A TEC mine does 700 points of damage, although this is reduced by all defensive technologies, including shield mitigation and armor. TEC mine's detonation range is 1500 and it can damage an unlimited number of targets within its blast radius.
TEC mine fields are bought and constructed in groups of ten for a cost of 570 135 70 per group. Unlike most other structures, several constructor ships can participate in the rapid deployment of a TEC mine field, although at most one constructor can be building an individual mine at any given time.
A very effective strategy is to construct a mine field where a phase lane meets a gravity well. Allied ships will be unharmed while enemy ships could be heavily damaged or destroyed. If the enemy breaks through the mines, be sure to construct several Gauss Defense Platforms and an Argonev Star Base to finish off the survivors before they come. This will enable you to distract the attackers and bring in reinforcements. This tactic works very well if the planet is a choke-point and has only one phase lane that goes in the direction of the enemy.
If you don't want enemy scouts to set off the mine fields, then you can also make the mines go in a circle around the planet, although you should put more mines in the direction of enemy phase lanes. That way, mines won't be used on scouts, and when enemy ships try to come in attacking range of the planet, they'll be greeted by mines. Be sure to have a proper defenses because mines don't always kill the enemy.
The mine does not become instantly phased out when it is built, so you do need to build them ahead of the enemy's arrival.