This document discusses garnet, a type of natural abrasive mineral. It describes garnet's composition, properties like hardness and crystal structure. There are different types of garnet classified by their chemical composition, such as aluminum garnet and iron garnet. Garnet is commonly used as an abrasive in applications like coated abrasive papers and cloths for wood sanding, and as loose grains for sandblasting due to its safety compared to alternatives containing silica. The document outlines garnet's production process including sizing, grading, heat treatment and testing to determine its abrasive quality for various end uses.

1. METALURGIA
DE
NO METÁLICOS
GARNET

2. ABRASIVES
DIAMOND,
INDUSTRIAL
CORUNDUM–EMERY
GARNET

3. GARNET
Natural Abrasives.
• Garnet:.
– The name “garnet” is given to a group of iron-aluninum silicate
minerals having similar physical properties, crystal forms, and
general chemical formula.
Composition.
In garnet, general formula 3RO-R2O3-3SiO2, the bivalent element
may be calcium, magnesium, ferrous iron, or manganese.
The trivalent element includes aluminum, ferric iron, or chromium,
rarely titanium.

4. Natural Abrasives.
Composition.
– There are tree prominent groups. These are as follows:
1. Aluminum Garnet
a) Grossularite, calcium-aluminum garnet
(3CaO.Al2O3.3SiO2).
b) Pyrope, magnesium-aluminum garnet
(3MgO.Al2O3.3SiO2).
c) Almandite, iron-aluminum garnet
(3FeO.Al2O3.3SiO2).
a) Spessartite, manganese-aluminum garnet
(3MnO.Al2O3.3SiO2).
2. Iron Garnet
Andradite, calcium-iron garnet
(3CaO.Fe2O3.3SiO2).
GARNET

5. Natural Abrasives.
Composition.
3. Chromium Garnet.
Uvarovite, calcium chromium garnet
(3CaO.Cr2O3.3SiO2).
GARNET

6. COLOR:
Color in garnet vary greatly but generally are as follows:
– Grossularite – white, pale, or yellow
– Pyrope – deep red to black
– Almandite – deep red, brownish red to black
– Spessartite – brown to red
– Andradite – black, green, and yellow green
– Uvarovite – emerald green.
PROPERTIES

7. HARDNESS.
Hardness ranges from 6 (grossularite) to 7.5 (almandite).
CRYSTAL SYSTEM.
Cubic, commonly as rhombic dodecahedrons or tetragonal
trisoctahedrons, or in combination of the two.
FRACTURE.
Garnets have a glassy structure and usually have a market
conchoidal fracture.
INDEX OF REFRACTION
The index of refraction of the garnet group ranges from 1.735 to 1.94.
PROPERTIES

8. TENACITY.
Aggregates composed of individual crystals of various size are brittle
and shatter readily.
CLEAVAGE.
Occasionally an indistinct dodecahedral cleavage is observed. Some
species of almandite possess a pronounced laminated structure; the
laminas are planes of weakness along which the mineral
separates.
FUSIBILITY.
Garnet having a high iron content, such as almandite, fuse at a
temperature of about 1200ºC.
White garnets containing a considerable percentage of chromium are
infusible.
PROPERTIES

9. OTHER PROPERTIES.
• Minerals of the garnet group have specific gravities that range from
3.5 to 4.2,
• luster that is vitreous, resinous, or dull,
• and are transparent to opaque.
PROPERTIES

10. Garnet commonly occurs as an accessory mineral in a large variety of
rocks, more particularly in gneiss and schists, but also in contact
metamorphic deposits, crystalline limestones, pegmatites, and
serpentines.
NEY YORK
IDAHO
FOREINGN COUNTRIES.
Garnet is or has been commercially mined in Madagascar, Japan;
Argentina, India and Tanzania.
States that reserves are significant in Australia, India, and the former
USSR.
OCCURRENCE

11. GRADING AND SIZING.
Practically 100% of the garnet sold today is a sized and graded product.
Sizing are governed by mining techniques and end uses.
They generally conform to specific standars: e.g., “Standards for Quality
Criteria” and “Standard Methods of Test, Rating, Certificatión”
established in cooperation with the National Bureau of Standars to
provide a uniform basic for fair competition.
PREPARATION FOR MARKET

12. HEAT TREATMENT.
• Practically all garnet grain used today is heat treated as a processing
funtion rather than for the purpose of improving inherent
abrasiveness. Impurities picked up during processing adhere to garnet
particle surfaces and destroy cleanliness and capillary needed in later
processing to give adhesion for bonding.
• By the early 1960s, scientific investigation had determined that
excessively high temperatures could destroy the abrasive quality of
garnet. It was concluded that heat treatment solely contributed to
cleansing of particle surfaces or, at most, it produced superficial
healing of microscopic fractures on the grain surface.
PREPARATION FOR MARKET

13. • TEST FOR ABRASIVE QUALITY OF GARNET.
There is no exact method for testing the abrasive quality of
garnet or any loose abrasive except by practical application.
However, several rought tests and examinations serve to indicate their
abrasives posibilities. Fracture, sharpness and shape of the grain,
character of the grain structure, and the presence of inclusions of
other minerals which weaken the grain structure can be studied under
the microscope.
.
PREPARATION FOR MARKET

14. • ABRASIVE PAPERS AND CLOTH.
Garnet coated papers and cloths are used primarily for wood
sanding, but also for finishing of leather, hard rubber, plastic,
glass, and softer metals.
Coated abrasives have been improved in recent years, mainly by
using resin bonds in place of glue bonds for certain applications.
Also, electrostatic coating is now in general use. In this process, an
electrostatic charge causes the grains to imbed upringht in the wet
bond on the backing. In efect, the shape cutting edges of the grain
are bonded perpendicular to the backing and due to individual
grain repulsion, individual grain particles are more evenly spaced.
Garnet paper and cloth products comprise approximately 10 to
11% of the total volumen of coated abrasives produced for the past
several years.
USES AND MARKETS

15. • GLASS SURFACING.
The properties that made garnet a competitive abrasive for this
application continue to produce new markets in the glass, plastics,
ceramics, and softer metals industries.
PREPARATION FOR MARKET

16. • LOOSE GRAINS.
A major nontechnical use of garnet in loose form is for sandblasting.
Garnet has the adventage of having no free silica and cannot
cause silicosis.
PREPARATION FOR MARKET

17. • MISCELLANEOUS USES.
Garnet, in different grain sizes, is also used for the manufacture of
grinding wheels, as tumbling media, for nonskid applications, and
as fillter media. In powder sizes, it is made into lapping and buffing
compounds.
PREPARATION FOR MARKET

18. • Markets and prices.
In general, every abrasive has an application for which its qualification
surpasses others, but there is keen competition, based on relative
cost and grinding and finishing abilities, in aplications where
qualifications overlap.
Garnet has substantially kept its relative position in the
expanding abrasives maket through exploration and application
of new technology.
MARKETS AND PRICES