Optical Microscopy

Illustrating some of our capabilities using transmitted and reflected light microscopy

Optical Microscopy

Optical microscopy is a relatively inexpensive method of providing information on ore concentrates, mineral prospects, mine waste and man-made materials including clarification of mineral phases present, modal abundance, textural controls and the estimated deportment of key elements. Optical microscopy can be used in conjunction with automated techniques, particularly to help verify mineral lists. Petrolab has a diverse staff with extensive experience of the use of petrographic microscopes to identify and interpret a wide range of sample types.

Methodology

Petrolab uses three standard petrographic microscopes (2*Nikon Microphot-FXA & 1*Nikon Labophot-2), to undertake transmitted and reflected light microscopy. Using these microscopes and staff experience, Petrolab is able to offer the following services:

  • Grab sample petrography, from prospective ores through to possible mine waste.
  • Aggregates, stone and slate – petrography to determine suitability for use.
  • Concretes, render and mortar – petrography to determine current material condition.
  • Smelter Slags – Analysis of smelter products to determine metal recovery prospects.

Petrolab Case Studies

Illustrating some of our petrographic capabilities

The following example case studies have been assembled to demonstrate the mineralogical / petrological information that Petrolab can supply using transmitted and reflected light microscopy.

Case study 1 – Industrial minerals

Scope

Produce a mineralogical assessment of a potash deposit using optical microscopy.

Analysis summary

Four standard petrographic thin sections were prepared from two potash samples using a water sensitive preparation method to avoid dissolution of mineral phases potentially present such as halite. Mineralogical characteristics were determined using a stereomicroscope and a polarising microscope.

Modal analysis

1000 points were counted using a Pelcon 64 channel electromechanical point counter to obtain modal mineralogy and weight percent of each phase calculated using specific gravity data (Table 1).

Table 1: Modal analysis of sample(s) obtained from point counting.

Phase, abbreviation General formula Specific gravity Wt. %
Sylvite, syl KCl 1.99 67
Carbonate, carb CaCO3 2.71 15
Clay, clay N/A ~2.60 13
Carnallite, cna KMgCl3.6H2O 1.60 3
Quartz, qtz SiO2 2.65 1
Gypsum, gy CaSO4.(H2O) 2.30 1
Baryte, bar BaSO4 4.48 <1
Haematite / limonite, hm/lm Fe2O3/Fe3+O(OH) ~4.55 <1

Phase description

Use of optical microscopy can provide information on the form and distribution of mineral phases within a sample (Table 2). In this study, sylvite was the dominant phase of significance for the project with minor carnallite. Both phases were present as composite grains with gangue carbonated and clay, sylvite as simple locks between 400 µm and 1 mm, carnallite as more complex lenses and veinlets.

Table 2: Phase descriptions including typical grainsize and phase form.

Phase, abbreviation Min Max Typical Prominent grain type
Sylvite, syl 20 µm 3 mm 1 mm euhedral grains
Description Coarse euhedral grains of sylvite commonly containing fine euhedral inclusions of carnallite. Sylvite forms minor composite grains with fine carbonate and clay. Individual crystals of sylvite have an average grainsize of 800 µm . Sylvite and halite could not be differentiated by optical microscopy due to their similar optical properties.
Carbonate, carb < 5 µm 1.2 mm 400 µm Fine aggregates
Description Fine aggregates of carbonate as liberated grains and composite grains with sylvite. Fine carbonate aggregates commonly host fine carnallite and clay as lenses and veinlets < 400 µm. The sample also contains a minor population of coarser carbonate with crystals < 300 µm.
Clay, clay < 10 µm 1.4 mm 500 µm Fine aggregates
Description Fine aggregates of clay, commonly associated with fine aggregates of carbonate. Clay aggregates commonly contain fine < 20 µm inclusions of quartz and also host lenses and veinlets of carnallite.
Carnallite, can < 10 µm 400 µm 100 µm Euhedral inclusions
Description Euhedral inclusions in sylvite with an average size of ~100 µm and as lenses and veinlets <400 µm in carbonate and clay.
Quartz, qtz < 10 µm 50 µm < 20 µm Fine inclusions
Description Fine sub-angular grains of quartz in aggregates of clay and carbonate.
Gypsum, gy < 10 µm 400 µm < 100 µm Euhedral inclusions
Description Rare euhedral inclusions in sylvite and carbonate. The sample also contains traces of euhedral baryte.
Haematite / limonite, hm/lm < 5 µm 60 µm < 10 µm Fine inclusions
Description Rare fine inclusions in sylvite giving crystals a pinkish colour and as opaque fine inclusions in clay rich aggregates.

Photomicrographs

Optical photomicrograph taken using transmitted light microscopy showing coarse crystalline sylvite (syl), clay and carbonate (carb) as liberated grains. The photomicrograph also shows coarse euhedral carnallite (carn) inclusions in sylvite.

euhedral carnallite (carn) inclusions in sylvite

Case study 2 – Ore deposits

Optical microscopy is a relatively inexpensive method of providing information on ore concentrates or mineral prospects including clarification of mineral phases present, modal abundance, liberation / locking of target phases and estimated deportment of target metal(s). Optical techniques can be used in conjunction with automated techniques.

Scope

Full quantitative mineralogical investigation of a Cu ore (concentrate) using transmitted and reflected light microscopy to determine distribution and liberation of Cu-bearing phases for mineral processing. A polished thin section and polished block were provided by the client.

Modal analysis

1000 points were counted using a Prior Model G point counter to obtain modal mineralogy and weight percent of each phase calculated using specific gravity data (Table 1).

Table 1: Modal analysis of sample(s) obtained from point counting.

Phase, abbreviation General formula Specific gravity Wt. %
Pyrite, py FeS2 5.01 76
Chalcopyrite, cp CuFeS 4.19 23
Sphalerite, sl (Zn, Fe)S 4.05 1
Quartz, qtz SiO2 2.65 <1

Phase description

Use of optical microscopy can provide information on the form and distribution of mineral phases within a sample (Table 2). In this study, sylvite was the dominant phase of significance for the project with minor carnallite. Both phases were present as composite grains with gangue carbonate / clay, sylvite as simple locks between 400 µm and 1 mm, carnallite as more complex lenses and veinlets.

Table 2: Phase descriptions including typical grainsize and phase form.

Phase, abbreviation Min Max Typical Prominent grain type
Pyrite, py < 10 µm 1 mm 400 µm Liberated massive grains
Description Massive grains of pyrite showing 60% apparent liberation. Pyrite is replaced by chalcopyrite creating complex intergrowth textures with a complete spectrum of chalcopyrite to pyrite dominated grains.
Chalcopyrite, cp < 10 µm 1 mm 300 µm Locked grains
Description Massive grains replacing pyrite. Chalcopyrite shows 25% apparentliberation. The chalcopyrite replacement of pyrite creates complexbrecciated textures of fine multiple pyrite inclusions in chalcopyrite. The average size of locked chalcopyrite is approximately 300 μm, however locking textures go through the complete size range, which is likely to make complete recovery very difficult.
Sphalerite, sl 20 µm 400 µm 100 µm Single crystal fragments
Description Crystalline sphalerite typically as simple locks with chalcopyrite +/- pyrite. Sphalerite also contains traces of ultra-fine (<10 μm) inclusions of chalcopyrite.
Quartz, qtz 40 µm 200 µm 100 µm Locked single crystals
Description Rare simple locks with pyrite +/- chalcopyrite. Individual crystals are devoid of inclusions of chalcopyrite.

Photomicrographs

Optical reports are provided with photomicrographs of key features. In this study, images of the key mineral phases and their typical form were included (Figure 1).

Key ore features

 

Figure 1: Photomicrographs showing key ore features. (left) All major sulphide phases with typical form of pyrite – chalcopyrite locks. (right) View showing fine chalcopyrite – pyrite intergrowths.

Case study 3 – Smelter slags

Petrolab has conducted numerous studies of smelter products to determine metal recovery prospects. This case study highlights a smelter project focussed on the clarification of the deportment of target metals identified by assay.

Scope

Full quantitative mineralogical investigation of a smelter slag using optical microscopy to provide mineral / phase identification, quantitative abundance and detailed qualitative phase descriptions including annotated photomicrographs. An assay provided by the client indicated that the sample contained significant Zn, Pb and Cu.

Modal analysis

1000 points were counted using a Prior Model G point counter to obtain modal mineralogy and weight percent of each phase calculated using specific gravity data (Table 1).

Table 1: Modal analysis of smelter slag phases obtained from point counting.

Phase, abbreviation General formula Specific gravity Wt. %
Slag (silicate glass), slg N/A ~2.90 54
Quartz, (qtz) SiO2 2.65 16
Pyrite / pyrrhotite, py / po FeS2 / Fe(1-x)S ~4.81 12
Fe oxides, FeO Fe3+O2 / Fe3+O(OH) ~3.80 8
Secondary coppers, 2y Cu Cu9S5 / Cu2S / CuS ~5.31 6
Lead, Pb Pb 11.37 4

 

Phase description

Use of optical microscopy can provide information on the form and distribution of mineral phases within a sample (Table 2). In this study, the Cu assay values provided by the client could be accounted for by the abundance of secondary coppers present in the sample. The lead assay abundance could only partially be accounted for by the presence of native lead; optical microscopy determined that lead grains were often strongly oxidised and could not be easily distinguished from iron oxides and were often smaller than the resolution of the point count. Additional analysis by SEM would potentially aid understanding of the metal distribution in this sample, although optical microscopy has provided valuable information on the possible target phases for this smelter slag.

Table 2: Phase descriptions including typical grainsize and phase form.

Phase, abbreviation Min Max Typical Prominent grain type
Slag (silicate glass), slg <10 µm 1 mm 500 µm Complex grains
Description Complex grains of undifferentiated slag, the composition of the glass could not be positively determined by optical microscopy. The reflectivity of the material is anomalous but relatively low and is in the region of magnetite and/or sphalerite, however as the grains show complex intergrowths and are largely isotropic the abundance of sphalerite, magnetite and zinc silicate could not be determined. The high iron content of the assay indicates that a significant amount of this material is likely to be magnetite. The slag phase hosts significant moderately sutured inclusions of copper and iron sulphides, lead metal and various oxides.
Quartz (qtz) 60 µm 500 µm 300 µm Liberated grains
Description Liberated (>80% apparent liberation) rounded grains of quartz and rare inclusion in complex slag grains.
Pyrite / pyrrhotite, py / po < 10 µm 1 mm 200 µm Locked grains
Description The primary sulphides appears to be pyrite and pyrrhotite, these occur as intergrown liberated grains (35% apparent liberation) and as moderately sutured inclusions in the slag phase. The iron sulphides only show traces of oxidation and very little association with the copper sulphides.
Fe oxides, FeO < 10 µm 1 mm 300 µm Complex grains
Description The overall proportion of iron oxide is difficult to determine due to the it being difficult to differentiate magnetite from the slag phases. The sample does however contain a significant amount of haematite as rims and coatings on slag and sulphide grains.
Secondary coppers, 2y Cu < 10 µm 500 µm 120 µm Complex locked grains
Description Complex locked grains of secondary copper sulphides (covellite and chalcocite) locked with the slag phase +/- minor sulphides. The secondary copper sulphides shows approximately 20% apparent liberation. The chalcocite and covellite also shows evidence of minor oxidation to secondary copper oxides. There are also minor amounts of bornite and traces of variably altered chalcopyrite. The copper minerals although locked are likely to response well to leaching.
Lead, Pb < 10 µm 400 µm 200 µm Locked grains
Description Rounded liberated grains of lead metal and rare grains of galena locked with slag +/- sulphides. The lead metal shows variable degrees of oxidation, with occasional oxidised grains containing <20 µm remnant veinlets of galena.

Photomicrographs

Optical reports are provided with photomicrographs of key features. In this study, images of the key mineral phases and their typical form were included, such as presence of secondary coppers and the oxidised nature of the lead metal.

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Point Counting

An optical microscopy investigation involving quantitative point counting.

Typically 500 or 1000 points are counted on the thin section to determine the proportion of the different phases or minerals that are present.

The adjacent video demonstrates this technique in operation at Petrolab’s laboratory.