What is the exact nature of Ming Guan kie? from the specimen and pictures i'm looking at it seems like a plate armor covering the upper body with a lamellar underneath. at least the Hu Jin seem like a plate instead of any lamellar or other type of armor?

Mingguang kai 明光铠 ('bright shining armour') is a type of armour developed in the Three Kingdoms and Age of Fragmentation, and used until the Tang. It was a suit of armour much more complete than the liangdang, in that it had shoulder guards, a neck guard and a skirt. Except for the breastplate and backplate, which were each a single sheet of leather, the other parts were of leather or iron lamellar. On the areas of the breastplate corresponding to the two halves of the chest, there were iron plates riveted on, called the hujing 护镜 (protective mirrors). These could be finely polished with water to achieve a brilliant shine, which was perhaps used to disrupt an enemy's vision during close quarters fighting. An additional hujing was often added to the abdomen area during the Tang.

In the below picture, the soldier on horseback and the man in the white cape on the left are both wearing versions of mingguang kai.

I always thought the plates were put over lamellar armor, but are you saying that the plates were the only armored part ?

Is that the form that is also called 'plate and cord' armor?

It's possible that the backplate and breastplate were lamellar beneath a layer of cloth, with the extra iron plates riveted on. But the material of the backplate and breastplate is not known for sure because we have only clay figurines to go on. Chris Peers and Liu Yonghua seem to believe that it was just two large pieces of leather, and I find this reasonable because leather lamellar is only superior to leather sheet in its flexibility - which is not necessary on the chest and back.

Cord and plate (or cord and plaque) was a development of minguang kai in which the front plate and back plate were tied together with a cord that first ran around the chest and then formed a yoke around the neck. This is probably because the weight of the extra iron plates made it difficult to be supported by just leather straps over the shoulders, as in the liangdang and earlier mingguang. In the above picture, the man in the white cape is wearing that kind of armour, but it is mostly covered up by the cape. In the below picture of Tang soldiers, the man on the left has a cord and plaque style.

what exactly are the plates made of? the osprey books referred to them as "cord and plaque".

The chest plates on the Mingguang kai are either iron or steel.

What is the difference?

I mean, I know that steel is iron + carbon, but as iron rusts, once people learnt rust prevention techniques (which they did quite early in history), they don't use pure iron for arms or armour anymore, right?

So what are being referred to when the term iron is used in arms and armour?

I'm no expert on metallurgy, and I'm sure Kenneth or Wujiang can give you a good answer. But one thing about Mingguang kai is that the chest plates were highly polished to shine in the sunlight, hence the name "mingguang" (brilliant light). So this would seem to indicate that they were of steel.

We have some more info on Mingguang armour on this thread: http://www.chinahistoryforum.com/index.php...opic=3224&st=15