GRANITE FOR SURFACE PLATES

In geological terms more commonly used, rocks for surface plates are Granite, Diabase / Dolerite and Gabbro. Granite consists of a fairly coarse grained aggregate of quartz and feldspar with small quantity of black or white mica. More often granite is identified by shades like black, grey, pink, etc,. The other and rather more commonly used rock for surface plate is the Diabase or Dolerite.

The properties, which help in selecting a rock for manufacture of surface plates, are hardness, density and the grain size. Uniformity of grains and structure is also a good feature. In general fine-grained Dolerite has better scratch resisting property than harder grey granites,and this has been confirmed by scratch test using a sharp diamond.

Another important property of rocks is the density. The compressive strength of a rock increases with an increase in the density. This higher density rocks have generally smaller grains and lesser porosity. Dolerite stays ahead in this category with very fine and smaller grains when compared with other igneous rocks. Diabase / Dolerite is a variety of a stone type known as Gabbro.

While it is closely related to granite, it is not the same. Dolerite lacks several of the minerals (such as quartz and mica) which give granite its distinctive grain structure. The lack of quartz means that Dolerite is slightly less resistant to wear than the hardest granite. However, this is offset by the lack of mica, which is very soft, and tends to flake out of granite plates, leaving pits. Dolerite is denser than granite, weighing about 190 lbs. per cu.ft. While most granite weigh between 160-170 lbs. per cu.ft.

Dolerite is less porous and more stable, which means that a smoother and more uniform surface finish can be achieved. It can used for multi-dimensional, very high-accuracy applications that require a very smooth, stable surface like angle plates, parallels, master squares, straight edges and so on.

BEING LESS POROUS, IT ABSORBS LESS MOISTURE THAN GRANITE AND THEREFORE WARPS LESS.

A medium to coarse-grained intrusive igneous rock composed of potassium and sodium rich feldspar, quartz, minor plagioclase, and small amounts of ferromagnesian minerals such as biotite or hornblende. It is the intrusive equivalent of rhyolite.
When we inspect granite specimen it shows orange to pink feldspar, white plagioclase, grey quartz with minor dark mafic minerals. Another form of coarse-grained granite consisting of white feldspar, grey quartz and black biotite it also contains an inclusion of dark metamorphic rock.

AVERAGE CHEMICAL COMPOSITION OF GRANITE (From Handbook of Physical constant)
Oxide	Weight
SiO2	70.18
TiO2	0.39
Al2O3	14.97
Fe2O3	1.57
FeO	1.78
MnO	0.12
MgO	0.88
CaO	1.99
Na2O	3.48
K2O	4.11
H2O+	0.84
H2O-	0.03
P2O5	0.19

MAJOR INGREDIENTS OF GRANITE
Quartz	38.8%
Orthoclase	16.7%
Albite	28.3%
Anorthite	9.2%
Corundum	0.3%
Hyperthene	4.3%
Ilmenite	0.6%

ADVANTAGES OF GRANITE SURFACE PLATES

ADVANTAGES OF GRANITE

1.	High degree of hardness (High quartz- high hardness)
2.	Low heat conductance
3.	Low temp sensitivity
4.	Non-magnetic
5.	Electrically Non-conductive
6.	Rust and acid resistance
7.	Matt surface, non-reflective surface
8.	Low water absorption
9.	High modulus of elasticity- Torsionally rigid, sag free, good slid's prop
10.	Dolerite - a better scratch resistance
11.	Modulus of elasticity - 26-86 GPa
12.	Compressive strength - 107 Mpa / 15600 PSI
13.	Water absorption - 0.07 - 0.31
14.	Density - 3g/cc
15.	Hardness - 6-7 Mohs scale
16.	Tensile strength 5 Mpa
17.	Co-efficient of thermal expansion - 6.1X10 -6/°C

It is  observed that fine-grained rocks have relatively higher density and hardness. They have higher resistance to scratch and are less porous. However stability of granite under varying temperature and humidity conditions is a major limitation even though the coefficient of thermal expansion is low compared to other engineering materials like cast iron and steel. If there is any small impact or scratch made over the granite surface plate, it results in crushing of localized zone and a broken piece or powder comes out without harming the surface accuracy. However in the case of cast iron or steel surfaces, under similar circumstances a bulge would result and the surface accuracy is affected and often these remain unnoticed by the naked eye.

The granite color alone is not an indication of the physical qualities of the stone. In general, the presence or absence of minerals determines the color of the granite and this may have no bearing on the qualities that make good surface plate material. There are pink, gray and black granites that are excellent for surface plates. However the consistency of several made from the same vein in a quarry can be unpredictable.this inconsistency is much less in pure black granite.

The aging qualities of these rocks are also excellent. This is conformed by a measurement carried out on a 1.6m X 1m surface plate kept in a metrology room immediately after its lapping and sparingly used over a period of four years. The variation in shape and the accuracy of this surface is negligible. Thus granite has been found to be a useful material for providing an accurate surface.