Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
Malphite does best when he has health and that is why he builds armor, magic resist, and health items. He wants to keep his health... he has to have that Armor and MR. Start out with the Mana item because it helps to use his Seismic Shard more often to harass the enemy. Eventually the enemy will become low enough with seismic shard hits and you and your lane partner can kill slowed enemy. Later in game Malphite becomes very hard to kill and thats why Sunfire Cape and Frozen Mallet are important. He has to make sure he has some sort of damage output so that he can effectively draw attention from your squishy team mates to yourself.