magnetic slightly magnetic

5 peridotitic garnets mixed with non kimberlitic garnets

PG

PG PG

PG

PG

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5 eclogitic garnets mixed with non kimberlitic garnets

EG

EG EG

EG

EG

MINERALS SIMILAR TO KIMBERLITIC GARNET

1. Common garnet

2. Pink zircon

3. Pink corundum (Ruby)

4. Pink spinel

5. Staurolite

6. Gahnite

PG

PG

PG

EG

NON ABRADED vs ABRADED

NON ABRADED •  Preserved surface

features:

•  kelyphitic rim •  resorption •  etching •  sharp fractured edges

ABRADED •  Not preserved or

partially preserved surface features

•  absence of kelyphitic rim •  polished resorption

features, etching •  abraded fractured edges

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NON ABRADED vs ABRADED

fresh slightly abraded well abraded

proximal distal

NON ABRADED PERIDOTITIC GARNET

Sculptured surface

Rounded with coarse resorption

Mixed with kimberlite Kelyphitic rim

NON KIMBERLITIC GARNET SOURCES

•  minette

•  melnoite

•  eclogite lenses in ophiolites

•  granulites

•  pegmatite,

•  granite, etc.

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Kimberlitic garnet review quiz 1  Kimberlitic garnet has a metallic luster

FALSE

2  Crystal shape is typical for kimberlitic garnets FALSE

3  Green colored garnets can be kimberlitic TRUE

4  Inclusions are frequent in kimberlitic garnets FALSE

5  A kelyphitic rim is one of the characteristics of kimberlitic garnets TRUE

6  Megacrystic garnets are indicative of good diamond potential FALSE

7  "G10" garnets are a bright red color FALSE

8  Megacrystic garnets are an orange red color with red fire TRUE

9  Lamproitic and kimberlitic garnets have the same visual features TRUE

10 Eclogitic garnets are easily mixed up with epidotes FALSE

PICROILMENITE

Mg Ilmenite Mg,Fe,Ti,Oxide with elevated Cr, Mn, Al

VISUAL FEATURES

•  Opaque •  Lack of crystal shape and cleavage •  Coating (leucoxene, perovskite, anatase) •  “Bumpy” surface •  Conchoidal fracture •  Black scratch powder •  Lack of inclusions and adhering grains

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PICROILMENITE MORPHOLOGIES

1. Fractured picroilmenite (brittle, sharp, conchoidal fractures, no surface preserved)

2. Matte, smooth surface picroilmenite (could be similar to chromite except for the metallic and not glassy luster)

3. Coated picroilmenite with a“bumpy surface” (leucoxene, rutile, anatase and/or perovskite coatings)

4. Polygranular picroilmenite (with or without coatings, multiple crystal growth, rare but present)

Fractured

Smooth surface

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Coated surface

Polygranular

PICROILMENITE vs ILMENITE PICROILMENITE

•  lack of crystal shape •  lack of cleavage •  lack of inclusions •  conchoidal fractures •  lack of holes or any

depressions •  often coated

ILMENITE •  crystal shape •  visible cleavage and

layering •  inclusions present •  flat fracture along cleavage •  possible presence of holes

and depressions •  rarely coated

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MINERALS SIMILAR TO PICROILMEITE

1. Common Ilmenite

2. Magnetite

3. Hematite

4. Chromite

5.Columbite/Tantalite

6. Euxenite

7. Rutile

8. Amphibole

Mix of picroilmenite and chromite

3 picroilmenite and 3 chromite

CHROMITE

PICROILMENITE

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1 picroilmenite and amphibole

IL

NON KIMBERLITIC ILMENITE SOURCES

•  Picroilmenite: basalts ultramafic lamprophyres carbonatites

•  Common ilmenite: gabbros norites anorthosites, etc

Picroilmenite review quiz

1. The picroilmenite is the most distinctive kimberlitic OXIDE mineral

2. Genetically the picroilmenite in a kimberlite is a MEGACRYST 3. Morphologically picroilmenite may appear as a SINGLE or

POLYGRANULAR crystal 4. A typical surface feature of the picroilmenite is COATING 5. The picroilmenite has a BLACK streak 6. Morphology of picroilmenites is used in ABRASION studies 7. Compared to the common ilmenite, the picroilmenite does not

have a CRUSTAL SHAPE and INCLUSIONS 8. Layers on fractured surfaces are typical for COMMON

ILMENITES 9. The picroilmenite has a METALLIC luster 10. The picroilmenite has CONCHOIDAL fractures

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KIMBERLITIC CHROMITE

Chrome spinel, Cr spinel Mg,Cr,Al,Fe oxide with elevated Mn, Ti

VISUAL FEATURES

•  Black mineral but isotropic at the transparent, thin edges •  Resorbed octahedron •  Smooth, satin or matte, sheen surface •  Possible zoning •  Glassy and conchoidal fractures •  Brown and soft scratch powder •  Lack of inclusions, adhering quartz, pyroxene, sulfide,

etc

CHROMITE MORPHOLOGIES

1. Resorbed octahedrons with “pitted” surface similar as diamond resorption

2. Rounded to subrounded with reminiscent of octahedron shape

3. Angular, anhedral, matte surface with “sheen”

4. Twined resorbed octahedrons (very rare)

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Resorbed octahedron

Rounded

Angular with matte surface (sheen)

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TWINNED

KIMBERLITIC vs NON KIMBERLITIC CHROMITE

always look as a population not as a single grain!

KIMBERLITIC •  Resorbed octahedron •  Absence of layering •  Smooth, “matte” surfaces •  Rare polycrystalline phases •  Lack of inclusions or any

adhering grains

NON KIMBERLITIC •  Sharp octahedrons •  Layered facets •  Glassy surfaces no matte •  Often polycrystalline phases •  Possible presence of inclusions or

adhering grains

MINERALS SIMILAR TO CHROMITE 1. Common Ilmenite/Picroilmenite

2. Columbite/Tantalite

3. Rutile

4. Euxenite

5. Andradite

6. Magnetite

7. Tourmaline

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Chromite and picroilmenite 0.50mm

chromite picroilmenite





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NON KIMBERLITIC CHROMITE SOURCES

•  chromitite

•  basaltic rock

•  meteorites

•  serpentinite

•  olivine rich peridotite, etc

Kimberlite chromite review quiz Which population of grains (1, 2, 3, 4) are kimberlitic chromite?

1. PICROILMENITE


2. NON KIMBERLITIC CHROMITE

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3. SPHALERITE


4. NON KIMBERLITIC CHROMITE



5. KIMBERLITIC CHROMITE

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CHROME DIOPSIDE

Clinopyroxene Ca, Mg silicate with elevated Fe,Cr,Al,Na

VISUAL FEATURES

•  cleavage •  emerald green color •  “cockscomb feature” •  rounded, subrounded or elongated •  lack of inclusion and adhering grains

Chrome diopside and pyrope assemblage local geotherm!

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CHROME DIOPSIDE MORPHOLOGIES

1. Bright emerald green, angular, clear, transparent

2. Gray green, angular, clear

3. Frosted, non transparent, pale emerald green

4. Rounded, nodular, grass green, coated

Bright emerald green

Gray green

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Frosted

Coated

MINERALS SIMILAR TO CHROME DIOPSIDE

1. Emerald

2. Uvarovite

3. Cosmochlore

CD

CD

CD

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NON KIMBERLITIC CHROME DIOPSIDE SOURCES

•  melnoite

•  pegmatite

•  alkaline olivine basalt

•  ophiolite

•  minette, etc

Chrome diopside review quiz Identify 5 chrome diopside?

CD

CD

CD

CD

CD

KIMBERLITIC OLIVINE

Forsteritic olivine Mg silicate with Fe, Ni, Mn

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VISUAL FEATURES

•  Beige, pale green, pale yellow and orange yellow color (due to oxidation)

•  Rounded, subrounded, subangular, often fractured

•  Conchoidal fractures

•  No cleavage but “partitioning”

•  Surface resorption features

KIMBERLITIC OLIVINE MORPHOLOGIES

1.  Subrounded to rounded, pale yellow-green colored with smooth surfaces and fine or coarse resorption

3.  Subangular to angular stained pale orange colored with frosted surfaces

5.  Subrounded to subangular beige coloured with frosted surfaces

4. Angular pale green colour with intensive parallel partitioning/”etching” (late stage)

Resorption

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Pale orange and frosted surface

Pale beige and frosted surface

Partitioning

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MINERALS SIMILAR TO OLIVINE

1. Apatite 2. Epidote

3. Clinopyroxene

4. Grossular

NON KIMBERLITIC OLIVINE SOURCES

•  dunite

•  olivine basalt

•  lamprophyre

•  basalt, etc.

Kimberlitic olivine review quiz Identify 5 kimberlitic olivine?

OL OL

OL

OL

OL

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CONCLUSION

single KIM grain can lead to many diamonds

Spiked KIM grains

visual features of kim

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