COMMERCIAL SENSE

SCIENTIFIC SENSE

 GRANITES

MAGMATIC ROCKS:
GRANITES, SYENITES, DIORITES, GABBROS, CHARNOCKITES

METAMORPHIC ROCKS: 
GNEISSES, MIGMATITES, GRANULITES, QUARTZITES (polishable), METACONGLOMERATES

PETROGRAPHIC TYPE

As mentioned, commercial granites have different origins and compositions:

MAGMATIC ROCKS were formed by crystalization from magma. Many commercial granites are magmatic (igneous) rocks created some kilometers under the earth‘s surface. These so-called plutonic rocks are composed of visible mineral grains of equal size (e.g. RUBY RED) or some grains larger than others - porphyritic structure (e.g. ROSSO SANTIAGO).

The grains are typically without orientation (e.g. BLANCO CRISTAL), but there are also oriented plutonic rocks (e.g. SOLAR WHITE).

Part of magma – so called lava crystalizes on earth‘s surface as volcanic rocks. These rocks do not belong to commercial granite group because they are not well polishable and have large water absorption.

Classification of plutonic magmatic rocks are based on mineral content as follows.

Granite group - rich in quartz (20 – 60 %) and feldspars (K-feldspars, Na-Ca-feldspars – plagioclases).

• Granite – feldspars (K-feldspars more than plagioclases), quartz, a little mica. It is of light grey, pink, red, white, yellow, brown colour.

• Alkali granite – feldspars (K-feldspars and Na-plagioclases), quartz, a little mica and amphibole (e.g. ASA BRANCA). It is of light colour.

• Granodiorite – feldspars (plagioclase more than K-feldspar), quartz, a little dark mica – biotite (e.g. BARRE GREY). It is mainly of grey colour.

• Pegmatite – very coarse rock of granite composition (e.g. AZUL ARAN) created in dykes (veins).

• Aplite – finegrained light rock of granite composition (PEARL WHITE) created in dykes (veins).

Syenite group - rich in K-feldspars with less plagioclase and quartz (up to 20 %).

• Syenite - feldspars (K-feldspars more than plagioclases), less biotite, hornblende. 

• Alkali syenite - feldspars (K-feldspars and Na-plagioclases), less biotite, hornblende and pyroxene (e.g. CAFE IMPERIAL).

• Larvikite (type of alkali syenite) – special feldspars, small quantity of quartz, pyroxene, magnetite (e.g. BLUE PEARL).

• Quartz syenite - feldspars (K-feldspars more than plagioclases), less quartz, biotite, hornblende.
  

• Monzonite - feldspars (K-feldspars = plagioclases), less biotite, hornblende, pyroxene.
  

• Quartz monzonite - feldspars (K-feldspars = plagioclases), less quartz, biotite, hornblende, pyroxene.
  

• Sodalite syenite – mainly feldspars (K-feldspars, plagioclases) and sodalite of blue colour (e.g. AZUL BAHIA)

• Diorite - plagioclase, biotite, hornblende and pyroxene, small amount of K-feldspar and quartz.
• Quartz diorite - plagioclase, quartz, biotite, and hornblende, small amount of K-feldspar (e.g. FAVACO).
  

Gabbro group - rich in plagioclase, pyroxene and olivine. It is usually of dark-grey to black colours.  

• Gabbro - plagioclase, clinopyroxene and less olivine.  

• Norite - plagioclase, orthopyroxene and less olivine (e.g. BLACK GALAXY).  

• Anorthosite – plagioclase, small quantity of pyroxene and olivine (e.g. VOLGA BLUE).  

• Dolerite – fine-grained rock of gabbro composition formed in dykes (e.g. SWEDISH BLACK). Older name – diabase. It is transition between gabbro and basalt.

Charnockite group is rich in feldspars (K-feldspars, plagioclases) with less quartz and pyroxene (e.g. VERDE UBATUBA). This special group can be mainly added to granite group or to syenite group according to their composition. The rocks are often metamorphosed.

A table showing the composition of the most frequent magmatic rocks used as commercial granites is below

Notes:
1. Plagioclases are from total feldspar content
2. Quartz is from total light minerals (quartz and feldspars)
3. red = granite group
4. violet = syenite group
5. blue = diorite group
6. green = gabbro group
7. charnockites belong to granite or syenite groups
8. blank areas are rock almost unused as natural stones

METAMORPHIC ROCKS were formed by the recrystalization of sedimentary or magmatic rocks under the earth‘s surface during raised pressure and temperature conditions, from low to high grade metamorphism. 

Mainly high metamorphic rocks such as gneisses belong to the commercial granites. Commercial slates are metamorphic rocks exposed to low metamorphism. 

Gneiss group contains rocks with high pressure and high temperature metamorphism, which have strongly oriented, schlieren, banded, and wavy structures such as paragneisses, orthogneisses, garnet gneisses, migmatites and granulites. Migmatites and granulites are added to gneiss group only because of their similar structures.

• Paragneisses (e.g. BARENTS BLUE) were formed by strong metamorphism of clayey sedimentary rocks (slates, clayey    
               sandstones).

• Orthogneisses (e.g. GIALLO VENEZIANO) were created by strong metamorphism of light magmatic rocks (granite group, light 
                  volcanics).

• Garnet gneisses (e.g. SANTA CECILIA) are rich in garnet & can be formed like para- or ortho-gneisses.

• Migmatites were formed by partial melting of gneisses (e.g. PARADISO).

• Granulites were created by very strong metamorphism (higher one than at orthogneisses) of mainly light magmatic rocks (e.g. KASHMIR WHITE).

Metaconglomerates (e.g. VERDE MARINACE)  are formed by metamorphism of conglomerates - sedimentary rocks composed of rounded grains over 2 mm (4 mm in Europe).

Quartzites are almost exclusively composed of quartz, metamorphosed from quartz sandstones (e.g. AZUL MACAUBAS). They are sometimes separated as an individual commercial stone group, when they are not polishable and splited as slates.

GEOLOGICAL AGE

The age of the stone is determined in the table by the geological time unit when the stone was created by the geological process. In some cases the older stone the larger strength and smaller water absorption is presented.

The basic geological periods with their ages are:

Quaternary (0 - 2 million years)

Terciery

• Neogene (2 – 25 million years)
• Paleogene (25 – 65 million years)

• Cretaceous (65 – 135 million years)
• Jurassic (135 – 205 million years)
• Triassic (205 – 250 million years)

• Permian (250 – 295 million years)
• Carboniferous (295 – 355 million years)
• Devonian (355 – 415 million years)
• Silurian (415 – 440 million years)
• Ordovician (440 - 495 million years)
• Cambrian (495 - 540 million years)

OTHER NAMES AND VARIETIES 
In the column, less used or less suitable names and varieties are presented. Many of them are not quite correct because the word order is reversed or they are incorrect because of  wrong spelling or have been mistaken for another stone.

SIMILAR STONES 
This column can help stone specialists and customers to find similar stones. A stone is similar to another stone by its colour(s), grain size and particle arrangement. A stone is partially similar to another one by colour(s) or appearance. In this column, there are also additional stones not covered in the Commercial Name and Other Names columns. Use Ctrl+F or Find in Page feature of your computer to find your stone in the Other Names or Similar Stones columns.

BASIS OF NAME 
The commercial names have different origins. The majority of names are derived from a geographic area or colour. The geographic site could be the quarry place, nearby town or the state. The colour is described in English, Italian, Spanish or other languages. Some stones have their names according to colours of precious stones, plants or animals.

Explanation of some terms in the text above:

K= Potassium, Mg= Magnesium, Fe= Iron, Al= Aluminium, Na= Sodium, Ca= Calcium

Term explanation:
Al – Aluminium
Alumosilicates – silicates, where Al partially substitutes Si
B - Boron
Biotite – dark mica – K, Mg, Fe, Al silicate 
Bronzite – pyroxene type
Ca - Calcium
Chlorites – green minerals – Fe, Mg, Al silicates 
Cl - Chlorine
Cleavage – the tendency of some mineral crystals to break along definite planes of weakness
Conglomerate – sedimentary rock composed of rounded fragments over 2 mm
Cordierite – mineral – Mg, Al silicate 
Dumortierite – mineralof blue colour - Al, B silicate
Fe - Iron
Feldspars –  light minerals – K, Na, Ca alumosilicates 
Garnet – mineral of red to brown colour – Fe, Mg, Ca, Al silicate
Hematite – ferric oxide
Hornblendes (Amphiboles) – group of dark minerals – Ca, Na, Mg, Fe, Al silicates
K – Potassium
K – feldspars – orthoclase or microcline minerals - K alumosilicates 
Limonite – ferric hydrate
Mg - Magnesium
Micas – scaled minerals – muscovite and biotite
Mohs Hardness - determined according relative scale which consists of ten minerals arranged in order of their increasing hardness
Muscovite – light mica – K, Al silicate 
Na - Sodium
Olivine – dark mineral – Mg, Fe silicate
Oriented granites – columnar or tabular minerals are distributed according to a certain orientation
Petrography – branch of geology concerned with rock description
Plagioclases - minerals – type of feldspars - Na, Ca alumosilicates
Plutonic rocks  - formed by crystalization from magma under earth`s surface in large massifs
Porphyritic – the texture in which larger crystals or grains occur in smaller ones
Pyroxenes – group of dark minerals - Ca, Mg, Fe, Al silicates
Quartz – mineral – silicon oxide
Schlieren – the texture with discontinuous almost parallel bands
Sodalite – mineral of especially blue colour – Na, Al silicate with Cl
Translucent – light passes through the mineral but the object viewed is not clearly outlined

My faculty is able to provide the following services:

1. determination of petrographic name
2. petrographic description
3. mineral composition
4. chemical composition
5. physical mechanical properties.